DailyDriver
Member
.
Last edited:
Question about downshifting MS3, double-clutching needed?
- Thread starter DailyDriver
- Start date
I've never owned a car that was happy shifting into first unless stopped or damn near close. In fact, I've had cars that locked it out entirely. So I have never made a habit of this behavior. If I am in a situation like the one you describe here, I will go for second gear. First gear is so short in this car anyway, I see no reason to use it unless it's from a complete stop. I realize this car is turbo and has some lag, but it seems to do just fine in 2nd gear at low speeds. Even from a complete stop, I start in 2nd gear every now and then. If you are smooth on the throttle and engage the clutch at the right moment, it doesn't bog nor do you have to slip it.
I suspect the noise you are hearing is the synchros. I'm sure if you revmatch properly it would slide in easier...
I suspect the noise you are hearing is the synchros. I'm sure if you revmatch properly it would slide in easier...
Last edited:
Silver Ecstasy
Member
- :
- 11 MS3 Sport
Helpful tip, don't EVER shift into 1st while the car is still moving.
If anything, you can always re-engage 2nd, should the light turn green and you're still coasting, but 1st is too large of a gear (in size) and too small (in ratio) to even bother with, unless you're at an entirely DEAD stop.
It's painful to be a passenger in a car where the driver will force the shifter into 1st while the car is still coasting to a stop. Here's an example (right at 0:12)
<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/O82UxYEENO4&hl=en_US&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/O82UxYEENO4&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>
*EDIT*
LOL we posted at the same time. +1 to all your tips. And yes, synchros arn't happy in that scenario.
If anything, you can always re-engage 2nd, should the light turn green and you're still coasting, but 1st is too large of a gear (in size) and too small (in ratio) to even bother with, unless you're at an entirely DEAD stop.
It's painful to be a passenger in a car where the driver will force the shifter into 1st while the car is still coasting to a stop. Here's an example (right at 0:12)
<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/O82UxYEENO4&hl=en_US&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/O82UxYEENO4&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>
*EDIT*
LOL we posted at the same time. +1 to all your tips. And yes, synchros arn't happy in that scenario.
Silver Ecstasy
Member
- :
- 11 MS3 Sport
I forgot to add, even if I am going to shift into 1st at a dead stop, ill release the clutch and re-engage.
In my opinion, don't shift into 1st, do it to 2nd as jeebusm3 suggested...
Anyways, your double cluthing might do it smoother, but the key is rev-matching (blip the throttle) before you release the clutch and engage the lower gear.
If you're familiar with it, preferably heel-and-toe always, you'll go much smoother while shifting, more rapid changes and less clutch wear as well... Plus, it'll make you look good with the people you ride with in case they know cars...
PS - Don't ride the clutch, never, the more you press the pedal, the more it'll wear out. If you're on neutral, while lowering you're speed, or if you've stopped, don't keep pressing the pedal, release it
Anyways, your double cluthing might do it smoother, but the key is rev-matching (blip the throttle) before you release the clutch and engage the lower gear.
If you're familiar with it, preferably heel-and-toe always, you'll go much smoother while shifting, more rapid changes and less clutch wear as well... Plus, it'll make you look good with the people you ride with in case they know cars...
PS - Don't ride the clutch, never, the more you press the pedal, the more it'll wear out. If you're on neutral, while lowering you're speed, or if you've stopped, don't keep pressing the pedal, release it
Last edited:
Stavesacre21
Member
- :
- 2010 Mazdaspeed3
If not in the MS3 manual, I KNOW i've read in other manuals that you are directed explicitly NOT to shift into 1st gear over speeds of usually 20-25 or transmission damage can occur.
I wish I had the rationale for this, but I can tell you that there is a lot of truth to it. There's no need to ever be downshifting to 1st gear above speeds of 20ish MPH. In most cases, 2nd gear will suffice if you're downshifting to hop out of an apex. Short of stopping completely, 1st gear would be obsolete anyway. And even when coming to a complete stop, you've got gobs of time to throw it into 1st gear below 20.
I wish I had the rationale for this, but I can tell you that there is a lot of truth to it. There's no need to ever be downshifting to 1st gear above speeds of 20ish MPH. In most cases, 2nd gear will suffice if you're downshifting to hop out of an apex. Short of stopping completely, 1st gear would be obsolete anyway. And even when coming to a complete stop, you've got gobs of time to throw it into 1st gear below 20.
DailyDriver
Member
.
Last edited:
Every car I've ever driven, with the exception of a mini-van, has been equipped with a manual transmission. Probably 10+ cars...give or take. Some of them have been a little cranky at getting them into 1st while still rolling. Some of them have not. In EVERY case, I have been able to put the car into 1st gear WITH THE CLUTCH PEDAL PRESSED while doing anywhere from 5 to 20 mph with the intent that I'll be stopping or coming very close to stopped before I release the clutch and accelerate again (basically what you're doing DailyDriver). I have *NEVER NEVER NEVER* had a problem with a clutch. Never ever.
Silver Ecstasy
Member
- :
- 11 MS3 Sport
The clutch is a guaranteed wear-and-tear item. It's going to wear as it's a necessary part in the car.
I think what you meant to say is, "Let out the clutch if it's not necessary to hold it in. You'll reduce wear on the throwout bearing."
Double clutching is a thing of the past. New cars don't need it in any application. Just downshift and revmatch and your set. I never downshift into 1st gear and rarely into 2nd.
In DD if im going relatively fast (50mph) I will downshift from 5th-4rth and then to 3rd. Then ill just coast in and pop it in neutral before stopping.
In DD if im going relatively fast (50mph) I will downshift from 5th-4rth and then to 3rd. Then ill just coast in and pop it in neutral before stopping.
Stavesacre21
Member
- :
- 2010 Mazdaspeed3
My quote above was also referring to the same...not engaging the clutch, that is. I've just always heard to never even downshift into 1st unless your pert-near at a complete stop. Just getting it into gear can be almost more of a problem then engaging.
the reason you are NOT (and I mean -NOT-) supposed to shift into first gear at speeds above 20ish mph is because despite what most people think the transmission is still spinning when the clutch is disengaged
the transmission consists of three shafts: the input shaft, the lay shaft, and the output shaft
the input shaft connects to the engine and drives the layshaft
the layshaft contains one of two of each gearset
the output shaft contains the second cog in each gear set, the gear selection collars, and hooks up to the differential (which in laymans terms means it's hooked up to the wheels directly)
the gears on the layshaft are directly attached to the layshaft, and spin at the engine's speed at all times
the gears on the output shaft are placed on roller bearings and are connected to the layshaft gears at all times...this allows them to spin at the engine's speed at all times while the wheels spin at a different speed
the gear selection collars are attached to the output shaft via a spline and spin at the same speed of the output shaft at all times...but they are still able to slide along the output shaft as well...when you put a car into gear that pushes the collar along the output shaft and mates it up to the gear you are selecting, which then locks that gear up to the output shaft and allows power to travel through that gear to the output shaft and then to the wheels
that being said, and the fact that every gear spins at engine speed at all times, not wheel speed, you might notice one issue: how do you engage each gear? each gear spinning at different speeds from the wheel speed means that when the collar needs to mate up to the gear you want it's going to destroy itself when it touches that gear...which brings us to what double clutching was originally needed for and furthermore to why it is no longer needed:
double clutching was used when synchros did not exist...you needed to bring the engine speed up to a certain rpm so as to match the speed of the gear selection collar with the speed of the gear you want so they shift into each other smoothly...thus you would push the clutch in, let it out, raise or lower the rpm's accordingly, then push the clutch in, shift, and let it out again
THIS IS NO LONGER NEEDED IN MODERN TRANSMISSIONS AND WILL NOT AFFECT WHETHER OR NOT YOU CAN SHIFT INTO A GEAR
which brings us to a synchro and furthermore to the reason why you cannot shift into 1st above a certain RPM:
a synchro is the same thing as a gear selection collar in that it performs the same task, how it does that is slightly different...what a synchro does is allows the gear and the collar to make frictional contact prior to letting the collar teeth (called dog teeth) to engage with the gear...the synchro housing rubs up against the gear when you begin placing the car into gear, causing the gear to begin to spin at the speed of the output shaft, then when you finally lock the gear in (which you can feel through the lever) the dog teeth have engaged into the gear...then when you let the clutch disengage the engine rpms have to catch up to the input shaft's speed to match the speed of the gear on the layshaft
now, a gear is a ratio of the speed of the wheels to the speed of the engine...a pretty common 1st gear ratio is about ~2.3:1, which means that if the engine is spinning at 2300 rpms the output shaft would be spinning at about 1000 rpms, where as a common 5th gear ratio would be around ~.9:1 which means that if the engine is spinning at 900 rpms the output shaft would be spinning at about 1000 rpms....now let's run some simple math to calculate the RPM of a standard 17" wheel with a 215/45 profile tire (pretty common, but we will round the 45 profile up to 4" from 3.7" for easy calc)....which means that at 20 mph the wheel is spinning at about 43 rpms
now that means that if we calculate that down to the diameter of the output shaft (we will say 2" in diameter for good measure, it's smaller than that) it would be roughly spinning at 3,000 rpms (actually much higher because the output shaft is not 2" in diameter, it is smaller than that)
now in first gear at 20 mph and the output shaft spinning at 3,000 rpms that means that the engine would need to be spinning at 2.3 times that (according to what is a really common 1st gear ratio of 2.3:1)...which places engine speed at 6900 rpms if you were to engage it at that speed, which is probably either above or just barely at your redline considering unless you drive a honda your redline is probably 6500 rpms....so to spin the layshaft and input shaft up to a speed of 6900 rpms from a speed of around 1000 rpms takes a lot longer than the typical gear change, which puts a LOT of stress on the synchro....as a result most car companies place a lockout on downshifting to first above a certain rpm, and also as a result causes the gear to grind when you try to place it into first if there is no lockout
bottom line:
unless you are stopped or traveling under ~10mph use 2nd gear, it's usually around 1.5:1 and will give you MORE than enough usable power to the ground to get out of the way of something in any weird traffic situation you can think of...especially with the amount of torque that the speed3 produces....first gear is only there to get the car in motion from a stop, once you have any slight amount of momentum you can use 2nd gear 95% of the time and usually can use 3rd if you really want to...the point of such a high gear ratio of 2.3:1 is to get a 3000lb hunk of metal moving from a stop...if you have ever had to push a dead car from a stop you would remember that getting it moving is always a lot harder than keeping it in motion because you are not changing it's inertia once it is in motion because it has momentum now...in comparison to 1st gear a common 2nd gear ratio is closer to 1.5:1, which is MUCH MUCH lower for a reason...1st gear should not be downshifted into most of the time, and if you are too impatient to wait until you are moving slower to get the car into first even if you aren't engaging it into first then you probably shouldn't be driving a manual, sorry
remember, production cars are designed with idiots in mind...if they don't place lockouts on these kinds of things then some idiot would be downshifting into first every day, placing lots of stress on that synchro and also possibly bringing his engine above redline, and then the car company is liable to replace the blown engine/trans under warranty...which is bad for business, and obviously why they choose to just lock those things out...anyone with a brain who actually understands the basics of how cars work, or at least the basic rules of how to drive stick, will never even notice the lockout because they won't be trying to downshift into first at those speeds because they have second gear instead
if you wish to bring the rpms up to near redline to downshift into first and your car is not equipped with a lockout then sure, that may work...but due to the high gear ratio you may still have trouble getting into gear and you will just look like a jackass driver to your passengers when you are traveling at 20mph and redline when 2nd gear will suffice just fine
and here's a diagram of a standard manual transmission:
and a diagram of how a synchro works:
if you need more info on manual transmissions, howstuffworks.com has a pretty good article on it:
http://auto.howstuffworks.com/transmission.htm
the transmission consists of three shafts: the input shaft, the lay shaft, and the output shaft
the input shaft connects to the engine and drives the layshaft
the layshaft contains one of two of each gearset
the output shaft contains the second cog in each gear set, the gear selection collars, and hooks up to the differential (which in laymans terms means it's hooked up to the wheels directly)
the gears on the layshaft are directly attached to the layshaft, and spin at the engine's speed at all times
the gears on the output shaft are placed on roller bearings and are connected to the layshaft gears at all times...this allows them to spin at the engine's speed at all times while the wheels spin at a different speed
the gear selection collars are attached to the output shaft via a spline and spin at the same speed of the output shaft at all times...but they are still able to slide along the output shaft as well...when you put a car into gear that pushes the collar along the output shaft and mates it up to the gear you are selecting, which then locks that gear up to the output shaft and allows power to travel through that gear to the output shaft and then to the wheels
that being said, and the fact that every gear spins at engine speed at all times, not wheel speed, you might notice one issue: how do you engage each gear? each gear spinning at different speeds from the wheel speed means that when the collar needs to mate up to the gear you want it's going to destroy itself when it touches that gear...which brings us to what double clutching was originally needed for and furthermore to why it is no longer needed:
double clutching was used when synchros did not exist...you needed to bring the engine speed up to a certain rpm so as to match the speed of the gear selection collar with the speed of the gear you want so they shift into each other smoothly...thus you would push the clutch in, let it out, raise or lower the rpm's accordingly, then push the clutch in, shift, and let it out again
THIS IS NO LONGER NEEDED IN MODERN TRANSMISSIONS AND WILL NOT AFFECT WHETHER OR NOT YOU CAN SHIFT INTO A GEAR
which brings us to a synchro and furthermore to the reason why you cannot shift into 1st above a certain RPM:
a synchro is the same thing as a gear selection collar in that it performs the same task, how it does that is slightly different...what a synchro does is allows the gear and the collar to make frictional contact prior to letting the collar teeth (called dog teeth) to engage with the gear...the synchro housing rubs up against the gear when you begin placing the car into gear, causing the gear to begin to spin at the speed of the output shaft, then when you finally lock the gear in (which you can feel through the lever) the dog teeth have engaged into the gear...then when you let the clutch disengage the engine rpms have to catch up to the input shaft's speed to match the speed of the gear on the layshaft
now, a gear is a ratio of the speed of the wheels to the speed of the engine...a pretty common 1st gear ratio is about ~2.3:1, which means that if the engine is spinning at 2300 rpms the output shaft would be spinning at about 1000 rpms, where as a common 5th gear ratio would be around ~.9:1 which means that if the engine is spinning at 900 rpms the output shaft would be spinning at about 1000 rpms....now let's run some simple math to calculate the RPM of a standard 17" wheel with a 215/45 profile tire (pretty common, but we will round the 45 profile up to 4" from 3.7" for easy calc)....which means that at 20 mph the wheel is spinning at about 43 rpms
now that means that if we calculate that down to the diameter of the output shaft (we will say 2" in diameter for good measure, it's smaller than that) it would be roughly spinning at 3,000 rpms (actually much higher because the output shaft is not 2" in diameter, it is smaller than that)
now in first gear at 20 mph and the output shaft spinning at 3,000 rpms that means that the engine would need to be spinning at 2.3 times that (according to what is a really common 1st gear ratio of 2.3:1)...which places engine speed at 6900 rpms if you were to engage it at that speed, which is probably either above or just barely at your redline considering unless you drive a honda your redline is probably 6500 rpms....so to spin the layshaft and input shaft up to a speed of 6900 rpms from a speed of around 1000 rpms takes a lot longer than the typical gear change, which puts a LOT of stress on the synchro....as a result most car companies place a lockout on downshifting to first above a certain rpm, and also as a result causes the gear to grind when you try to place it into first if there is no lockout
bottom line:
unless you are stopped or traveling under ~10mph use 2nd gear, it's usually around 1.5:1 and will give you MORE than enough usable power to the ground to get out of the way of something in any weird traffic situation you can think of...especially with the amount of torque that the speed3 produces....first gear is only there to get the car in motion from a stop, once you have any slight amount of momentum you can use 2nd gear 95% of the time and usually can use 3rd if you really want to...the point of such a high gear ratio of 2.3:1 is to get a 3000lb hunk of metal moving from a stop...if you have ever had to push a dead car from a stop you would remember that getting it moving is always a lot harder than keeping it in motion because you are not changing it's inertia once it is in motion because it has momentum now...in comparison to 1st gear a common 2nd gear ratio is closer to 1.5:1, which is MUCH MUCH lower for a reason...1st gear should not be downshifted into most of the time, and if you are too impatient to wait until you are moving slower to get the car into first even if you aren't engaging it into first then you probably shouldn't be driving a manual, sorry
remember, production cars are designed with idiots in mind...if they don't place lockouts on these kinds of things then some idiot would be downshifting into first every day, placing lots of stress on that synchro and also possibly bringing his engine above redline, and then the car company is liable to replace the blown engine/trans under warranty...which is bad for business, and obviously why they choose to just lock those things out...anyone with a brain who actually understands the basics of how cars work, or at least the basic rules of how to drive stick, will never even notice the lockout because they won't be trying to downshift into first at those speeds because they have second gear instead
if you wish to bring the rpms up to near redline to downshift into first and your car is not equipped with a lockout then sure, that may work...but due to the high gear ratio you may still have trouble getting into gear and you will just look like a jackass driver to your passengers when you are traveling at 20mph and redline when 2nd gear will suffice just fine
and here's a diagram of a standard manual transmission:
and a diagram of how a synchro works:
if you need more info on manual transmissions, howstuffworks.com has a pretty good article on it:
http://auto.howstuffworks.com/transmission.htm
Last edited:
Stavesacre21
Member
- :
- 2010 Mazdaspeed3
the reason you are NOT (and I mean -NOT-) supposed to shift into first gear at speeds above 20ish mph is because despite what most people think the transmission is still spinning when the clutch is disengaged
the transmission consists of three shafts: the input shaft, the lay shaft, and the output shaft
the input shaft connects to the engine and drives the layshaft
the layshaft contains one of two of each gearset
the output shaft contains the second cog in each gear set, the gear selection collars, and hooks up to the differential (which in laymans terms means it's hooked up to the wheels directly)
the gears on the layshaft are directly attached to the layshaft, and spin at the engine's speed at all times
the gears on the output shaft are placed on roller bearings and are connected to the layshaft gears at all times...this allows them to spin at the engine's speed at all times while the wheels spin at a different speed
the gear selection collars are attached to the output shaft via a spline and spin at the same speed of the output shaft at all times...but they are still able to slide along the output shaft as well...when you put a car into gear that pushes the collar along the output shaft and mates it up to the gear you are selecting, which then locks that gear up to the output shaft and allows power to travel through that gear to the output shaft and then to the wheels
that being said, and the fact that every gear spins at engine speed at all times, not wheel speed, you might notice one issue: how do you engage each gear? each gear spinning at different speeds from the wheel speed means that when the collar needs to mate up to the gear you want it's going to destroy itself when it touches that gear...which brings us to what double clutching was originally needed for and furthermore to why it is no longer needed:
double clutching was used when synchros did not exist...you needed to bring the engine speed up to a certain rpm so as to match the speed of the gear selection collar with the speed of the gear you want so they shift into each other smoothly...thus you would push the clutch in, let it out, raise or lower the rpm's accordingly, then push the clutch in, shift, and let it out again
THIS IS NO LONGER NEEDED IN MODERN TRANSMISSIONS AND WILL NOT AFFECT WHETHER OR NOT YOU CAN SHIFT INTO A GEAR
which brings us to a synchro and furthermore to the reason why you cannot shift into 1st above a certain RPM:
a synchro is the same thing as a gear selection collar in that it performs the same task, how it does that is slightly different...what a synchro does is allows the gear and the collar to make frictional contact prior to letting the collar teeth (called dog teeth) to engage with the gear...the synchro housing rubs up against the gear when you begin placing the car into gear, causing the gear to begin to spin at the speed of the output shaft, then when you finally lock the gear in (which you can feel through the lever) the dog teeth have engaged into the gear...then when you let the clutch disengage the engine rpms have to catch up to the input shaft's speed to match the speed of the gear on the layshaft
now, a gear is a ratio of the speed of the wheels to the speed of the engine...a pretty common 1st gear ratio is about ~2.3:1, which means that if the engine is spinning at 2300 rpms the output shaft would be spinning at about 1000 rpms, where as a common 5th gear ratio would be around ~.9:1 which means that if the engine is spinning at 900 rpms the output shaft would be spinning at about 1000 rpms....now let's run some simple math to calculate the RPM of a standard 17" wheel with a 215/45 profile tire (pretty common, but we will round the 45 profile up to 4" from 3.7" for easy calc)....which means that at 20 mph the wheel is spinning at about 43 rpms
now that means that if we calculate that down to the diameter of the output shaft (we will say 2" in diameter for good measure, it's smaller than that) it would be roughly spinning at 3,000 rpms (actually much higher because the output shaft is not 2" in diameter, it is smaller than that)
now in first gear at 20 mph and the output shaft spinning at 3,000 rpms that means that the engine would need to be spinning at 2.3 times that (according to what is a really common 1st gear ratio of 2.3:1)...which places engine speed at 6900 rpms if you were to engage it at that speed, which is probably either above or just barely at your redline considering unless you drive a honda your redline is probably 6500 rpms....so to spin the layshaft and input shaft up to a speed of 6900 rpms from a speed of around 1000 rpms takes a lot longer than the typical gear change, which puts a LOT of stress on the synchro....as a result most car companies place a lockout on downshifting to first above a certain rpm, and also as a result causes the gear to grind when you try to place it into first if there is no lockout
bottom line:
unless you are stopped or traveling under ~10mph use 2nd gear, it's usually around 1.5:1 and will give you MORE than enough usable power to the ground to get out of the way of something in any weird traffic situation you can think of...especially with the amount of torque that the speed3 produces....first gear is only there to get the car in motion from a stop, once you have any slight amount of momentum you can use 2nd gear 95% of the time and usually can use 3rd if you really want to...the point of such a high gear ratio of 2.3:1 is to get a 3000lb hunk of metal moving from a stop...if you have ever had to push a dead car from a stop you would remember that getting it moving is always a lot harder than keeping it in motion because you are not changing it's inertia once it is in motion because it has momentum now...in comparison to 1st gear a common 2nd gear ratio is closer to 1.5:1, which is MUCH MUCH lower for a reason...1st gear should not be downshifted into most of the time, and if you are too impatient to wait until you are moving slower to get the car into first even if you aren't engaging it into first then you probably shouldn't be driving a manual, sorry
remember, production cars are designed with idiots in mind...if they don't place lockouts on these kinds of things then some idiot would be downshifting into first every day, placing lots of stress on that synchro and also possibly bringing his engine above redline, and then the car company is liable to replace the blown engine/trans under warranty...which is bad for business, and obviously why they choose to just lock those things out...anyone with a brain who actually understands the basics of how cars work, or at least the basic rules of how to drive stick, will never even notice the lockout because they won't be trying to downshift into first at those speeds because they have second gear instead
if you wish to bring the rpms up to near redline to downshift into first and your car is not equipped with a lockout then sure, that may work...but due to the high gear ratio you may still have trouble getting into gear and you will just look like a jackass driver to your passengers when you are traveling at 20mph and redline when 2nd gear will suffice just fine
and here's a diagram of a standard manual transmission:
and a diagram of how a synchro works:
if you need more info on manual transmissions, howstuffworks.com has a pretty good article on it:
http://auto.howstuffworks.com/transmission.htm
(omg)
UH, YEAH...what he said.
{which means that at 20 mph the wheel is spinning at about 43 rpms
now that means that if we calculate that down to the diameter of the output shaft (we will say 2" in diameter for good measure, it's smaller than that) it would be roughly spinning at 3,000 rpms (actually much higher because the output shaft is not 2" in diameter, it is smaller than that)
now in first gear at 20 mph and the output shaft spinning at 3,000 rpms that means that the engine would need to be spinning at 2.3 times that (according to what is a really common 1st gear ratio of 2.3:1)...which places engine speed at 6900 rpms if you were to engage it at that speed, which is probably either above or just barely at your redline considering unless you drive a honda your redline is probably 6500 rpms....so to spin the layshaft and input shaft up to a speed of 6900 rpms from a speed of around 1000 rpms takes a lot longer than the typical gear change, which puts a LOT of stress on the synchro....as a result most car companies place a lockout on downshifting to first above a certain rpm, and also as a result causes the gear to grind when you try to place it into first if there is no lockout}
The diameter of the output shaft has nothing to do with wheel rotation speed. The output shaft will turn faster than the the wheel speed because the differential has final drive ratio gearing. Lets say it's 3:1. Using your example of wheels turning at 43 rpm, the transmission output shaft will be turning at 3 times wheel speed or 129 rpm. That will be true regardless of the output shaft diameter.
Don't know how you came up with a wheel speed of 43 rpm? Tire calculators say the 215/45/17 tire will rotate at 845 turns/mile. At 60 mph it would also be 845 rpm. 20 mph would require 3 minutes for a mile. At 20 mph you would go 1/3 mile/minute or 1/3 of 845 rpm= about 282 rpm at 20 mph. Now the transmission output shaft ( regardless of diameter)would spinning at about 846 rpm. Engine speed in first gear would be the the first gear ratio X the output shaft speed, assume a first gear ratio of 3:1, 846 X 3 = 2538 rpm, so there should be no problem shifting into first gear at 20 mph. That said many cars have poor shift linkages that can make shifting into lower gears difficult.
http://www.1010tires.com/TireSizeCalculator.asp?action=submit
Clifton
now that means that if we calculate that down to the diameter of the output shaft (we will say 2" in diameter for good measure, it's smaller than that) it would be roughly spinning at 3,000 rpms (actually much higher because the output shaft is not 2" in diameter, it is smaller than that)
now in first gear at 20 mph and the output shaft spinning at 3,000 rpms that means that the engine would need to be spinning at 2.3 times that (according to what is a really common 1st gear ratio of 2.3:1)...which places engine speed at 6900 rpms if you were to engage it at that speed, which is probably either above or just barely at your redline considering unless you drive a honda your redline is probably 6500 rpms....so to spin the layshaft and input shaft up to a speed of 6900 rpms from a speed of around 1000 rpms takes a lot longer than the typical gear change, which puts a LOT of stress on the synchro....as a result most car companies place a lockout on downshifting to first above a certain rpm, and also as a result causes the gear to grind when you try to place it into first if there is no lockout}
The diameter of the output shaft has nothing to do with wheel rotation speed. The output shaft will turn faster than the the wheel speed because the differential has final drive ratio gearing. Lets say it's 3:1. Using your example of wheels turning at 43 rpm, the transmission output shaft will be turning at 3 times wheel speed or 129 rpm. That will be true regardless of the output shaft diameter.
Don't know how you came up with a wheel speed of 43 rpm? Tire calculators say the 215/45/17 tire will rotate at 845 turns/mile. At 60 mph it would also be 845 rpm. 20 mph would require 3 minutes for a mile. At 20 mph you would go 1/3 mile/minute or 1/3 of 845 rpm= about 282 rpm at 20 mph. Now the transmission output shaft ( regardless of diameter)would spinning at about 846 rpm. Engine speed in first gear would be the the first gear ratio X the output shaft speed, assume a first gear ratio of 3:1, 846 X 3 = 2538 rpm, so there should be no problem shifting into first gear at 20 mph. That said many cars have poor shift linkages that can make shifting into lower gears difficult.
http://www.1010tires.com/TireSizeCalculator.asp?action=submit
Clifton
Last edited:
The op NEVER stated that he was shifting into first gear while going 20ish mph then letting the clutch out to begin accelerating again. He stated:
(excerpted)
For those of you arguing the shortcomings of downshifting into first at 20ish mph, then letting the clutch pedal out and accelerating...you're discussing something completly irrelevent to the OP's question. He's not accelerating, he's shifting into first at around 15mph, CONTINUING to hold the clutch pedal in, then coming to a complete stop.
Here's an example of when he, or I for that matter, may want to do something like that:
ex: I'm heading into work first thing in the morning during rush hour traffic and I'm approaching a 4-way intersection. I will be making a right turn in the right-turn lane. There are two additional lanes to my my immediate left, that are going straight but our lights have JUST turned red. I know the other lanes from the side street will now be going straight so I need to come to a complete stop and check to make sure that I'm not about to pull out in front of someone coming across the intersection. I quickly get to the light. As I slow down to around 15ish mph just before I stop, I push in the clutch and shift into first. A split second later, I'm stopped at the light and looking to my left. Now I see that traffic from that sidestreet is beginning to come across the intersection. I'm already in first gear, and stopped, so no need to spend that extra second shifting into first. I quickly launch into my right turn and continue cruising on down the road, just ahead of that traffic.
(I hope that example makes some kind of sense. heh)
(excerpted)
For those of you arguing the shortcomings of downshifting into first at 20ish mph, then letting the clutch pedal out and accelerating...you're discussing something completly irrelevent to the OP's question. He's not accelerating, he's shifting into first at around 15mph, CONTINUING to hold the clutch pedal in, then coming to a complete stop.
Here's an example of when he, or I for that matter, may want to do something like that:
ex: I'm heading into work first thing in the morning during rush hour traffic and I'm approaching a 4-way intersection. I will be making a right turn in the right-turn lane. There are two additional lanes to my my immediate left, that are going straight but our lights have JUST turned red. I know the other lanes from the side street will now be going straight so I need to come to a complete stop and check to make sure that I'm not about to pull out in front of someone coming across the intersection. I quickly get to the light. As I slow down to around 15ish mph just before I stop, I push in the clutch and shift into first. A split second later, I'm stopped at the light and looking to my left. Now I see that traffic from that sidestreet is beginning to come across the intersection. I'm already in first gear, and stopped, so no need to spend that extra second shifting into first. I quickly launch into my right turn and continue cruising on down the road, just ahead of that traffic.
(I hope that example makes some kind of sense. heh)
DailyDriver
Member
.
Last edited:
justanothermp5
Banned
- :
- Rust Bucket
dont shift into 1st while moving and you wont have a problem
and +1 to whatever neox said lol
and +1 to whatever neox said lol
