Pics of my MSP6 w/ 2 tone int.

Da 6 · Nov 26, 2005

don't know...depends if he is going backwoods with it

J/K Just don't get a lift kit!

crossbow · Nov 26, 2005

Pretty light for 18s

As one 6 owner proved at the track. Its the size, not the weight that matters most.

Going from his lightweight 18's to his stock heavy 17's (23 lbs 17 vs 19 lbs 18's), he went 0.2 seconds faster on the 1/4 consistently. (faster).

Another member tested the difference between 19 lb 18's, and 15 lb 16's, and got consistent repeatable benefits of an average of 0.5 seconds faster. (depending on the launch).

The 6 loves 16's to death, and we've got 8-10 timeslips proving it.

0.2 and 0.5 doesn't sound like much, but ask someone who goes to the track regularly what they'd give for that type of speed increase.

flat_black · Nov 26, 2005

Uhm, just saying '16's good' isn't really conducive to a reason. =) What size tires was it wearing, and what size are they, stock? The wheel/tire combo not only needs to have a known weight, but if you're having an overall smaller diameter tire/wheel combo, you end up with the lower gear ratio thing in your advantage, since your wheels are your true final gear. Whereas taller wheels will give you a longer gear ratio, which can be better in some circumstances, too.

Da 6 · Nov 26, 2005

0-60 isn't one of them

crossbow · Nov 26, 2005

Gearing was actually superior on the 18's. 225/40/18 vs 215/50/17. The 18's also were 2 lbs lighter, and were performance summer rubber instead of all-seasons.

Best time with stock 17's

60' - 2.281
330- 6.277
1/8- 9.557 @ 74.91
1000 12.375
1/4- 14.750 @ 94.94

Best time with 18's:

60' - 2.291
330- 6.384
1/8- 9.677 @ 74.49
1000 N/A
1/4- 14.929 @ 93.15

I'd post a list of articles, forums, and timeslips, but anyone with larger wheels always automatically discounts them so I feel its pointless to even argue.

Da 6 · Nov 26, 2005

good idea...difrent when a car is designed for 18's vs one designed for 16's

FX-MAN · Nov 26, 2005

A car design has nothing to do with how much rotational force is required to spin larger rims. Crossbow is right, diameter is exponentially more important than weight (in the physics world). It's much harder to spin a larger rim of equal weight no matter what engine is spinning it.. It all comes down to force, and spinning something when the mass is in the center is always easier (think about an ice skater, you pull your arms IN to spin faster, out to spin slower, the reason for this is equal kinetic energy applied to an object with more centered mass... it spins faster).

For handling, larger wheels are better though, since the tire is lower profile, you get less tire compression and shift on turns, the car hugs curves much better (which noticable as well with the 17s). So it depends on what is more important. If you were doing autocross, I think you would acutally want larger rims with lower profile rubber.

The main thing I want, is to make sure the 18s I put back on there this summer are lighter than the heavy-ass, ugly, impossible to clean, stock 18s that Mazda threw on the car (I much prefer the RX-8 and regular 6 wheels). The fact that the car will perform better with my winter 17s is fine with me, I am not getting rims for go... ALL SHOW. I know I will lose a little speed compared to the 17s, and I did notice gains with the 17s. The goal is to go lighter than stock when I switch back to my summer rubber. I think the 17s are nice but they look small on the car.

Da 6 · Nov 27, 2005

RX-8 would be perfomace due to the fact they are *cough* Eight Wide

roll fenders...

crossbow · Nov 27, 2005

FX,

Aye larger wheels are better if you lower the profile...but there isn't any reason you can't do that on a smaller wheel and retain the previously mentioned benefits.

ie if you start with a 215/50/17, you can easily go to a 225/45/17 or a 235/45/17, or even a 245/40/17 on the 6. Same with a 16. 245/45/16's do fit the car (standard 6) if you pick the right wheel offset. Then of course, going too small a profile has its disadvantages as well. Around 40-45 seems to be the best bang for the buck. When you get into the 30's the car starts becoming twitchy.

Rainman · Nov 27, 2005

crossbow said:
FX,

Aye larger wheels are better if you lower the profile...but there isn't any reason you can't do that on a smaller wheel and retain the previously mentioned benefits.

ie if you start with a 215/50/17, you can easily go to a 225/45/17 or a 235/45/17, or even a 245/40/17 on the 6. Same with a 16. 245/45/16's do fit the car (standard 6) if you pick the right wheel offset. Then of course, going too small a profile has its disadvantages as well. Around 40-45 seems to be the best bang for the buck. When you get into the 30's the car starts becoming twitchy.

If you take a larger wheel with a smaller profile tire and put it against a smaller wheel with a taller profile tire, shouldn't you get the same times as long as the outside diameter of the wheel is the same?

Different aspects of each combination are favourable, but it comes down to which advantage is more significant in the end.

For example, if you take a larger wheel (assuming that the weights of all tires and the wheels are the same) it has a larger radius. This makes it harder to turn. However, it increases the moment arm of the force that is applied at the point the wheel contacts the tire. If you also assume that there is no drivetrain loss at the wheel tire interface, the degree of change in performance is a function of the decreased turning velocity of the rim as a result of its larger radius vs the increased torque transmission to the edge of the wheel and thus the tire. I don't know the mathematical equation that defines this relationship so I am unsure whether both of these factors have a linear or an exponential relationship. Of course, this combination requires that the engine work harder for a given weight, however, if the engine has sufficient power this point shouldn't be a major issue.

Likewise, if you take a smaller wheel (again assuming equal weights all around) with a smaller radius, it is easier to turn. But, the moment arm is also smaller which means that the torque is less at the tire/wheel and the wheel/pavement interfaces (again assuming no drivetrain loss at the tire/wheel interface).

This means, either faster spinning small wheel with less torque available at the pavement, or slower spinning large wheel with more torque available at the pavement. Each could be beneficial in a given situation if ideally matched to the weight of the car.

Alas, I am still confused about the whole thing. (boom07)

R

crossbow · Nov 27, 2005

Rainman · Nov 30, 2005

Had my summer rims and tires in the trunk still cause I hadn't yet had an opportunity to take them out. Put them in the house last night and I gotta say, the stock rims and tires are friggin' heavy. I would guesstimate that they are between 40 to 50lbs each. They are significantly heavier than the rims and tires off the MSP. I'll bet that you could shave off between 50-60 lbs total with a different rim selection. I'll have to put these badboys on a scale when I get a chance.

R

FX-MAN · Dec 6, 2005

Logically, I can see what you are saying with same size (larger rims, smaller wheels = same diameter and same weight, you would THINK that you should run the same times...) This is actually not correct though.....

Problem is that the little 1" strip of rubber you are losing weighs A LOT less than the 18lbs (or so) of Alloy or Steel you are pushing an inch farther out. The alloys weighs more (per inch) than the rubber, and wheels have a LARGE majority of the weight on the outside of the rim so expanding that, basically pushes all that weight out while taking away only an inch or 2 of rubber (you have the same tread in the same spot either way). The distance an object is from the fulcrum is actually exponentially more important than it's weight... So in practice.. the answer is no.. you don't keep the same times, weight dispersal goes to the outside of the wheel.

Ideally what crossbow is saying is 100% true, you want lightweight SMALL diameter rims (for speed) and relatively low profile tires (for handling).

colin949 · Dec 6, 2005

The backseats fold down!!!
there is just a bar there with with a triangle effect you can still put stuff through it. it's got a couple scews it takes 30 seconds

Rainman · Dec 7, 2005

FX-MAN said:
The distance an object is from the fulcrum is actually exponentially more important than it's weight... So in practice.. the answer is no.. you don't keep the same times, weight dispersal goes to the outside of the wheel.

Ideally what crossbow is saying is 100% true, you want lightweight SMALL diameter rims (for speed) and relatively low profile tires (for handling).

I am working through all of this in my head and I am interested in the physics behind this. F=M*A and Tq=F*r (where r=radius of the lever arm, in this case the radius of the rim in question). This is a linear relationship rather than exponential. Centripital acceleration involves an exponential relationship however. Is this what you refer to here?

I am trying to picture the two scenarios on a top fuel drag car. I guess that you never see a dragster with huge ass wheels with a thin strip of rubber around them. Then again, you don't usually see track cars with tires that have huge sidewalls either. Educate me.

R

crossbow · Dec 7, 2005

Dragsters only go in a straight line (hopefully). Same with other drag cars. Forward traction is their only concern.

Smaller sidewall profiling prevents distortion of the tread under cornering situations. Since drag cars don't need to turn the wheel except to try and stay in a straight line, it doesn't matter if they have giant sidewalls. They only use a few psi (2-4) in those giant tires, so the tire has maximum contact patch with the road surface.

Check out the massive distortion in the sidewalls. They tires HAVE to be that big just to absorb the massive torque. Those cars are outputing 7,000 bhp+.

Thats a simple explanation. If you want the physics and such, google has a bunch of links if you start searching.

FX-MAN · Dec 10, 2005

The torque equasion refers to how much force you can apply due to your current leverage (a wrench on a nut, and the length of the wrench is linearly important to how much force you can put on the nut), Don't confuse this leverage as the same concept as loss of torque due to mass being on the outer edge.. torque is spinning that edge (the edge is not what is generating torque).... it's a very different equasion when you are talking leverage radius for a force applied to spin an Axis point (what the engine is doing), VS something being spun around an Axis at a radius causing a loss in Intertia.

what we are talking about here is the loss in Inertia due to radius taking into account an object's Mass. You need to use the "moment of inertia" equasion for a hoop or solid cyllinder rotating about it's diameter. In this equasion Inertia=1/2Mass*Radius^2 (radius squared).. this is what causes the loss in torque due to mass being pushed farther out. The radius of the wheel is twice as important as 1/2 the mass, which is why the same weight wheel if it's larger will take more torque to generate the inertia. Using this equasion you can calculate how much lighter a wheel would HAVE to be to maintain the same inertia with the same torque applied at the hub.

lets say I have a 20lb 17inch wheel, and I want an 18" wheel that won't cost me torque (and lets say the wight distribution along the radius of those 2 wheels is the same.. although it won't be exact, this is still ROUGHLY close).... so 1/2N*18"^2 = 10*17"^2
N=17.83

So to maintain the same amount of torque on the road, I need a 17.83lb 18" rim to replace my 20lb 17inchers (or my 26.6lb 15" wheels)

now, you start talking something like 12lb 14" Motegi RT5s? VS an 18" rim?
You would need 7.25lb 18s to match torque on the road.... LOL

nalspeed · Dec 23, 2005

men I want one arghhhhhh

Falango · Dec 24, 2005

I love watching them tires on top fuels grow as you floor it. Damn I love the sound of em too! It's crazy how much torque those cars put out. So much that the rim spins before the tire moves. Thank god for beadlocks!

Rainman · Dec 24, 2005

FX-MAN said:
The torque equasion refers to how much force you can apply due to your current leverage (a wrench on a nut, and the length of the wrench is linearly important to how much force you can put on the nut), Don't confuse this leverage as the same concept as loss of torque due to mass being on the outer edge.. torque is spinning that edge (the edge is not what is generating torque).... it's a very different equasion when you are talking leverage radius for a force applied to spin an Axis point (what the engine is doing), VS something being spun around an Axis at a radius causing a loss in Intertia.

what we are talking about here is the loss in Inertia due to radius taking into account an object's Mass. You need to use the "moment of inertia" equasion for a hoop or solid cyllinder rotating about it's diameter. In this equasion Inertia=1/2Mass*Radius^2 (radius squared).. this is what causes the loss in torque due to mass being pushed farther out. The radius of the wheel is twice as important as 1/2 the mass, which is why the same weight wheel if it's larger will take more torque to generate the inertia. Using this equasion you can calculate how much lighter a wheel would HAVE to be to maintain the same inertia with the same torque applied at the hub.

lets say I have a 20lb 17inch wheel, and I want an 18" wheel that won't cost me torque (and lets say the wight distribution along the radius of those 2 wheels is the same.. although it won't be exact, this is still ROUGHLY close).... so 1/2N*18"^2 = 10*17"^2
N=17.83

So to maintain the same amount of torque on the road, I need a 17.83lb 18" rim to replace my 20lb 17inchers (or my 26.6lb 15" wheels)

now, you start talking something like 12lb 14" Motegi RT5s? VS an 18" rim?
You would need 7.25lb 18s to match torque on the road.... LOL

I realized this after watching a program discussing the disadvantage of big ass disc brakes which can often make a cars stopping distance worse if they do not take into account the factors you mentioned.

Using your equation though radius is not twice as important as the mass, it is exponentially more important. That is, the effect of changes in radius act exponentially on the inertia while changes in mass act only linearly on inertia. All this physics brings me back....way back...LOL! Didn't give a s*** about it then, but now it comes in handy every once and a while.

(deadhorse)

All in all, this is making me want to get 17" rims for the summer rather than 18" shoes. The 18" lo-pro look good, but I'm kinda getting used to the 17" look as my snows are this size and the car still looks mean as hell. With some bad-ass 17" rims and a subtle drop it'll even be sicker!

R

Pics of my MSP6 w/ 2 tone int.

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