Scrub Radius

[Ghetto Mods]

O.K. so I'm trying to find out a bit more about this since it is affected by wheel offset.

Specifically, I had to pass on a really nicely priced set of rims b/c the offset was like 36 mm and our cars typ. prefer ~50 or so.

So can anybody tell me specifically what is the effect on the car's handling of altering the scrub radius? Specifically using a lower offset wheel which will increase (or make less negative) the scrub radius?

Thanks in advance suspension gurus.

PS Search function and google were not much help here.

 

[Ghetto Mods]

(cricket)

anybody???

 

[ms3kar]

Google scrub radius and torque steer. There are a few pretty good articles out there.

 

[bacarl]

You may/maynot be aware, but the fact that you can't mount an et+36 wheel on our cars is more a packaging issue than anything else. Absolutely, torque steer and handling will be affected, but more importantly, with a +36 offset, your fender will be in your tire.

May help: http://en.wikipedia.org/wiki/Scrub_radius

 

[Ghetto Mods]

Well aware of the fender clearance issue. Looking for a bit more on how running lower offset would affect handling - aside from torque steer.

The few resources online allude to this phenomenon, but nobody has weighed in with a solid opinion on this.

You may/maynot be aware, but the fact that you can't mount an et+36 wheel on our cars is more a packaging issue than anything else. Absolutely, torque steer and handling will be affected, but more importantly, with a +36 offset, your fender will be in your tire.

May help: http://en.wikipedia.org/wiki/Scrub_radius

 

[ms3kar]

Torque steer is really the biggest problem as far as handling is concerned. Since I doubt anyone (except mazda engineers) knows the exact suspension geometry of the ms3 it's hard to say how the scrub radius would change with a specific change in wheel offset though.

 

[dan7225]

i think... the smaller the scrub radius the better (except for negative).

 

[nhluhr]

Old thread, I know, but I can also explain some of the many effects of scrub radius.

All these effects consider going to a more positive scrub radius (i.e. the tire is farther out than normal, like when you put on a smaller than stock offset wheel):

1) Torque steer is increased
2) Track width is increased, allowing for slightly less load transfer in corners
3) Steering weight is increased (may not be noticeable, but the power steering pump will be working harder and the fluid may overheat in some situations)
4) increased tramlining feel on rough or grooved pavement
5) increased wheelbearing wear rate
6) increased steering instability when braking

If for some reason the scrub radius is not identical left to right, the vehicle will pull strongly.

That said, McPhearson Strut style suspensions generally benefit from a small amount of negative scrub because of the relatively high amounts of SAI and caster inherent to these suspensions. Also, negative scrub is frequently employed on FWD cars just for the torquesteer reduction.

There is an issue with having 0 scrub as well. Quoting from an online resource:
"If the scrub is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a welded differential, inducing a condition called 'squirm'. In a straight line the tire tends to be stable and tracks well. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the contact patch. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes 'grabby' causing tire wear to increase and the steering to become unstable."

So in short, going with less offset than stock will give a very small benefit in terms of load transfer (and thus available cornering grip) but will also yield a myriad number of minor issues as listed above ranging from increased tire wear to stability problems. I have found that the tiny increase in grip is almost never enough to outweigh the speed you find from a confidence-inspiring stable handling car so I always stick with as close to OE offset as possible.

Keep in mind, if you do go with a low-offset wheel and want to maximize your benefit, you should utilize a strut-top camber plate type device to alter the SAI and bring your scrub radius back to stock value. This will prevent the negative effects while giving you the benefit (and also you'll have more negative camber for uber cornering)

 

[MS3077]

^ This seems to be obsolutely correct as I'm riding on wheels with a +42 offset. I get great grip around corners but I also get the negatives as nhluhr listed
Someone throw some $$ my way for new wheels...

 

[bacarl]

...

1) Torque steer is increased
2) Track width is increased, allowing for slightly less load transfer in corners
3) Steering weight is increased (may not be noticeable, but the power steering pump will be working harder and the fluid may overheat in some situations)
4) increased tramlining feel on rough or grooved pavement
5) increased wheelbearing wear rate
6) increased steering instability when braking

...

Wow, good stuff, thanks for chiming in sir. This is pretty remarkable really, I recently installed 235/40-18's on an 8"+48 wheel, and I really felt that my torque steer worsened dramatically. Even to the point that I'm leery of adding power mods. I also noticed that tramlining worsened, esp while braking, and even that straight-line (on-center) steering effort increased, although I've been driving another car lately with very light efforts, so I can't be sure. I chalked it up to added front grip, or wacky tire pressures (I'm still experimenting) but now I wonder if my decreased offset could have changed my scrub radius to cause this stuff.

Is there a way to measure scrub radius? I'm curious to know if our stock set-up yields a negative value, like nhluhr says our MacPherson suspensions would benefit from. I'd really like to see an illustration like the one dan7225 posted, but of our MacPherson suspension or one like it, as opposed to an SLA.

Hopefully Ghetto Mods still looks at this thread because you answered his OP question perfectly!

 

[MS3077]

These prior WRX owners / owners (sperry & nhluhr come to mind) have been a great contribution to the forum I must say. Lot's of knowledge! Thank you

 

[nhluhr]

Wow, good stuff, thanks for chiming in sir. This is pretty remarkable really, I recently installed 235/40-18's on an 8"+48 wheel, and I really felt that my torque steer worsened dramatically. Even to the point that I'm leery of adding power mods. I also noticed that tramlining worsened, esp while braking, and even that straight-line (on-center) steering effort increased, although I've been driving another car lately with very light efforts, so I can't be sure. I chalked it up to added front grip, or wacky tire pressures (I'm still experimenting) but now I wonder if my decreased offset could have changed my scrub radius to cause this stuff.

Is there a way to measure scrub radius? I'm curious to know if our stock set-up yields a negative value, like nhluhr says our MacPherson suspensions would benefit from. I'd really like to see an illustration like the one dan7225 posted, but of our MacPherson suspension or one like it, as opposed to an SLA.

Hopefully Ghetto Mods still looks at this thread because you answered his OP question perfectly!

Keep in mind that many of the effects you mentioned are also side effects of wider or grippier tires (especially tramlining and steering weight). Also, grippier tires will amplify any increased torquesteer from scrub radius because there's more of a force difference.

Measuring scrub radius is accomplished by determining the SAI and then projecting that onto the pavement and looking where the contact patch centerpoint hits in comparison. I can't really think of an easy way to measure this at home (but i've never really tried or looked into it)

 

[ericrapp]

Another thread in suspension that deciphers the pieces in the big picture of handling. nice. and i do mean nice. What is tramlining? Trying to learn. This stuff is kinda deep for me but the explanations are pretty darn good !

 

[bacarl]

Tramlining is when a car wants to follow the "ruts" in the road caused by heavy trucks and such. Basically it wants to act like a train following a track, and you have to fight the wheel to keep it pointed in the right direction. Check out the following tirerack article: here

I was reading a study performed by my employer that examined the effect of scrub radius on drift/pull (very similar if not identical to tramlining). A BWM M3 was fitted with wheel spacers of varying thicknesses in order to change the scrub radius from -10mm to +15mm (+3mm was the stock configuration). The car was driven back and forth on the same stretch of road and objective data was taken (steering wheel torque, steering wheel angle, yaw, lat accel). No measurable differences were found.

The study went a bit further to compare three similar vehicles with identical wheel offsets, scrub radii, and kingpin offsets, but all three had different tires. Differences in subjective evaluations were significant for these vehicles, leading to the conclusion that tires have a much more drastic effect on drift/pull.

Given the aforementioned, it can be concluded that only subtle differences exist in road rut drift/pull response when scrub radius changed. The largest factor observed for road rut sensitivity appear to be tires

We were kinda on to this anyhow, but I found it interesting and relevant, especially in my situation here

 

[ericrapp]

Interesting and relevant for sure. I had forgotten why I naturally drive on the peaks of road " ruts ". The wheel spacer comment surprised me but the tire, well anyone who has played with tire selection knows that there can be dramatic differences between tire manufacturers and models. luckily I have found a favorite. Thanks for making me read and think damnit bacarl and nhluhr

 

[MS3077]

My brain hurts

 

[ericrapp]

My brain hurts

I hear that, brother.

 

[jp4130]

Tramlining is when a car wants to follow the "ruts" in the road caused by heavy trucks and such. Basically it wants to act like a train following a track, and you have to fight the wheel to keep it pointed in the right direction. Check out the following tirerack article: here

I was reading a study performed by my employer that examined the effect of scrub radius on drift/pull (very similar if not identical to tramlining). A BWM M3 was fitted with wheel spacers of varying thicknesses in order to change the scrub radius from -10mm to +15mm (+3mm was the stock configuration). The car was driven back and forth on the same stretch of road and objective data was taken (steering wheel torque, steering wheel angle, yaw, lat accel). No measurable differences were found.

The study went a bit further to compare three similar vehicles with identical wheel offsets, scrub radii, and kingpin offsets, but all three had different tires. Differences in subjective evaluations were significant for these vehicles, leading to the conclusion that tires have a much more drastic effect on drift/pull.



We were kinda on to this anyhow, but I found it interesting and relevant, especially in my situation here

Very interesting.
What I would like to see is this same test being done on a high powered front wheel drive car equipped with Macphearson strut. As opposed to a rear wheel dive and I'm guessing it was a double wishbone set-up on the front of the bmw.

What kind of company do you work for bacarl, sounds like you guys study some interesting things.

 

[nhluhr]

Very interesting.
What I would like to see is this same test being done on a high powered front wheel drive car equipped with Macphearson strut. As opposed to a rear wheel dive and I'm guessing it was a double wishbone set-up on the front of the bmw.

What kind of company do you work for bacarl, sounds like you guys study some interesting things.

bmw sedans have strut front suspensions.