How much positive scrub radius until screwing things up?

[Calum]

Originally Posted by 7thgear

depending on how the camber is achieved, you might actually be angling the strut to compensate for the increased offset so the actual effect on scrub is much smaller than the raw offset value.

Reading this and a couple other posts, it looks like adding camber from the strut mount reduces scrub radius? And adding camber from the clevis doesn't affect scrub radius?

I thought scrub radius was a measure of the tires center line where it meets the road, to the pivot axis between the strut mount and ball joint. By this, wouldn't increasing camber (more negative) from the clevis also increase the lever arm, but increasing camber from the strut mount shouldn't affect scrub radius?

Thanks for any help.

[7thgear]

Originally Posted by Ultramaroon

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Originally Posted by Calum

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staring from a perfectly perpendicular tire 90*, and a 70* (purple) strut, observe the relationship of black and purple on the ground


if you angle the strut more by 5* with camber plates but do nothing at the wheel attachment point, then observe the relationship of red and green at the ground


if you angle the wheel at the wheel attachment point by 5*, and not touch your camber plate, then observe the relationship of green and purple at the ground


if you angle at the camber plate, but then use the wheel attachment point to angle in the opposite direction, then observe the relationship between red and black lines.

this is assuming no changes to offset have been made... feel free to do that in your head.

[Calum]

Originally Posted by 7thgear

staring from a perfectly perpendicular tire 90*, and a 70* (purple) strut, observe the relationship of black and purple on the ground


if you angle the strut more by 5* with camber plates but do nothing at the wheel attachment point, then observe the relationship of red and green at the ground


if you angle the wheel at the wheel attachment point by 5*, and not touch your camber plate, then observe the relationship of green and purple at the ground


if you angle at the camber plate, but then use the wheel attachment point to angle in the opposite direction, then observe the relationship between red and black lines.

this is assuming no changes to offset have been made... feel free to do that in your head.

Awesome!! Thank your for the explanation.

Great, now I'm considering getting camber plates.

[7thgear]

Originally Posted by Calum

Awesome!! Thank your for the explanation.

Great, now I'm considering getting camber plates.

the only thing I'd want to know... is where to we actually start at?

[Calum]

Originally Posted by 7thgear

the only thing I'd want to know... is where to we actually start at?

Id also like to know if there is an ideal? If we adjust from the strut top it will also affect inclination angel. Is that making IA better or worse? If worse, which has more of an effect?

I'm presently driving with MPSS tires, T0 coilovers with standard springs, group-n strut mounts, stock stabilizer bars, -2.5/-1.5 camber front/rear, zero toe everywhere, and about 31 psi tire pressure because our roads are horrible.

Steering feel is good, but it was better with less camber and more tire pressure. I don't want to lose anymore steering feel.

The good news is the the car tracks as straight as an arrow despite the aliment. Another thing I don't want to screw up by adding to the scrub radius.

I love this topic, but every time I learn something new it only brings up two more questions. We should change the name of this forum from suspension to Hydra.

[Ultramaroon]

Originally Posted by 7thgear

the only thing I'd want to know... is where to we actually start at?

Intuitively, I've wanted to start with only camber plates for the very reason you illustrated. I see negligible effect on scrub radius by changing the angle of the line intersecting top hat and ball joint.

That's assuming we have the same goal.

Never mind what I said about fore and aft. Duh.

[drewbot]

Well I guess it's good I got my camber from plates. Thanks for the math

Sent from my A0001 using Tapatalk

[Racecomp Engineering]

Yes, you can use camber plates to reduce scrub radius. There is also a very slight benefit to pushing them in...it raises your roll center a little.

However, angling the strut inwards more with camber plates and increasing your SAI has some drawbacks that IMO outweigh the scrub radius issue, namely the change in camber for the outside wheel as you increase steering angle. It's not as big of a deal if you add caster at the same time, but it's important.

In most cases I recommend getting most of your camber from the hub. The change in scrub radius is pretty small. For what it's worth, a lot of the really high end dampers will come with some neat camber adjustment at the lower mount, either specially designed bolts for a slotted lower mount or "keys" (I'm not sure what else to call them) that go in the slot. The Nurburgring Challenge STI that I saw in Japan for example simply has (roughly) center fixed location lowering top mounts and got it's camber at the lower mount. Group N rally cars get their camber at the lower mount as well. Part of it is application specific I'm sure (higher steering angles having something to do with it) and many very fast cars still do get a lot of camber from plates.

With regards to scrub radius, a very large value can feel pretty awful in terms of steering kickback, effort, and also do some funky things with toe change. A wider track width is generally a good thing, but you have to keep in mind the scrub radius issue.

I don't have any set numbers for you guys, but -10mm in offset from stock is certainly fine IMO.

- Andrew

[solidONE]

^ also there's the amount of camber gain (minute) under compression with the different ways to get camber. Or rather, the lack thereof

[Ultramaroon]

Originally Posted by Racecomp Engineering

angling the strut inwards more with camber plates and increasing your SAI has some drawbacks that IMO outweigh the scrub radius issue, namely the change in camber for the outside wheel as you increase steering angle. It's not as big of a deal if you add caster at the same time, but it's important.

...and you confirm my suspicion that I was oversimplifying. Thanks, Andrew.

[Calum]

Originally Posted by Racecomp Engineering

Yes, you can use camber plates to reduce scrub radius. There is also a very slight benefit to pushing them in...it raises your roll center a little.

However, angling the strut inwards more with camber plates and increasing your SAI has some drawbacks that IMO outweigh the scrub radius issue, namely the change in camber for the outside wheel as you increase steering angle. It's not as big of a deal if you add caster at the same time, but it's important.

In most cases I recommend getting most of your camber from the hub. The change in scrub radius is pretty small. For what it's worth, a lot of the really high end dampers will come with some neat camber adjustment at the lower mount, either specially designed bolts for a slotted lower mount or "keys" (I'm not sure what else to call them) that go in the slot. The Nurburgring Challenge STI that I saw in Japan for example simply has (roughly) center fixed location lowering top mounts and got it's camber at the lower mount. Group N rally cars get their camber at the lower mount as well. Part of it is application specific I'm sure (higher steering angles having something to do with it) and many very fast cars still do get a lot of camber from plates.

With regards to scrub radius, a very large value can feel pretty awful in terms of steering kickback, effort, and also do some funky things with toe change. A wider track width is generally a good thing, but you have to keep in mind the scrub radius issue.

I don't have any set numbers for you guys, but -10mm in offset from stock is certainly fine IMO.

- Andrew

Thanks!

[JDM4E]

Originally Posted by Racecomp Engineering

Yes, you can use camber plates to reduce scrub radius. There is also a very slight benefit to pushing them in...it raises your roll center a little.

However, angling the strut inwards more with camber plates and increasing your SAI has some drawbacks that IMO outweigh the scrub radius issue, namely the change in camber for the outside wheel as you increase steering angle. It's not as big of a deal if you add caster at the same time, but it's important.

In most cases I recommend getting most of your camber from the hub. The change in scrub radius is pretty small. For what it's worth, a lot of the really high end dampers will come with some neat camber adjustment at the lower mount, either specially designed bolts for a slotted lower mount or "keys" (I'm not sure what else to call them) that go in the slot. The Nurburgring Challenge STI that I saw in Japan for example simply has (roughly) center fixed location lowering top mounts and got it's camber at the lower mount. Group N rally cars get their camber at the lower mount as well. Part of it is application specific I'm sure (higher steering angles having something to do with it) and many very fast cars still do get a lot of camber from plates.

With regards to scrub radius, a very large value can feel pretty awful in terms of steering kickback, effort, and also do some funky things with toe change. A wider track width is generally a good thing, but you have to keep in mind the scrub radius issue.

I don't have any set numbers for you guys, but -10mm in offset from stock is certainly fine IMO.

- Andrew

Thanks a lot and especially thanks for for some number.