How much positive scrub radius until screwing things up?
 

Many was written here about scrub radius change with different ET rims / spacers, but mostly there are fears and questions than answers...

In scenario, where you want to use lets say stock tires but you hate hate hate the tucked in setup, how much you can go until things go badly? I don't want to make the car worse... Will 10-15mm increase cause problems with handling/braking/leverage/power steering/feedback/safety/etc?

Thanks if anyone know

curious about this as well

Negative effects of excessive scrub radius are rarely written about.

Quite a good explanation:

http://www.pistonheads.com/gassing/topic.asp?t=1004532

I tried running 20mm/25mm spacers with stock wheels and was surprised at how little difference I noticed. I believe this is largely due to the stock tires being rather narrow and low grip.

In theory, a substantial change like that should be immediately noticable, but in reality it didn't seem to be the case. One additional concern is bearing load, but with all the aggressive/wide wheel fitment and sticky rubber, and stance etc, bearings don't seem to be a widespread problem.

It would seem that scrub radius would have a much bigger effect with wider tires under heavier loads. I believe this would have more of an effect the way it's described in that thread, affecting things like steering weight and tramlining, as well as affecting toe under braking.

Do we know the OEM scrub radius?

That's because it's typically mental masturbation.

Does it affect things? Sure. Is it particularly noticeable? Not really.

okay

on the street or on the track/autocross

Street, canyons, and track.

My BRZ is my daily driver, mostly driven in town. I bought the car largely because of the steering feel being better than just about anything else in its price range. With some trepidation, I switched from the stock set up (+48 offset) to 225 MPSS on 17x8 +35, so a 13mm difference in offset/scrub along with more grip and more tread width, but not huge differences. I really do notice a difference in steering feel and weight. My sense is that the bulk of the additional steering weight, once up to speed, is from the offset. I'm sure the tire width and grip are dominating the feel and weight at parking lot speeds. (Three point turns are a workout!) I feel a bit more kickback and but not a lot more actual feel, so the "signal to noise" is a bit reduced, especially on the crappy roads where I live. I'd love to try the same tires but with +48 offset rims. Too bad no one makes negative thickness spacers...

Overall, I much prefer the grip and intensity of the MPSS tires over the Primacies on the street and I am willing to live with the slight increase in kickback (and big increase in noise), especially since my particular wheel/tire combination is a few pounds lighter than stock at each corner and didn't cost and arm and a leg.

I personally wouldn't want to go much further outboard with the tire patch on the street. On track might be a different story, but not my area of expertise.

I love you. Seriously.

Your constant snarky questions and condescension whilst never providing any of your own input is awesome.

Yikes, really? 9.5 ET 38 with 255/35 Direzza IIs.

If you're finding parking lot turns a workout, I guess hit the gym? I have no problem using one hand to get around, or are you just exaggerating for effect?

My Miata with a depowered rack on 205 Rcomps was an order of magnitude more difficult to get around the paddock then the BRZ on wide sticky rubber.

I provide plenty of input, you just have selective reading.

and if I had a shop and all manner of testing equipment I'd input even more.

but dismissing this discussion as "mental masturbation" makes you the only snarky marvin around here.

What testing equipment do you need? Are you going to put a string pot on the steering column and start parsing individual foot pounds of torque necessary to make the wheel reach 45° at X speed with different offset wheels?

Stock Miata wheels 14x5.5 ET 45, there was no discernible change in effort at speed going to a 15x8 ET 25 wheel.

My point is, if you can't feel the car behaving differently, pointing out that it changed 1.38%.......what does that get you? What help has it provided?

Maybe just a little! :) Seriously, though, we have a very long, narrow driveway, between our house and a fence, that I don't like backing out of, so I usually end up doing a 5 point turn in the back where it is a bit wider. Cranking the wheel from one stop to the other while stopped with the MPSS on tumbled interlocking brick is a minor workout! If I only turn the wheel when the car is moving, it ends up being a 9 point turn...

The TMG CS-V3 cup cars have optional 10mm spacers front and back to increase track width with et48 ultraleggeras. In case you didn't know, TMG are Toyota's offical motorsport team in Europe that build the le mans cars and won all those rally championships in the Celicas.... if they say its okay to run those spacers. ...

I've run stock (+48), +45, +40 and +35 offset wheels from 0 camber to -3.2* degrees and there was very little difference in steering effort required between the different offsets. Not sure how much effect it has on tire wear characteristics, but steering effort, not so much.

A personal very negative effect for me is the greatly increased possibility of turning what would be minor curb rash into a catastrophic accident.

I don't remember how much positive scrub I had on my 240Z but whenever I barely touched a curb it would rip the steering wheel right out of my hands. I was always afraid of tapping a curb at any speed over a creep.

That's how I understand the possible troubles with scrub radius. The curb could suck your steering wheel and makes you turn more than what you want.

The question is: when does trouble start ?

Good question. No idea but I have first-hand experience of when it's over the top. :(

My only experience is with LFS simulator. The only one I know where you can test different scrub radius.
The effect is scary.

good call, I couldn't find the 10mm info tho. How do you know? Anyway I agree that if it is good for them...

10mm spacers.

First mentioned here:
http://www.toyota-motorsport.com/mot...tmg-gt86-cs-v3

and can be seen here on page 55 of the user manual:
https://www.google.co.uk/url?sa=t&rc...nTTLlxtcbYPbyQ

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.

The camber was added from camber bolts, so the increased negative camber probably had the opposite effect of increasing scrub radius a little, if anything.

This.

Exactly. IIRC 2 degrees of negative camber (strut camber not bolts) at the front moves the scrub radius out by about 5mm. I cant find my friggin spreadsheet now actually I need it :bonk:


So you might actually want spacers if you add significant camber depending on the wheel offset.

I believe I've found it easier to lift a rear wheel just by adding 10mm spacers. Of course, we're talking about hauling ass up steep driveways. The other thing I noticed is bigger scrub radius exacerbates body roll. Even at standstill, turning the wheel lock to lock causes noticeable roll. Maybe 1-1.5" as measured at the roof?

I'm not picturing this unless you're talking about changing, what is it with camber plates fore-aft, rake?

...well, plates in the strut tower that can do both.

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.

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.

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.

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.

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

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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

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

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

Thanks!

Thanks a lot and especially thanks for for some number.