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Inside Tire Shoulder Wear

 
Old 04-22-2018, 05:39 PM
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Default Inside Tire Shoulder Wear

Relevant threads from way back:
https://www.miataturbo.net/wheels-ti...uestion-84957/
https://www.miataturbo.net/wheels-ti...r-comps-84432/

I've noticed a pattern lately on my last two sets of tires (RS4, Rival S) of the inside shoulder wearing quite a bit more than the outside. It has a very distinct wear in a 2" band. I just chalked this up to the slight toe out I was running and didn't think much of it except I realized the rear of the car is doing it as well and I'm running toe in. I also picked up a couple sets of SM7's from a very competitive racer and he was hitting cords on the inside edge within 8 heat cycles. there was probably 50% tread life left on the rest of the tire.

You can see what I'm talking about on the right hand side of this tire.




If you ever look at Miata's mid corner you'll see them with the inside tire hiked into this arrangement with the inside edge being dragged.



My question is, do you think this is how this odd wear is happening? What is it about our suspension geometry that causes this?

My initial thoughts are to run more caster since that will dial out camber on the inside tire. I'm also not opposed to changing up the overall geometry if someone can confirm what it is that's causing it.
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Old 04-22-2018, 07:32 PM
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What are your alignment and suspension specs? And why are you running toe out in the front...
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Old 04-22-2018, 07:39 PM
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Dual purpose Autox and track car with 2-3k worth of street miles per year.

2.75*F 2.25*R
5.5* Castor.

​​​​​​Now running 0 toe in the front 1/8" toe in the rear. I was running 1/16" toe out in the front because it helps turn in quite a bit in an auto x setting.

1000/500 AFCO'S (was on DIY Bilsteins 800/450)
Stock sway bars

2250lbs without me in it
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Old 04-22-2018, 07:41 PM
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Stock sways....? 1000/500 Afros don’t sound great either.
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Old 04-22-2018, 08:08 PM
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Listen I get I don't follow the prescribed "setup" that everyone around here nut swings off of but how exactly would my setup contribute to the wear shown? Why would multiple Spec guys have the same issue? I'm not after a quick answer of what to change to fix it, I'm after the HOW and more importantly the WHY.

As for my setup, I'm trying a soft bar heavy spring setup, so far it seems to have less brake dive than a big bar lighter spring setup (700/375) It also keeps the inside rear tire on the ground better which makes the torsen work.....but again how would spring rate cause the tire to not be flat on the road? Are you suggesting I'm having huge amounts of body roll? I would really like to hear how the AFCO's would contribute to this......
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Old 04-23-2018, 12:56 AM
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When my tires did this, it was because I was running 3.5 degrees negative camber up front as an autox alignment. Hoosiers at the autox, Neovas on the street, and the Neovas got pasted. Hoosiers, OTOH, wore the outside edge faster.

Photo:



(wheels + then-junk tires were mounted on the Locost just to move it around the shop, all the wear in question was done as street miles on the Miata).

--Ian
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Old 04-23-2018, 07:11 AM
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I think you are using too much camber for your chosen tire, but I don't know anything.

A Hoosier will stick more and put more of the outside edge in contact during a corner than a less sticky tire at the same camber. A less sticky tire therefore needs less camber. All of this should be verified with a pyrometer.

Verify also the suspension bushings aren't worn and allowing movement under hard braking that would change the toe.

Also know that camber is a trade between optimal contact patch during cornering versus braking. It is entirely possible to have enough camber to achieve a good contact patch in the corner but predominantly using the inside half of the tire during hard braking. In my mind it is similar to Emilio's recommendation of high horsepower cars using less rear camber for better traction under acceleration. But I'm no expert.

Again, pyrometer and lap data will likely yield the answer.
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Old 04-23-2018, 10:50 AM
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My low grip snow tires do this, only running 1.8 camber all the way around. 4.5 caster, 5in pinch, 800/500 Xidas, biggripkit, delrin bushings, giken.

My re71r tires were also doing this at 3.4/2.8 camber, 8 caster, 4 in pinch. Perfect pyrometer spread.
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