Tire Wear and Suspension Geometry

[kivnul]

I have had a confluence of multiple variables over a short about of time. The result is a very worn tire.
1) Had a MFactory LSD installed.
2) Had medium wear OEM tires start bouncing. Roadforce balanced.
3) Front Alignment specs as checked after tire wear: (LT on left, RT on right)
Camber -2.1, -1.8
Toe in 0.55deg, 0.55deg
Caster whatever, pretty even & in spec

Both tires started at even wear. After a month, the right tire was near bald, the inside was very bald. I am somewhat confused because:
a) Thought a LSD would help the tires wear more evenly LT vs RT.
b) The RT tire has less camber than the LT, but it wore the inside stupid fast.
The left tire shows normal wear. More wear than the rears over that month, thinking due to the messed up toe.

So my question/thought is, could somehow the excess toe, coupled with the left tire having a smidge more grip & camber, coupled with the RT tire being the "drive" tire, have caused the tire to slip more thus causing the weird wear? I have adjusted the toe a bit by eye until I get new tires on the car. I plan on getting an alignment soon to get the Camber & Toe fixed (have slotted coilovers so camber is adjustable.) Is there something else that could cause wear like this? I do have some groaning in the front end, engine load independent (very audible just creeping along at 1 mph over an uneven surface). I plan on unhooking the tie bar to see if it goes away. Otherwise I'm gonna have to figure out what is causing it. Fuel economy dropped by around 3mpg during the last couple months. Daily driven tires, no autocrossing.

 

[jeffreylyon]

That's seems like a whole lot of toe - almost .250" for the old school of us. Camber seems ok - right at the limit for a street setup, I'd think. The LSD is only going to help w/ tire wear when it's engaged and it's only engaged when 1 tire is trying to spin. Anything with the engine or gearbox that would cause uneven wear would also cause a ton of tugging on the steering wheel. Did you notice the car pulling to one side?

You can measure toe with a couple of pieces of angle iron (or aluminum) and two tape measures. Center the steering wheel, stick the sticks of angle iron on top of a couple of beer cans on either side so the flats are flush against the tire. Measure the distance between the front and rear sticks, right up against the tire. Subtract, divide by 2 = toe (or close to it). Eye-balling toe is really hard on our cars given the fender lines.

 

[MRX330]

I thought an lsd was posi until one tire is spinning?

 

[kivnul]

I can hear the LSD just driving constant speed, straight down the highway, sounds kinda like a truck with knobby tires. Noise goes away if I lift off the throttle. It is not loud, but it is noticeable. The toe was definitely too much. I adjusted in 1/2 rotation of the adjusters for now. As to the Camber, the car was twitchy, and it pulled depending on the road surface (both directions). One direction seemed more than the other so I adjusted camber blindly until the pulling felt even. I think I added -1mm to the left, and -2mm to the right, measured at the top plate of the strut. (wanted more camber overall)

 

[jeffreylyon]

I can hear the LSD just driving constant speed, straight down the highway, sounds kinda like a truck with knobby tires. Noise goes away if I lift off the throttle. It is not loud, but it is noticeable. The toe was definitely too much. I adjusted in 1/2 rotation of the adjusters for now. As to the Camber, the car was twitchy, and it pulled depending on the road surface (both directions). One direction seemed more than the other so I adjusted camber blindly until the pulling felt even. I think I added -1mm to the left, and -2mm to the right, measured at the top plate of the strut. (wanted more camber overall)

Hmmm... twitchiness is usually cased by too much toe out. Toe will change with suspension movement (bump steer), making a car dart on uneven surfaces. I can't visualize how slight differences in camber would cause twitchiness.

You're flying blind trying to set alignment by feel and, remember, you're changing toe when you change camber.

 

[jeffreylyon]

I thought an lsd was posi until one tire is spinning?

Not sure what you mean - Positraction is a form of limited slip.

 

[Mikey456]

Just as a side note. Does the car understeer a little more with the lsd because it does not allow the slip of the inner tire?


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[jeffreylyon]

Just as a side note. Does the car understeer a little more with the lsd because it does not allow the slip of the inner tire?

It has no effect until you roll on the throttle and start to break the inside tire loose. I don't have one (yet), but the report is that the front end sticks hard and pulls into the turn.

 

[MRX330]

Not sure what you mean - Positraction is a form of limited slip.

With an lsd both tires are pulling until one spins and when that happens it sends power to the wheel that is not spinning.

 

[jeffreylyon]

With an lsd both tires are pulling until one spins and when that happens it sends power to the wheel that is not spinning.

There's a bunch of different LSD tactics: https://en.wikipedia.org/wiki/Limited-slip_differential

 

[kivnul]

Hmmm... twitchiness is usually cased by too much toe out. Toe will change with suspension movement (bump steer), making a car dart on uneven surfaces. I can't visualize how slight differences in camber would cause twitchiness.

You're flying blind trying to set alignment by feel and, remember, you're changing toe when you change camber.

The camber changing was mostly trying to figure out why the car was pulling more one direction than the other. I fully anticipated taking it into a shop to get it dialed in. The overall 1.1deg toe in is most likely caused by my addition of negative camber.

Just as a side note. Does the car understeer a little more with the lsd because it does not allow the slip of the inner tire?

I have not noticed any change in under/over steer due to the LSD