Caster, Camber, Scrub radius and tire width inter-relationship

[Bow Tie 67]

My setup,

3 1/4 Caster

1.8* Camber

1/32" Toe out

3 3/8" scrub

275mm width tire ( both 17" and 18" rims same height )

The above setup runs fine with R compound tires, 17 x 9.5

When I bolt on my street rims / tires ( 18 x 10 ) the scrub increases to 3 3/4", and I get a vibration ( high frequency ) in the steering wheel above 70 mph. The vibration seems to change with pavement type / condition.

My question is can more caster dampen this? How much would toe settings help? Would less camber change this? Is the softer tire less inclined to show the vibration or is the extra 3/8" scrub the culprit?

I'm starting to think the new rims may have a problem with being concentric.

Matt

[funcars]

More castor might help, but it sounds like there is another problem or a significant difference between the wheels or tires. Are both sets of wheels balanced the same way? 18's have more weight at the perimeter. Have you tried adjusting the toe for street tire use to zero or very slightly in? That might help too and it's very easy to go back and forth if you mark the tie rods.

Good luck

[sik68]

+1 I'd also like to know.

I have this issue on my daily driver subaru. I've had my wheels balanced multiple times and multiple tire rotations (same size on all 4 corners), but I still get a steering wheel vibration at 65+ that seems to be based upon road surface and other random factors (temperature, uphill/downhill, straight aways no turning).

My subaru runs 0.0 camber up front and +1.5 caster.

[Bow Tie 67]

I did try 0 and slight toe in with no noticeable difference.
Both wheels are balanced the same way although the 18's are heavier. I jade them rebalanced and they were slightly off. I'm keeping my fingers crossed but doubt it will help.


More castor might help, but it sounds like there is another problem or a significant difference between the wheels or tires. Are both sets of wheels balanced the same way? 18's have more weight at the perimeter. Have you tried adjusting the toe for street tire use to zero or very slightly in? That might help too and it's very easy to go back and forth if you mark the tie rods.

Good luck

[406 Q-ship]

Check the wheel bearings for play. The added weight of the 18's might just be enough to bring out the wheel bearings vibration.

[Bow Tie 67]

Check the wheel bearings for play. The added weight of the 18's might just be enough to bring out the wheel bearings vibration.

I did adjust them and there was no difference.

[zerotofear]

After spending 20 plus years in racing, with more than my fair share of steering wheel holders, we need a definition of your vibration. Underload, the rear steer steering components are compressed, front steer components are being for a lack of a better term are stretched. Anything that increases or decreases that load can cause a kind of vibration as the load fluctuates. Positive Caster in all honesty wants to turn the wheels around, more castor the more load , the more load, the weaker links start to fail. Scrub, can increase the load and cause a load ,unload situation as the driver holds the wheel. Toe-in typically with good components causes a high frequency wobble, some drivers call a vibration, as the parts wear the car looses the vibration and starts to dance off of one wheel to the other or develops a wander. Increasing caster can mask this effect until the parts wear further and the vibration/wobble returns. Too throw another nut into the mix another source of vibration can also come from shock issues, that at the most violent feels like a tire out of round, or at the beginning of failure feels like a rumble in the steering wheel. Many times we baseline our cars, tune , and document, than establish duty cycles of each components based on our build sheets. I have been with teams that go with agressive front end settings, and you would be surprised which components started fail first and what the effects of those wore parts had for the driver.

[ArtosDracon]

You do have a lot of static camber and scrub, which should increse road noise, to a degree, but it doesn't sound like this is your issue to me; you are also a little small in the caster department, but if you haven't done the gulstrand mod, you're probably better off. What pre-load are you running on your front bearings?

[Bow Tie 67]

Ok, I need to clarify some things. I'm running a Chassisworks front subframe with upper adjustable " G-arms ". I dont know exactly how much more caster I can add without binding the arms. These arms use a turn-buckle type adjustment.

I drove the car 1600 miles with this annoying situation, and see no abnormal wear on the new tires. At around 70 - 75 mph the steering wheel gets a shake, rapid back and forth movement varying from about an 1" or less, depending on road surface. You can not feel this in the suspension or body. If I grip the wheel tight it disappears until I loosen my grip. Cornering does not seem to change it. Driving in the rain seems to dampen it. On one road with rain grooves running the direction of movement there was no shake at all. With my 17" rims it is non-existent.

I have checked all the suspension and made sure everything was tight, and there was no wear. I tightened the front spindle nut one more cotter pin hole so now I have just the slightest bearing play when rocking the brake rotor.

I'm wondering if the scrub difference combined with the large and heavy front wheel / tire assy, is just to much.

Or the balancing machine at Discount Tires is just not accurate enough for what I'm asking of the front suspension.

Both sets of rims were balanced at the same place, the 17" are corvette wheels with 54mm offset and use 1 3/4" spacers to fit my car. The budnik 18" x 10" have a 0 offset ( 5.5" back space ), they are using tape weights split in two rows, just behind the billet center and the inner edge.

Thoughts?

[John Wright]

Was the air pressures similar in the two setups? Wondering if the tire is bouncing like a basket ball and the shock isn't keeping that in check from one setup to the other.

[Bow Tie 67]

Was the air pressures similar in the two setups? Wondering if the tire is bouncing like a basket ball and the shock isn't keeping that in check from one setup to the other.

Interesting, I doubt it was bouncing as I could stop the effect with some hand pressure. But I will try and let some air out and see what happens. The pressures were within 2 psi of each setup, but the tire construction is completely different.

One note: I can let go of the wheel and the car tracks perfect at any speed. The only time I notice it is when I am resting my hand on the wheel. With any slight grip pressure I feel nothing in the wheel or the car. It really acts like there is a high speed oscillation being transmitted up the steering shaft. The steering setup is power, mustang 2 style, rack and pinion with two borgeson u-joints and an Ididit tilt column. The joint at the steering column end does have a vibration dampener.