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I don't expect anyone to necessarily agree with me, I'm just trying to help out with the issue based on my education and experience with my own car through trial and error.
According to the following books:
Chassis Design and Race Car Vehicle Dynamics by William and Douglas Milliken
and based on the design of the 1-series front MacPherson strut suspension I believe the following is taking place to cause a tramlining sensation:
- Tires with stiffer and shorter sidewall heights tend to transmit more load from the road through the sidewall to the wheel, and then ultimately to the steering wheel. I believe that wider tires will actually help combat this issue since there is more contact with the road and would have less of a tendency to follow small imperfections in the road compared to thinner tires. I also believe that switching from RFT to non-RFT would help, due to the change in sidewall stiffness.
- Low caster angle will increase tramlining, because as the wheel turns it creates less negative camber during cornering. The solution to this would be to increase caster angle in the positive direction. This will only create camber when the wheel is turned, which is good for handling as well as keeping tire wear in check. It also allows for better longitudinal grip during braking.
- Negative camber with zero toe or toe-in will increase tramlining due to the scrub radius. Negative camber with toe-out will have little to no affect on tramlining. I will address this later.
- Bushing stiffness will affect tramlining. Forces are transmitted from the road through the tire to the wheel, which is connected to the upright. The upright is supported by the control arms and strut. If the bushings in the control arms that are connected to the chassis are too soft, the upright will move at a different rate compared to the rest of the suspension. The tie-rod also connects to the upright as well as the rack and pinion. As the bushings deform and the upright moves, this will cause a force through the tie-rod that will be felt in the steering wheel. Increasing the bushing stiffness or using solid mounts will decrease tramlining. The problem with solid mounts is that it would cause extreme suspension and chassis wear as well as increase NVH. However, this would be the only way to completely control the suspension geometry. Many racecars use solid mounts for this reason.
- The 1-series front suspension is designed with a positive scrub radius. Spacers, wider wheels, and wider tires will all increase the scrub radius even more in the positive direction. The reason that I believe that toe-out will decrease tramlining is because if the suspension is set at toe-in or zero-toe, the positive scrub radius will rotate the wheel toward the toe-out direction automatically during deceleration, such as letting off the accelerator momentarily, and drastically during braking. Tires in a toe-in setting will constantly be pulling inward, this is why it is so stable. However, the combination of scrub radius and toe-in will cause the tire and wheel to rotate back and forth between toe-out and toe-in and this easily allows the tires to follow different irregularities in the road, but if the setting is already at toe-out then during deceleration the tires and wheels will remain stable at the toe-out condition not causing any extra force through the tie-rod.
For a stock 1-series I would suggest:
- Wider tires.
- Alignment: Front camber -1.5 degress, front toe-out 1/8", caster 7 degrees.
For a 1-series with moddified suspension including M3 control arms and sway, coilovers, camber plates, and adjustable endlinks:
- Wider tires.
- Alignment: Front camber: max approx -3 degrees, front toe-out 1/8", caster 9 degrees.
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