bherman
Well-Known Member
Hi Guys', this may seem to be a silly question that only has one answer, you don't want any, but is that true? First let me say that I'm not an expert on this subject but I do have some practical experience with it. My definition of "bump steer" would be toe change during suspension travel. Common thinking would be this toe change creates a steering input without moving the steering wheel. If this toe change occurs evenly on both tires the only negative effect is passing through zero toe, which is unstable all by itself. If the suspension travel is different from side to side (much more typical) the result is divergent toe patterns, that creates steering inputs not desired by the driver. This condition is even more apparent during body roll (cornering), causing "roll steer".
Many years back when we were working on road race cars we always attempted to tune all of the bump steer out of the cars. The cars were GTP cars, with carbon fiber "tubs", extremely stiff, very light, limited suspension travel and very fast. At the speeds those cars traveled (220+) any bump steer was not appreciated by the drivers, even small undulations in the track surface would make the vehicle twitchy, if they had any more than .010 bump steer (per side) through out the total suspension travel. This is very speed dependent and I'm not suggesting that our street cars need this kind of attention to bump steer geometry. If you're one of the racers on this site and your car is running 150 MPH you might want it as close as you can get it!
This gets me to the question that I really wanted to ask. More recently we work closely with people that are focused on production suspension design. In most modern front suspensions bump steer is used to create an under-steering condition. A small amount of bump-out under load reduces the steering angle and keeps us non-race car drivers from getting ourselves in trouble. This typically varies from .050 to .080 toe-out during roll steer. I'm currently tuning the bump steer on my car, do you think I should make it under-steer a little?
Checking bump steer, thanks for looking, Brian
Many years back when we were working on road race cars we always attempted to tune all of the bump steer out of the cars. The cars were GTP cars, with carbon fiber "tubs", extremely stiff, very light, limited suspension travel and very fast. At the speeds those cars traveled (220+) any bump steer was not appreciated by the drivers, even small undulations in the track surface would make the vehicle twitchy, if they had any more than .010 bump steer (per side) through out the total suspension travel. This is very speed dependent and I'm not suggesting that our street cars need this kind of attention to bump steer geometry. If you're one of the racers on this site and your car is running 150 MPH you might want it as close as you can get it!
This gets me to the question that I really wanted to ask. More recently we work closely with people that are focused on production suspension design. In most modern front suspensions bump steer is used to create an under-steering condition. A small amount of bump-out under load reduces the steering angle and keeps us non-race car drivers from getting ourselves in trouble. This typically varies from .050 to .080 toe-out during roll steer. I'm currently tuning the bump steer on my car, do you think I should make it under-steer a little?
Checking bump steer, thanks for looking, Brian