fatbillybob

What number of degrees of change through full suspension travel is considered ideal minimal Bumpsteer on the generic racecar?

thanks!

stownsen914

Usually you try to minimize bump steer, so there isn't an ideal amount other than as little as possible. With that said, I've heard some people try to optimize by designing in some toe in for bump travel and toe out in droop travel, or the other way around, but I haven't tried that.

2BWise

Depends on the tire this generic racecar is using, specifically how the mu-slip peak reacts with vertical load. Ideally the road wheel angle would work to match the slip angle at which peak grip is made and that angle is not necessarily a single value. It can change based on the vertical load on the tire.

fatbillybob

OK let me ask this another way. On the generic bump steer gauge like a long acre guage the BS is measured in thousands of an inch. Well what range greater than zero" is too much or requires correction?

One guy on youtube runs a shop aka a professional. He took a car with 0.050" of bump and made it worse with his proprietary bumpsteer kit increasing the bump to 0.060". He claims to have made it better because he changed a toe out in droop to a toe in on droop but yet increased the total bump and calls that a win! I don't buy that.

mj951

Longacre has a technical article on the subject. Couple quotes and link below.

"Some bump out can make the car more stable on corner entry. Bump in is almost always undesirable."

"My recommendation is to run .005 to .015 thousands of bump out but never allow the tires to bump in."

http://www.longacreracing.com/techni...aspx?item=8162

Bill Verburg

Hard to say on a generic, but here's the bump curve for a stock '84 RoW 3.2 Carrera, earlier 911 would have been similar the 2 red horizontal lines represent 2 different ride heights, the lower is a raised ride height as implemented on some SCs, The curve is benign in that it is linear and not too flat in the zones that these cars generally run. You mostly only care about bump on the compressed side for handling discussions, here at stock roW height 40mm of compression imparts -34' of extra toe, For these cars spec is 0° pressed, an extra 34' is quite a lot but considered normal.

An ideal bump curve would be a vertical trace and the worst bump curve a horizontal trace.

fatbillybob

I have never worried about BS before. I have never thought about it until this thread. I guess I just have a hard time wrapping my head around why so little makes any difference. It makes no sense to me. Longacre says keep BS under 0.50" on their gauge. The math says that on their 15" long tool that .050" = 0.19 degrees! That is a tiny amount of toe change. Also consider that the .050" is total over the suspension travel. So in bump or droop the toe change is even less! So as a wag 025" out is 0.095 of a degree of toe change. A generic carrera rear toe range is 0.08-0.25 degrees so you could be in the middle and with BS be within factory tolerance. That's just the math that makes 0.050" of bump seem insignificant. Then you got car dynamics, changes in the road, how the tires are loaded and yielding and 0.050" (0.19 degrees) makes a difference? What am I missing?

Mahler9th

I don't think that there is any such thing as a generic race car.

I have done some reading and believe that optimizing bump steer depends on a lot of variables that may be difficult to deal with for most situations in amateur racing. I am sure that at the pro level, say for example with factory efforts like those of Porsche for their RSR cars, there is a lot of institutional knowledge and a lot of simulation work.

My car is a '75 911, and has what in my experience are typical adjustments and modifications to help with bump steer. This includes spacing up the rack, putting appropriate bends in the strut steering arms and the use of the ERP (Eisenlohr Racing Products) bump steer kit (tie rod ends).

This winter I hope to go further and measure and shim to get an even more optimized bump steer curve. I have the tools and hopefully will have the time. I will likely use my custom string set up to measure toe change versus travel.

So how will I know the curve to target? Well I will again tap into the brain trust that raced this kind of car in pro situations back in the day. Sure, it will still be a macro level optimization, but it may make a difference and put just a little more performance potential into the car.

So I think that one way to approach this is to find experts that have already been motivated to work toward optimization for a pro level effort with a similar car. And have (and are willing to share) what they consider to be a good target range for a bump steer curve.

Disclaimer: my impressions about bump steer optimization are heavily influenced by what I have read in the Puhn book.