va122

Hey,

Can someone please explain to me what bumpsteer at the front end feels like seat of the pants?

My GT3 is experiencing some jerking of the wheel over bumps. I'm not looking for a diagnosis of my suspension or seet up info at all. I'm looking for an explanation of the term "bumpsteer" so I can communicate with my tech accurately.

Thanks in advance.

Victor

roberga

http://www.derpca.com/Porscheforme/911Tech6-01.htm

Larry Herman

To add a little more to the explanation there are other things that can produce the same feel. Severe bumpsteer can be felt as you travel in a straight line and one wheel hits a minor bump, and the steering wheel jerks in that direction. Minor bumpsteer tends to be felt most often when you have the car loaded in a turn, and the undulation of the road causes the front end to dart around. A similar feel can occur when the rebound is set too high in the front shocks and the grip changes because of unloading of the suspension. This will also produce a darty sensation. Finally, toe-out at the front can produce the same thing.

mark kibort

generally, you might not even be able to tell if you have a bumpsteer problem, especially if the car is tightly sprung. in a hard turn, the inside tire would be drooping, as it is supposed to be, and the difference of toe for that wheel vs the highly compressed outside tire, (which is toeing out) will result in some scrubbing of the inside tire's inside edge. Sometimes this is not an issue, because you may even be lifting that inside wheel off the ground in high G turns.

Under straight line braking, it could result in toe out that could make the car more "darty". bad bump steer issues are a concern as the angles of the tie rod to steering rack increase, those angle changes for a given suspension compression are greater. This is the problem that the bumpsteer kits try and solve.

mk

va122

Thanks Guys! That was just what I needed. The quote message in reply button doesn't work for some reason i don't care enough about to fix.


http://www.derpca.com/Porscheforme/911Tech6-01.htm
lol, i should have just asked him when i saw him. I'm SD region too.

A similar feel can occur when the rebound is set too high in the front shocks and the grip changes because of unloading of the suspension.
Ok, know what that feels like. not it.

This will also produce a darty sensation
Nope straight as an arrow.

Finally, toe-out at the front can produce the same thing.
Of course! Duh! Thank you I have very aggressive toe out (2mm)

This is the problem that the bumpsteer kits try and solve.
Oh, good don't need to spend any more money, I'm only 10mm lower then stock.

va122

Damn, I'm afraid to ask because The car is perfect on track so i'm hesitant to change anything...

I'm running 2mm toe out in front and 2mm toe in rear at present. I had good results with 1mm out and 3 in f/r with the stock suspension.

I'm currently all monoball, moton tripples, rsr dogbones and toe links cup control arms yadda yadda and at 100/120mm ride height.

2 questions:
1, will taking one click HS Rebound in front only increase or decrease understeer?
2, what will going back to the +1mm/-3mm toe f/r feel like relative to what I have now +2mm/-2mm f/r?

Larry Herman

#1 is a tough question because there are so many areas that shock changes affect. Based upon my understanding and experience, reducing the front rebound will add grip under conditions where the suspension is decompressing. So you may have less understeer after the apex under full throttle. It probably won't do much under corner entry.

#2, if the car is darty, reduce the front toe out. I run my cars at 0. Best compromise for braking and straight line speed vs turn-in. If the back feels good at 2mm toe-in, don't increase it. The less toe in the back the better as long as it does not become unstable.

SundayDriver

In regards to #1, reducing front rebound may really mess up braking. Front rebound keeps the front loaded under braking - if you back off too far, you will lighten the front as it bounces upwards and will lock the front brakes. That is one way you know you have gone too soft on front rebound. Personally I have never gotten to that point in a production based car, but certainly have experienced that in sports racers which are for more tunable.

On #2 I tend to agree. Few production cars like front toe out. It is standard set up on most purpose built race cars.

Larry Herman

Interesting. As you say, it could be very much based upon the weight of the car (i.e. lightweight racecars vs heavy production based cars), as I have found that too much front rebound will cause locking when braking over uneven or rippled surfaces. The rebound has a much greater effect on suspension movement in a heavier car. In a lighter car, the movement of the body has greater significance.

SundayDriver

You are on the right track though I think it is not really weight of the car, rather relative spring rates. With very stiff springs, the shocks have far more influence over the car. My former car ran rates that are equivalent to a modern 911 using ~10,000 lb/inch springs. OF course you would never really get there as there is just too much chassis flex in a production based car to use springs that stiff. And the only reason to go that stiff is to manage ride height for ground effects as those sorts of spring rates reduce mechanical grip.

As far as braking, I was really referring to the dynamics on a smooth surface. Add bumps and you are forced to compromise the setup in more ways than on a smooth track and your experience matches mine. I think we are saying the same thing in different ways and there are an infinite set of combinations that a racer/engineer has to try to deal with.

va122

Thanks guys, I really appreciate it. I personally like the toe out in front as the car turns in like nobodies business but as far as how much exactly, I need to call Stanton anyhow since i'm seeing him in Jan. His tech set up the suspension and he dialed it in on the track so I'll pose the question to him since he has about 20 years more experience in Porsches then i have.

I need to back off the fronts anyway one click, b/c I'm getting some shuttering (i dont' know the term, but car feels like it's on a pogo stick in front. Like stock actually) when i go from RA-1s to R6s due to the stiffer sidewall i'm sure.

Thanks again and have a merry new year.

RedlineMan

Hmmm...

FIRST off, you have to you have to DEFINE bumpsteer. I define it as any time that the toe is changing measurably with suspension movement; a bump is casuing a geometry change. We know this is caused by the angle of the tie rods passing horizontal, sloping down toward the rack, and toeing the wheels in or out (depending on whether your tie rods attach to the spindles in front of or behind the axle) as the suspension cycles. There are many kinds of dartiness, but to call them all bump steer is incorrect. Maybe that is obvious, but I didn't see it mentioned.

Victor, if I were you, I would do as Larry suggests and go with 0 toe in front. I do not see any huge benefit to going with toe out. You do theoretically get improved turn in, but at the expense of inner tire wear, as Mark K suggests. Porsches do not generally suffer from sluggish turn in. Using toe out to solve for something that is not there is silly. I was silly for a while, but have gone back to 0 to help with inner front tire "drag wear."

va122

Thanks John for your definition, I appreciate it and will consider backing off on the toe in front. I'm not sure how to exactly quantify why but i didn't like 0 toe, i found it to be too darty if that make sense.

va122

after re-reading my post it doesn't make sense, sorry.

SundayDriver

There are other factors at play and often what people think is bump steer is actually a camber gain problem. As the suspension travels, camber changes. Ih it's simpleet form think of a suspension that is double A-arms with the top one quite a bit shorter than the bottom. As the suspension travels upwards, the shorter (upper) A-arm move further inward than the lower and you get an increase in negative camber. At the very least, you want a camber gain (more negative) as much as the body roll you get. The designers did this based on stock spring rates and stock suspension height & geometry.

So if it is messed up due to ride height, compliance or design compromises, you hit a bump in a turn and either get a lot of unwanted negative camber or it goes the other direction and you lose camber. Either way, you are now running on a much narrower tire than intended with a corresponding loss of grip, The car changes direction and tends to feel exactly the same as a bump steer problem. To know what you have, you really need to measure the bump steer to see how much is there.