mark kibort

im not trying to argue at all. sorry if its coming across that way. i really am asking the question. you might be a little too sensitive.
im trying to understand the effects if you are riding on the bump stops or shock body, in a smooth turn (no undulations in the road).
im thinking its something analogous to applying the brakes and hitting the bump stops as well, even before max braking has been reached. would that be comparable, and maybe easier to see the comparison to effects on lateral accelerations (albiet there are a couple of more factors involved there)

scott, if you ready my responses, you will see i was answering the assumptions of those responding. one talked about mcpherson struts (i dont have them), you talked about making bump stops, (ive done that), someone asked about tire temps (done that too), and one also mentioned the bump stop rubber, works like a progressive spring, and i agreed , in some cases it can be used effectively... .proof, ive seen no ill effects when hitting the bump stops in teh corkscrew, and im wondering if i am in some other pushy turns. I dont think i am based on markings ive made on the shocks
I really want to know what i can do about the push, assuming that im not on the bump stops and it goes back to your original post i copied here, about roll coupling distribution. maybe tightening up the rear swaybar or higher spring rate in the rear?
what i do know, is with a good set of front tires, the problem goes away for a while. problem is, i want to be able to balance the car with not so good tires up front. either after a few sessions or toward the end of the race.
.,

Veloce Raptor

Sure you were . You asked why this would matter on a smooth surface. My point is the physics don't change

mark kibort

Smooth, meaning without bumps. I should have said "flat", or "consistent". rough vs smooth would make no sense to talk about. If you understood physics, you might have realized that. That's a friction coefficient factor. not really the question here. But you have a hard time reading between the lines... i get that.....

winders got it, based on his response saying "even on a smooth turn, the effects......" it would make no sense to be talking about surface quality here.

so, what "physics " doesnt change. If the car is going over bumps vs it being flat or smooth (no bumps..... nothing to do with surface quality), the physics dont change... physics doesnt ever change! (the factors can though. ) so, what are the effects on the handling?

333pg333

Mark

I noticed all the steering angle you had in that recent thread. Seemed too much just on a visual sense.

What you're describing sounds like something I went through a few years ago with some KW shocks. Mostly on a few slower / tighter turns. Played around with quite a few things trying to discover and fix this issue. In the end we replaced the KW adj camber plates for the Racers Edge version which are designed in such a way as to increase droop. Pretty much fixed the situation instantly. We figured that I was just into the bump stops and ran out of travel which inhibited the amount that the nose could go down to increase frontal grip. I would agree that this is a relatively simple place to start. Not sure what options are available to the 928 though?

mark kibort

yep... its kind of a problem with the 928 steering rack. the good news, is that it desensitizes any knee jerk movements so it's a tame car for DE guys, but a little more work to race. (and more movement than i like for sure. a contrast to something like a Vantage, that has about half the steering input for the same wheel movement)

so, you are saying that more droop on the opposite side, allows for that side to be more involved in the turning forces? from the "fender cam, i dont think we are getting near full droop, even though that the droop of the Koni, is much less than a stock set up.

go to 12:37 on the video and see one of the pushiest, left turns, that doesnt show the inside tire needing any more droop on that side.

Did you mean , to increase compression range? like , getting a shorter shock body to keep from hitting the bump stop? Just want to be clear with what you did to improve the situation. again, i think you might have a strut suspension, so there are no camber plates that would or could work.

Veloce Raptor

I am trying to help you here . No reason to get pissy. Smooth without bumps is what I am talking about. The benefits of camber and toe are there irrespective of the surface characteristics. Which is what I said.

Matt Romanowski

There is no such thing as a smooth surface. You might not see or feel the bumps, but they are there.

I work with cars that run on bump stops. They are not easy to set up or a set and forget kind of thing. You have to do a lot of work and be experienced with them to make it work.

Veloce Raptor

You're right, Matt.

I guess it's funny the surprise that a car that is randomly on the bumpstops is under steering lol !

333pg333

Well essentially I just mean that if you were going into your bumpstops and reducing travel this would/could have a detrimental effect on the steering. I have a vague recollection of the front end suspension on a 928 but am not sure what mods are available to it? How are your other 928 race buddies setting up their suspension? Is there any aftermarket suspension product available for it? You'd think that someone else has run into a similar problem even though there's not a lot of 928s running around hard on the track.

mark kibort

Dave, you are not trying to help, you said... That, by anyone's standards, is being "pissy". Now, if you are.... then you have an interesting way to show it.

Again, the discussion isnt talking about surface quality, we are talking about the effects of suspension compression at the end of travel, and roll couple distribution changes.

The question im acting, is that if the surface is perfectly smooth, would you even need a suspension. (i.e. running on the bump stops during braking or corning) if you are at high g loading , does toe really matter that much. after all the inside tire is barely touching the ground. and yes, camber is important, but ive already shown there are no hot spots on the tire. very well distributed and wear characteristics.

the real question is there a way to really get rid of the push. easy to make the rear loose. thats not what i want. I want to fix the push, without having to buy a new set of front tires every 2 races.

maybe as Scott mentioned, i could stiffen the rear bar and change the roll couple distr. thats what im leaning toward first.

mark kibort

sure, at the limit, many small road changes will induce vertical accelerations that will reduce adhesion. you can see from the video the tire moment . in the rear , there is tremendous amount of tire shift left and right. in front, they are beyond their slip angle and loosing grip at the speed that the rears are working fine..... obviously slowing down entry speeds, fixes the problem, but relative to other tire sets, its slower than i need to go.
Its not hitting the bump stops on the corners in question.

Again, if you want to help, can you lose the attitude? the bump stop hitting is one corner on all the race tracks i visit. and there is no problem on those corners. (by the way).. its the corners where i dont hit the bump stops that are the issues.

you are advising or trying to help when you haven't asked any of the right questions. one thing that sticks out, is that the 928 has a weisache rear end. it fights turns and rotation because it has an auto-toe-in functionality. i could counteract it by pinning the rear end or adding more rear toe.

mark kibort

that's the issue. ive driven the only other two 928s that are faster than mine, and I think mine handles very close, (sans the push) . the main thing they have over mine is 150more HP, and several inches wider front tracks with 2" to 2.5" wider front rims and tires. the faster of the two, took on more of my settings and seems to handle even better now. Plus, they run real slicks on top of that. one of the things that they both have done , is change the roll center heights by using extensions on the lower control arm to level out the angles. it gets rid of some of the bump steer effects. But, there are very few other modifications that can be made to the front suspension. Its actually quite ideal when those angles are corrected. unequal double wishbone is a pretty good place to start and that's stock and very tough.

audipwr1

Suggest you read tune to win - walks through all of this and how to test for it

mark kibort

I have it... probably have had it and read it more than most. discussing here on the list for a different perspective.

fatbillybob

All arm chairing aside. isn't the only way to ferret out this bump stop thing is to raise the ride height to ensure full suspension travel at all times and see what that does to laptimes? Who cares if it is tight or loose if it is fast?