You said "we are not talking about surface quality here" and I said "sure you were". And you still are, even though you contradict yourself, as in bold above.

You said "If you understood physics, you might have realized that. But you have a hard time reading between the lines". Tell me again who is pissy?????

There's a reason a suspension has bump stops. If you truly believe you can use them exclusively as your suspension, knock yourself out.

As for the Weissach axle...if its tendency is to toe in, why is that a bad thing? Most race cars benefit from a bit of rear toe in anyway, to help stabilize the back end under braking and trail braking. Not seeing what that has to do with your original question about understeer and the bumpstops.

Yep. Occam's Razor...

 

Dear Santa,

Since you screwed me in 2015, perhaps this year you can provide me with the patience to read any thread of Kibort's without an increase in blood pressure, and/or the desire to make rude comments. Indeed, his threads are like potato chips - I keep going back for more even though I know they are bad for me. Also, get Kibort some softer bump stops since he seems to load them up regularly.

 

..

 

Mark,

You said the mustang guys have suggested that touching the bump stops may be part of your understeer problem. You also suggest that you don't believe this is the problem due to the "bumpy vs. smooth" surface argument you outline above in bold.

VR then points out that surface quality has nothing to do with the impact on roll couple due to touching the bump stops. He is correct. While the impact would likely be more unpredictable (big variances in roll couple) on a bumpy track, the physics are the same.

It doesn't matter whether you bottom the suspension hard via bumps or you barely graze the bump stop due to high-g load compression. As soon as the suspension bottoms (or even starts to bottom), you can get a dramatic change in spring rate and roll couple.

It's the same thing as instantaneously stiffening the front roll bar. The front roll bar doesn't care whether the track is smooth or bumpy, it does it's job via spring rate and relative stiffness. The bump stop is "replacing" the coilover completely. The spring is no longer part of the equation (for the most part), the spring rate of the bump stop itself becomes the new controlling factor in terms of relative roll stiffness. The factor that causes you to touch the bump stop, bumpiness or high Gs, is irrelevant.

On the other hand, I don't think this is your problem (smooth vs. bumpy aside). You describe a progressive increase in understeer that occurs with tire wear. This doesn't sound like a bump stop issue, which would be MORE likely to occur with a stickier tire and higher G loading. In any case, it's a good idea to check and confirm.

This sounds like a front-engine issue. Your front tires have to handle 54% of the lateral loading plus steering inputs. The car is going to tend to understeer in general. I have a 911, it wears out the rear tires first. Such is life. You can deal with this a couple of ways:
1) change the roll couple which will increase the tendency to oversteer.
2) change your driving style to accommodate the front bias (ie more trail braking or an earlier apex).

Does the car understeer on corner entry when you are trying to get it rotated before the apex or after you get on the throttle?

 

Well the book clearly says don't ride bump stops as the instant infinite increase in roll stiffness screws up car not sure why we need to discuss more - more ride height or cut the bump stops lower or order a softer progressive bumpstop - add pot to your motec or aim system to measure shock stroke and you are all set to go

 

There are tools in this thread; ideas, concepts, literature that can lead you to finding a solution. Nixing all of them simply because you don't think that's what is going on shows a fair amount of ignorance. Based on your understanding of bump stops/bump rubbers you have some reading to go do. Even that said, there are others here that have adequately described the piece of the puzzle you're missing, yet you refuse to acknowledge their ideas as relevant.

 

yep and willing to test that , but it doesnt seem like its a possiblity, due to it not being on the bump stops on the turns in question. one proof of that is, when the sway bar is disconnected, i lift a inside rear wheel and it spins maddly. i dont think it could do that if on that pushy turn, the front under compression was on the bump stop. kind of an acid test i would think

 

what we have here is a failure to communicate. how many times to I have to clarify. im not talking about surface quality. smooth or rough for example.. im tallking about undulatios , vs flatness. surface quality that would cause vertical acceleations of the tire. can i make it more clear??? there is no contractions... you just cant grasp that "smooth" can mean a mirrror, or rough road that doesnt have any bumps. again, you are getting "pissy" here.
key word: vertical acceleation ...... i know, its too much of a physics term for you to understand. think of this. im talking about your coffee you are holding in a cup and the forces make the coffee fly upward. (i.e not a smooth surface)
vs coffee staying in cup. (smooth surface). compared to the surface quality of rough to smooth. big diff.

then you end up talking about using the bumpstops "exclusively" agian, your comprehension is amazing!! i said ,(and you can read it again) that i ONLY think im on the bump stops through the corkscrew at laguna and its not pushing there. so, thats not using the bumpstops exclusivley, now is it.

also, there are others that have posted about using them on occasion which can work if its expected and in your tuning design.

then you ask why is toe in bad in the rear? really? it tightens things up, and that can cause the rear end to not help in the car rotation and cause a "push" characteristic. that seems obvious, but what do you not understand there, or do you disagree?

 

I dont know why you would say that. Im totally listening to all that have posted with adivce. im only reflecting and correcting any misundersandings of the environment or conditions. because i say i have double wishbone vs mcpherson, or that i dont have a push when i possibly am on the bumpstops,(for example) doesnt mean im nixing advice. im clarifying conditions. I have the literature and have opened the discussion to those that may have the same situation. again, im not nixing anythign, but will make changes based on advice here that seems to apply or that im capable of doing. (there are some limitations and things i cant do based on the dissimilarities to others set up advice, like camber plates, etc)
So, what do you advise? so far, its jack up the front of the car to get more clearance incase im running on the bump stops, and i can do that, but i want to know why raising the CG in front would help, expecially if im not on the bump stops. and, going back to my first question, would that even help, if on the bump stops, if the turn was very smooth (no vertical accelerations ,VR )


and i want to understand why, if on a smooth (no vertical accelerations) surface. how does this infinte increase (and its not infinite, the tires are still adding some compliance here) effects handling and why.
I dont have data aqu systems and i have no sensors. however , i might be able to mount a camera up front, like i didn in the rear to see what happens during extreme conditions.

 

Change thread title to You're Wrong I'm not Pissy, IMO.

 

Thanks for the response. appreciate you thinking this through for the sake of the discussion.

so, yes, im thinking that if you are on a smooth surface and in full compression, does it matter if you are riding on the bump stop, if there is no vertical accelerations? i understand that you are saying, even as you approach that point of the bump stop, all is good, but when you reach it, the spring rate goes way up. Not infinite, as you still have the bumpstop, which is a spring and the tire to act against the tire. But, i get the point that suddenly you increased the spring rate at that point , dramatically. this changes the roll couple distribution.

forget about what VR said. im not talking about road surface quality. im talking about vertical accelerations if on the bumpstop.. in other words, if you have a dip in the road, the tire will droop and then come into a high velocity compression, controlled by the shock that puts an upward acceleration force on the chassis.... you then loose contact with the road if that motion is not controlled by the shock and spring. But, if its smooth, that dynamic doesnt exist, and what does exist, is the roll coupe distribution change as you press , even gradually, on that bumpstop.

Yes, i do think its a front engine thing, but i guess it would be analogous to the "rear engine thing" of a 911. do all 911s oversteer during turns with no push?
if you have been doing this a long time, you know and as you say, driving style can cause all sorts of issues and fix some as well. i can make most any car push, and oversteer. but navigating turns as fast as you need to or think you should go, creates forces that the car cant handle. so yes, i can then trail brake even MORE. forcefully rotate the car, with a combination of steering and power on oversteer forces. but thats not always possible. in fact , this push even gets worse on flat turns like turn 3 or 4 at laguna, on the exit. even after hard braking, trail braking and then release at the proper angle to the turn exit, the power on understeer is really unnerving. knowing the speeds the car runs through those turns, its a tire thing that is totally cured by better tires. BUT, the point of all this is to make the car handle better with more oversteer when the tires are not new or newish.

so to answer your question and advise.. yes, ive done all the driving variations and, once im able to rotate the car and get it pointed correctly, it power on understeers. recently, that video at turn 6 at thunderhill shows, i really cant even get the car rotated. its pretty neutral in the constant speed turn prior. (more gas, and its starts to step out.. you can see my hand inputs) but braking and trail braking to the turn 6, it pushes with any turn input , pushes around the apex and im pushing out of the exit as well. again, with newer (not brand new) tires, its perfect through there. with bad tires i can have a push and exit oversteer. that's when all the tires go in the garbage.

I would like to try two things after some of the constructive input here.

change the roll couple... but what do you suggest? stiffen rear sway bar? i can loosen the front sway bar a little too. because the push happens mid turn and on the exit, i dont think shock stettings might help, but maybe less rebound in the rear and a little less compression up front to help with better turn in. (maybe a little less rebound up front as well). but to address the mid turn and exit push, you are suggesting changing the roll couple.. would that be the best first start.

of course , if I'm on the bump stops (and i can check this again on those critical turns) i can raise up the car .5" or something.

 

Again, all tracks have bumps. There is no such thing as a completely smooth track.

 

I know, but if you havent been to laguna, and see the problem areas for push at turn 3-4, its pretty darn smooth (oh, sorry VR, not bumpy)
im just trying to learn how this change in roll couple dist, can give me a push in those turns with out vertical accelerations. Ill see if i can dig up a fender cam for laguna to see what that outside tire is actually doing.

 

previous chassis, with more rear toe out and newer tires.

and to prove that the turns are not on the bump stops

go to about 10second in on this clip.

 

Pretty darn smooth is still bumpy. Here is data from a just repaved, smooth NASCAR track. If you really look, you can see a lot in these graphs. Notice in the second one, that even over relatively small spaces, their is a good amount of shock movement. These were at 200 Hz, so not slow at all.

Now, take those movements, and compare it to some bump rubber graphs like these http://www.fatcatmotorsports.com/iga...SpringRate.GIF and you could have a couple of hundred pound difference in spring rates on the front. Have you rated your bump rubbers?

In fact, the answer to your question is probably in what I just posted.