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Discussion in 'Suspension and Chassis' started by lakeside, Mar 20, 2018.
Looking for opinions on rear bump stops, how long and why
I cut mine down to the bottom section on my GT. Because my car sits low and is on short tires (25.6" dia.) I don't have any apparent bottoming or metal/metal clunking, but they certainly make contact with the arch in the frame rail. So they seem to be doing their job like that.
Boss is stock.
I only cut off about half a convolution because I was only going to be lowering the rear of the car by a little over half an inch. I over-bored the center hole with a tapered rotary rasp to add some flexibility and plugged/sealed the hole at the bottom. Simply cutting the stop makes it firmer, and when you do hit it as you eventually will, it'll hit harder.
I did not lower my car to lower it; that part was incidental to getting stiffer OE-style "big springs".
Too tall. Unbolt then and throw them away.
I think they're just about the right length for a suspension lowering of maybe 5/8" with springs about 50% stiffer than OE. I won't go any lower.
What’s to prevent your shocks from bottoming out?
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You're likely still getting into them from acceleration alone, hurting rear grip. I'd just remove them.
The bumpstop on the damper itself. I've never seen an s197 damper with a short enough damper body to bottom out before the axle hit the framerail.
I don't drag race. Ever. The gap is about what it was with the car in fully stock condition, and it goes up from there when I take everything out of the trunk for a track day. If I'm getting any loss of grip, it's minimal.
Koni yellows. At least anecdotally, they don't tolerate lowering as well as other choices. You should know that I did a little DIY work to soften the initial bumpstop contact and the first bit of b/s compression when I cut them down.
To some extent, I need/want it to be firmer. Mine is also relieved a bit on top with some cuts to the top. It also contacts the frame rail on the angled part behind the horizontal section.
With 350 lb./in. springs in back, I don't mind having half the clearance and travel and having the stops firmer on the order of double...seems to make sense to me. Also don't have any clunking like I did back with P springs + LS/Boss struts/shocks and bottoming out in the corkscrew at Laguna Seca. No clunks or bangs on the street either.
Good point on the damper bumpstop. I think the CorteX JRi units might have the clearance to hit if both are removed. Maybe.
I'm keeping mine in there, cut down as it is.
Who ever said drag racing?
You're likely getting into them hurting ride quality and rear grip.
you're definitely getting into the bumpstop exiting low speed corners. Try removing them between sessions and report back.
"just from acceleration" is going to be at its worst under drag-racing conditions (i.e. harsh upshifts) particularly in the lower gears. Having only 4.6L (NA) to work with may have something to do with it - track day datalogs show that it's not pulling past about 0.4g, and that only corresponds to about 3/8" rear squat. That's in 3rd gear on the track because with a 6000 rpm redline (and a 5900 rpm upshift indicator) 2nd gear isn't terribly useful.
Probably true as a binary either-or statement but of less significance when there are many 'shades of grey' in between. A little light contact with a softened bump stop isn't the end of the world, and so far it's not been noticeable as far as chassis inertial movements are concerned (with one possible exception if I get a bit sloppy and run too far wide at track out). Last time out, the car was still datalogging in excess of 1.2g laterally, on MPSS tires on their 12th track day plus 5000-ish street miles . . .
Maybe you're right...but I doubt it for my car. I think I'm running too much anti-squat right now for that to be a problem as-is.
I'm planning on setting up for more squat, but plan to raise the rear a bit for more clearance. Pretty sure I'm going to leave the axle stop there, but might cut it down more if it feels like I'm getting contact.
FWIW on my race car, with the rear springs being in the stock location (on the axle), I had the bump stops trimmed down to the top of the final part of the stop (the fat cylinder).
There are no on-axle bump-stops on the cambered axle setup I run now; the bump stops are on the JRi shock shafts like what is shown in @Grant302 's post above.
Either way, you want to give the suspension room to work. If you are hitting the bumpstops quickly, the shock may not have time to do much if anything. Similarly, you don't want the static ride height of the car to be too low as this limits available suspension travel in bump.
Not true. When exiting a corner, dh you wait until you're completely straight down the straightway to pick up the throttle?
No, when powering out of a lowspeed corner, you have combined loading of roll and squat. This will compression the outer rear damper far more than cornering or straight-line drag racing alone. "Acceleration alone" as in out of a corner and not considering vertical displacement from bumps, which when accelerating out of a bumpy low speed corner, would make you get into the bumpstop harder, further hurting rear grip at corner exit.
I think you'll find a much more compliant ride and better rear grip removing them.
I'd put $ down that you are even with those spring rates and lowering the axle side LCA significantly. Put some paint on the bumpstop and see if it's touching after a session .Or throw a GoPro to see how bad you're getting into it.
Of course not. But the amount of squat is minimal until I'm past the point where I'm getting max roll. Max roll by itself doesn't cause b/s contact either.
Like I said, only when I run too wide at track out (like, up on the curb all the way to the serrations and get that harsh buzz) do I feel like I might be getting some b/s contact.
NJMP (both courses) and Mid-Ohio really are pretty smooth, though. Perhaps especially for being in the north central/northeast portions of the country. At any rate, so far these modified bump stops aren't broken so there's no need yet to fix them.
You are getting far more compression in the outside rear tire when on throttle out of a low speed 2nd gear turn than you are from pure roll or acceleration alone. Even when hitting the exit curbing and "buzzing" the car (not much added vertical load).
While mid-o isn't bumpy, there's a ton of added verical load from elevation change in turns 4, 9, & 13 that will easily get you into the bumpstop. You're likely into it far more than you ever expected.
The great thing just there's no one forcing me to help you or forcing you to listen. If you're content with your curent performance and lap times, that's all that matters. Don't try anything different to make yourself go faster and just enjoy the drive.
Also, you should take anything you read on the internet with a grain of salt, especially free advice from random people.
I'm not doing any of that. Unless you're talking about enough $ to make it worth my while. I take paypal and if you're *that* sure...you're giving odds, right?
But seriously, I don't think it's 'sitting' on the bumpstop coming out of a low speed corner. A paint test as you propose would show contact for bumps, etc. for which I'm sure it is. My blue car is still a dual purpose street car, and I'm not removing the stops.
My static LCA angle is about 3º up and I think these pics show that the loaded side hasn't travelled all that much and I'm estimating just over an inch. Anyway, I've driven and rode in cars that are 'all over the bumpstops'...I don't think my setup is, on the track or on the street.
And like I suggested before, this side discussion is moot. I'm moving to more squat and need more travel for that alone. If I'm doing it right, the intent is to kiss the stops on low speed turns. Just my opinion, though.
Oh yea, you're definitely into the bumpstop already. Take your spring off and compress the axle until you're tucking that much tire to test for yourself .
I take PayPal as well.
3* front LCA mount higher than rear (on the axle)? That's not a lot of anti-squat, and with a tiny bit of yracel, you'll already be into pro-squat.
If I realistically had any 2nd gear corners . . . gearing, rpm limits, and tire size are such that on a traffic-free lap I'd only get about 7 mph and 750 rpm worth of 2nd anywhere.
Harshness of the buzz is my indication of getting any b/s contact at all.
Hard to say whether I'm getting contact, or how much if/when I am. What I do know is that any contact isn't hard enough to upset the chassis. Keep in mind that I've only got 315HP, 325 ft*lbs, and 3rd gear to work with here.
Should I get to the point where I can feel that the bump stops are holding me back - I think I'll be able to figure out how to recognize this - that's when I'll consider doing something about it. More than likely, that'll take the form of trimming another fraction of an inch off each b/s and maybe grinding the holes in the middle a little more. I might shim the rear springs up another 1/8" or possibly 1/4". What I really don't want to do is anything that would remove all protection against bottoming the Koni yellows out.
Now you gotta be pulling my leg. My car LIFTS in the rear off the line. You can still SEE the upward angle on the LCA in the pictures I posted. Driving the Boss (stock suspension) -2.2º LCA and GT back to back help me feel the difference. The GT is over 100% AS.