HighPSI_LowCC_Speed

Sorry for the long subject line, but it says it all...I am a novice HPDE student w/ an SS/SC and am experiencing a light rear end at high speed at high throttle during easy cornering over uneven pavement... (for those of you that care this is right under the bridge on the Bridge Straight of Summit Point-Shenandoah Circuit => Thanks for the track time, trackdaze_dot_com!!!). Needless to say, this is worrisome at high speeds, especially when I want to calm the car's suspension, winding through the gears before the drop/crest into the Karussell. So, I'm looking for a suspension fix for this, though I'm not too comfortable making an educated guess as to what is causing this/how to rectify it. My best guess is that I need more dampening in the rear, so as to scrub off the rear bouncing quicker. Any ideas on if I'm right / how to fix this would be much appreciated! Thanks a lot for the help, in advance!!

yellowltcoupe22

try lowerin the car and get some aftermarket sway bars. better yet spend the extra money and get some good coil overs.

rocketpunch1221

What kind of suspension set up and tires are you running? Just wondering and define "light rear end". Is the rear bouncing about or simply "nervous" at turn in?

That turn looks like a very easy corner but very bumpy, if your rear end is being thrown around on throttle going into and through the turn; your suspension set up (especially the rear) may be too firm.

HighPSI_LowCC_Speed

hmmm. the thing is, I'm 100% stock suspension-wise (SS/SC). tires (on stock rims) are 225x40x18 general exclaim UHPs. I don't think I'm too firm. What I'm feeling is a looseness in the rear caused by bouncing on uneven pavement at 90+ mph. it is not a turn in or turn out problem, only during the middle of the straight as you bear easy right while accelerating hard. a "nervousness" may be a good description of what I'm feeling. my instructor commented on the same thing. like i said, my best guess is that the spring rates in the rear may be slightly low, causing a slow reaction in dampening the uneven (left-to-right) wheel-shock over the uneven pavement. i would prefer not to lower the front and minimally in the rear, so as not to f- with the geometry. but it seems like springs may be the way to go.

yellowltcoupe22

get a good set of coil overs then man. you can keep the ride height in the front and lower the rear as needed and be able to adjust the dampin effect all at the same time.

qwikredline

TC struts and shocks, (better damping by far) Eibach springs and from OTTP a time attack style rear chrome moly sway bar....then get some better tires....you dont say what your tire pressures are, you can tune the stock car quite well with pressures, but you cant beat the TC damping...

HighPSI_LowCC_Speed

thanks for the opinions, guys! my tire pressures on track ~30 lbs. I may consider dropping the front pressures a bit more than this, since they get significantly warmer than the rears. let me kno if you have some personal insight on the tire pressures....and...yeah....I kno the tires suck, but I figure they'll be good novice tires to burn through at an affordable $80 a pop.

qwikredline

nonono... If you are seriously tracking a car, you need at least 36 psi front on a road race track and proably 40 psi rear. With a street tire initially you go up or down in 4 psi increments finding the right pressure. (less than that is hard to feel or measure a change) You are working to hot pressures, these suggestions are for cold settings (after standing for 1 hour or so) and you do not bleed down in between sessions. Look for around 45 -50 psi hot pressures.

You should figure out what a General tire wants for hot pressure, I have no experience with this tire, but for sure with all street tires, as they wear down to about 3/32 in tread depth it will give it best track performance. Initially after about 3 laps the street full depth tire will get squirmy, thats normal and lap times will get slower.

Until you can fix the flca with a spherical or stronger inner joint you will continue to wreck the outer rib of the left front tire on a generally clockwise rotation road race track. (opposite for counter clockwise) . Rotate the tires, as you can scrap a front tire in 20 laps going hard.

As you get smarter, you will look for alternative wheels (maybe 17's thats can fit your brakes) and then get used Hoosier DOT tires from racers, or buy new. Thats better for the track more grip and doesnt wreck your street tires. Dont forget to call Josh for a rear time attack bar...

HighPSI_LowCC_Speed

again, good info. thanks! thats a lot more hot pressure than I was going for originally (aimed for 38-40) but I'll definitely give it a try next time.

couple more ?'s:

would TC knuckles help the flca?

i have 16" steelies for winter tires, so the brakes fitting a 16 or 17 wont be an issue. i can only use street tires for novice HPDE. Should I attempt to reduce the overall diameter of my tire/wheel package while increasing width of the contact patch?

qwikredline

No LNF knuckles wont help the tiny little rubber bushing that gives up in the flca....best is to shave street tires, or buy used avons or pirellis that are worn but not beaten with burn outs...hard to find... best bet for tires is stick to the size it came with. on 17's look for 245 40 17 and try to get a wide wheel i use a 9.0 or 9.5 inch wide 17 again hard to find wide wheel is good to help tire...light wheel is good as well, but between lite and wide choose wide... again hard to find in 5x 110 good luck...

Platinum_Injected

i don't street race that much but i do find at high speeds like 180km/h or faster my back end feels as though it's not even contacting the road!! and is really sketchy

suburbanrobot

I Auto-X and do Lapping at the Autobahn CC here in IL. I guess I am not experiencing what you are as there is only one minor elevation change at that track on one of the straights.... I've never had the problem you are having..... my car is on 100% stock suspension and takes whatever I throw at her and is EZ to drive and very forgiving!

My thoughts are to bump up the tire pressure to eliminate sidewall squirm and flex.... I start out at the track at 40psi front and 38psi rear w/ the factory Pirelli's and usually don't touch it again

Next year I'm running the Kumho MX's on 17's.....we'll see how those work out.

please keep us updated w/ your progress

alleycat58

Ditto. I've never had that problem on stock equipment. I run different tires, so my pressures are a bit different.

DaBuzzard

I think I know what the OP is talking about. My daily commute is 65 km one way with 4 90* corners, 2 left and 2 right. I can rail 3 of these at 130+ kph, no sweat, no strain. The other one has a few really good expansion cracks running across the width of the road, and at 110 kph it feels like the back end is on the verge of snapping around over every crack. You can feel it in the front but it never feels like the tires are going to give up, little bump and they grip. The back feels like the tires are coming off the pavement as the back end steps out...feels like a foot but likely an inch or two. On smooth pavement both ends stick very well, understeer at the limit (as you would expect), really quite easy to drive quickly.The only vehicle I have driven around that corner that was worse is the F250 I drive for work sometimes. I have actually had the back end break loose over those cracks at around 90 kph....little scary to say the least.

I too am curious as to whether this is too much / too little dampening, too stiff a spring, a combination of both? Or something entirely different?

alleycat58

You're comparing two COMPLETELY different things. The OP is talking about driving on a race track. Pavement on a road course is much different than municipal street pavement. Usually different mixture, differeing friction coefficients, etc. PLUS you're talking about expansion cracks in the pavement where the tires are actually losing traction. It's ALWAYS going to feel that way regardless of car setup.

EDIT: You're also riding on a different suspension and different wheels, so the feel will be a bit different as well.

suburbanrobot

agreed 2X

Maven

iTrader: (1)

Besides tire pressures the cheapest remedy is gonna be to simply bolt on some TC dampers. If the course is very bumpy youll want to go with the springs too because since the TC dampers have different valving you can get rear axle lock down on bumpy courses using softer springs(usually parking lots, though....probably enough time for the shocks to rebound on a road course.)

qwikredline

I was thinking about this to myself today. And i said "self, Its mostly the tires."

I dont know the generals but this summer i lashed out and replaced the Avon 18s with Falken 452. On the street, ok fine. ON the car on the track changed from great to junk, rear floating every where, washing out, slow speed push lots of it , no balance.

came in put the 17" dunlop OEM tires, back to great again....so TC dampers and decent tires = cheapest solution...oh and please its an fwd car so a couple or 4 more psi in the rear than the front helps rotation, turn in, and balance...not the other way round...

HighPSI_LowCC_Speed

this is great stuff, guys. definitely tire pressures and I'll consider the TC switchouts. This brings another issue -- I'm poor as dirt, so I'd prefer to wait for **** to break before I upgrade and I'm thinking something is wrong with my suspension in the front....on cold mornings, when I go over relatively significant bumps at low or high speed (like speed bumps in parking lot or settling gradations on a road) the front squeaks/squawks on rebound pretty LOUD! Makes me think struts might be dead. It's definitely coming from the front, though.

Also, I have a stupid question (getting into flame-retardant suit): the SS/SC has struts in the front (spring + damper together) and shocks (damper separate from spring) in the back, correct?

Thanks again!

Maven

iTrader: (1)

Yup MacPherson fronts, shock and coil spring out back.

wordsthatiwrote

TurboTechRacing rear sway bar....?

DaBuzzard

True enough, although the "pavement" at the local track (Race City) is about as "smooth" as the local roads...except where the pavement has given up entirely and left a lovely hole marked with a pylon And the transistion from the chute (lumpy bumpy cracked asphalt) to the front straight (concrete drag strip complete with vht) is unbelievable.

Basic theory should be the same as far as getting a good balance should it not?

Did GM just set these things up with the idea that there would be more weight in the back (like passengers)?

idlingmike

As far as upgrades go, absolutely do not get "budget" coilovers of any kind. You will hear all sorts of claims from both manufacturer's and personal experiences. Unless they can back up their claims with shock dyno's or laptimes, it is not credible. References along the lines of "I can take this corner at X mph now" are also not credible. The entry level KW's (v1) is a good coilover for the price. Make sure you get the car corner weighted as well.

Assuming they do fit, one step down from there is probably the 08+ TC suspension setup. Just the shocks and springs are in the area of $500 from gmpartsdirect. (Again, this is assuming they actually do fit; also not sure if there are other parts that may be needed.) It's fairly unlikely aftermarket springs/shocks will be better than the 08 bilstein setup ; I bet nobody even makes an aftermarket linear-rate spring for the Cobalt.

However.... If your only real concern is the rear end stepping out on this one turn, you really shouldn't be unless it's also decreasing radius. Front wheel drive cars won't oversteer without a lot of suspension tuning, different tires or lifting/braking. So just stay on the loud pedal and the rear will stay put.

I'd also be careful adjusting tire pressures or alignment for more rear grip. As a front wheel drive car, it already understeers, adding more rear grip will only make it understeer worse and likely slow you down on the rest of the track. Doesn't hurt to experiment, just make sure you quantify your changes with laptimes.

-M

Maven

iTrader: (1)

Well....we'll know probably at the end of the week if thats true, once those springs up above get rate checked..

idlingmike

Care to elaborate?