Big tire setup and tire shake...

[LittleJ]

need a little help with some tire shake issues

i have a 7.50-7.60 10.5 car BBC,400,nitrous,4.56,3100lbs. up until the end of last year tire shake was very minimal using the stiff sidewall hooiser 33x10.5w. we run about 100% AS with the 10.5 tire and generally 60 ft. 1.17-1.20. we switched tires to 33x15 and the tire shake is out of control, we have dropped the AS to 65% with some improvement and started reworking the shocks. my question is 65% AS still high for this big tire? IC is about 61 out and 5 1/2 up, any help greatly appreciated.

[want-a-be]

What HPish is the BBC? Where are the Torque and HP peaks at in the RPM band? Whats the stall of the converter? Tranny type? Rear gear? What tire pressure are you running? What is the weight of the vehicle? How much of it is on the rear end? Rear Shocks? Front Shocks? Rear Springs? Front Springs? Height of the lower 4-link bar? Is the Lower bar level or pointed down a bit? Do you carry the front tires at all?

Where does the tire shake come in at?

Don

[LittleJ]

around 1500 with the nitrous.
torque and hp peaks ill have to get back to you.
turbo 400.
4.56 gear.
8 1/2 lbs. air pressure now
3100 lbs.
48.9 on the rear
strange double adj. rear/koni struts front.
lower bar slightly downhill.
carries the tires about 25 feet about 9-10 inches up.

the shake seems to come in worst when she comes off of the wheelie bars then most of the way to second gear.

[want-a-be]

Does the nos activate when the brake releases? Meaning,...are you leaving on the nos?

What is the rear spring rate? The front spring rate? What is the converters stall?

Don

[LittleJ]

the car leaves on full nitrous with the transbrake

138 lbs springs rear
250 lbs springs front

[want-a-be]

I'd try some lighter srpings on the rear for a start. 110# maybe. If the springs are too stiff....then the tire side walls end up being your shocks. Are the rear shocks adjustable? Have you played around with them if they are?
You might want to shorten up the IC just a tad...maybe around 58" with about the same height. What pressures have you tried in the rear tires?

Don

[Longo]

Don i see it's not only our car having the same problem ! Bud stick with Don, he Knows his stuff!!

[LittleJ]

remapped the link 54" 4.5 high, tires between 8 1/2 to 9 so far, the rear is soft in compression and maybe still a little too loose in extension.

realized today my partner ordered non "S" tires so we have been running soft sidewalls the whole time.....could be part of it.

racing this saturday ill let you guys know how it goes.

[want-a-be]

Will be looking forward to hearing how things go.

The IC may be a bit low. If you don't like how things react, you may want to map out the same IC length but raising it up to about 6 inches or so. But where you have it ought to be a very good starting point.

Don

[LittleJ]

nothing but shake...at the launch,first gear, second gear, high gear. went from a pretty consistent 7.60- 7.50 car to complete garbage this weekend.

got some info on the tire my partner mounted Hoosier 33.0x15x15 (d05)....no tubes either!

thuroughly disgusted going to bed... any words of advice would be good

[want-a-be]

What all did you try last night?

Did you try raising the ic any at all?
Play with air pressure any?

Don

[LittleJ]

air pressure from 8,8 1/2, 9, 9 1/2 no change

never change IC height but we had run higher IC's with this set up and still shook

the shake got so bad at one point the tire reached into the back of the wheeltub and smashed the bellypan, with tire clearance i would estimate it pushed back about 5 inches.

[want-a-be]

How wide a rim are you running with those 15" wide tires?

don

[LittleJ]

15x15 beadlock

[want-a-be]

I have been told by some pretty talented people in the past that it's best to run a rim thats 2" narrower then the tire you're running. From what I've been told is it allows the side wall to do it's job better.

Don