jnywadd

Hello everyone, I have this 81 vette that I put a lot of time, money and effort in and now it is race car fast but I have this one problem. Not like you are supposed to do this but when you are traveling down a long stretch of road where nobody is around and get up to an excessive speed sometimes the front of the car will look like it is bouncing as the hood is flexing. The steering is rock solid and the car is smooth as silk but the hood looks like it is flexing sometimes during high speed and when you let off of it, problem goes away. New shocks, ball joints, tie rods, bushings, bearings and tires, and had it professionally aligned. Does anyone know how to fix this or has experienced the same things?

doorgunner

Rookie guess: Is your car supposed to have a rubber or thick foam seal between the top of the radiator and the hood? If it does not have a seal there the air through the grills could be going over the top of the radiator and is trying to "lift the hood".

ALSO......let's be clear that the front of the car isn't "FLOATING" at high speeds!

jb78L-82

You should change the title of this thread to hood flexing at high speed, NOT front end bounce.

jnywadd

You should if in fact that is exactly what is happening, it looks like the whole front end is flexing at high speed not just the hood

jnywadd

I don't think that the front end is floating but when you are driving it looks like the whole front of the car is flexing and then stops when you slow down. Sometimes a bump in the road or irregular surface gets it started especially if it is unusually windy.

jb78L-82

Did you replace the front springs during the rebuild and what spring rating are the springs that are installed? What shocks did you use? Hopefully, Bistein Heavy Duty/Sports or an equivalent high performance shock and NOT a mass market one...

SH-60B

Look at the bottom of the radiator core support. If the mounts are loose or rusted thru I suppose its possible for lots of body flex at high speed. Probably see it on a good bump too.

MelWff

How fast are you going when this problem occurs?
Has anything been changed to the nose of the car in the area under the bumper?

Rescue Rogers

Sounds like shocks to me

jnywadd

I put new kyb shocks on the back and the front but kept the original springs

jnywadd

85-90 usually is when it occurs and the only thing that I have changed is that I took off the spoiler on the bottom, could that be it?

jb78L-82

I had KYB Gas A just on my 78 and the car floated all over the place...not a fan of them...Bilsteins HD's in front with 550 springs and Bilstein Sports in the rear with a 360 monospring now...Car is very stable....

I would definitely look at those OEM springs and think about changing those KYB's out for some shocks that offer more performance.

mikem350

Couple other things:
wheel balance

cowl shake...also search "spreader bar"

bashcraft

So, it's only the hood, right?

jnywadd

It really looks like the front of the car is bending,and felexing when looking at the top of the hood on the driver's side if that makes any sense. The tires are brand new, balanced and have been aligned

bashcraft

OK, so the suspension is not shaking? If it was, you would certainly feel it in the steering wheel.

jnywadd

No, it almost feels like the front of the car is bending but the steering is absolutely smooth and solid with no vibration

7T1vette

A '78 C3 with 'stock' front springs is NOT "race ready".

You need to have a qualified mechanic do a thorough check out of the front end suspension and frame to make sure there is not something broken/loose/worn and causing problems. Not doing this and then driving the car at 90+ mph "could be hazardous to your health."

Do you have a front air dam mounted to the lower front valence (as intended by the car's design)? If not, that's another issue...

jnywadd

No I don't, the previous owner removed it but still have it. I should reach install it and see if it helps.

7T1vette

That's a good idea...PLUS it will deflect cooling air into the radiator from below, as was intended.