First of all I would like to say whats up, I'm new to the form(07 Z).

Ok here's the problem. Went to the strip tonight and ran a 15.4(yank yank) but when I took off the rear end bounced really bad all through 1st and I just spun all through 2nd. I know that a lot of it is just rookie mistakes of driving. As far as the bouncing goes will a new differential fix that or is that my driving also?

 

It's your driving. I assume this is your first manual car? The bouncing could be due to your dropping the clutch. Also, did you remember to turn traction control off?

 

I don't have a traction control button that I know of. I just have the base model. I've never had the problem before and this is my 4th manual. So your saying that the bouncing is from dumping the clutch?

 

What RPM are you launching at and do you have stock rims and tires?

 

I took off at idel I didn't up the rpms before I left the line and yes stock rims and tires.

 

If you are bouncing in 1st, it's called axle hop and it's really bad for IRS rearends. And you said your spinning all the way through 2nd?? My car hooks on the street in first and second with no wheel spin? Someone must have dumped some oil on the track or your tires are shot. Maybe you need to lay off the 100 shot off the line. LOL

 

I feel dumb for asking but....IRS rearend? Whats that?(independent rear suspension)

 

Welcome. You are experiencing wheel hop: the tires grab, slip, grab and slip setting up a serious vibration in the rear axle. It puts lots of stress on the rear axle.

My advise if you are experiencing wheel hop: back off the throttle, and abort the run.

 

Yes.

Great for autocross. Not so great for drag racing.

 

Try getting as much weight on your rear tires before burying the gas pedal. I get wheel hop occasionally when I dump the clutch too fast. I'm also on stock tires though. As far as spinning throughout second, that beats me. Shifting into second I usually get a nice chirp then quickly grip and accelerate nicely.

 

You had wheel hop because you didnt lower your PSI in the tires.

I ran 14.4 FIRST TIME in the Z last week ...with the stock PSI in the rear...i experienced the same as you through 1st.

I lowered the PSI to 24 and that was alot better, NO NO NO hop..just good ol' tire burning

I lowered the PSI to 24 and ran 13.811 @ 102


FYI..ihe lowere PSI was not the only reason i ran better times lol!

 

^^^^ I'll try that out if you say that it works.

 

It doesn't work.

The sidewalls are really stiff, so lowering pressure will decrease contact patch area, and make the tire more prone to wheel hop. He was probably more consious of what he was doing and got better as a driver because of placebo effect. I get MUCH better launches on stock tire pressures. I was actually mad, I got to the track and measured 32.5 psi cold last night and didn't have any way to get to 35, the cold nights dropped my tires a bit.

 

Since contact patch area is purely a function of weight on tire and psi in tire.
800#/35psi= 22.857 sq inches
If psi goes down the area has to change since the weight is fixed unless you have created some new math rules.

 

it does work. I lowered the PSI and raced 5 times and ZERO wheel hop ...when i raced with stock PSI ..wheel hop FTL.

 

While your math is correct, and your interpretation is correct, it is quite an outdated method - those rules apply to bias ply tires. The only bias ply tires we see on the roads nowadays are drag slicks (wrinkle walls), some highway truck tires (becoming less common) and the occasional vintage tire used on a classic car to keep it in "period."

The vast majority of tires nowadays are radials. Radials have strength in the sidewall, steel banding is now quite often even skewed a degree or 2 between the 2 or 3 plies that exist in the sidewall, adding strength without losing flexibility or adding weight. What this means, is that if you decrease the air pressure in the tire, the center portion of the tread gets decreased pressure - but the edges - near the sidewall actually get an increased pressure placed on them. They are loaded more because the sidewall is acting similar to the sidewall on a runflat tire. It doesn't let the tire squat down in an equal manner, so the center tends to lift away from the surface of the track.

Grip theory (I know....yawn right) says that contact patch size has nothing to do with the amount of tractile force produced. Only the normal force as a vector, and the coefficient of friction. Softer tire = more grip without any other changes. But tires tend to bond to asphalt on a molecular level - VHT increases this significantly, causing a measured increase in coefficient of friction, but instead of being related to normal force on the tire, its related to surface area on the track.

So - you decrease the tire pressure in the tire, the center of the tire lifts (notice how an underinflated tire wears) - the sidewall carries more of the load because its acting like a spring with the skewed steel bands in it - the shoulder of the tire carries more load - meaning less is to be carried by the main bits of tread - meaning less contact patch.

Less contact patch on VHT means less gained grip.

Now - wheel hop is a result of the tire gripping - then sliding causing the suspension to load and unload.

Having less grip could potentially prevent this, but I don't believe it, I get more hop in rain slicked roads than anywhere I drive. My bet is still on the placebo effect. The only solid way to eliminate wheel hop is through suspenion modification, the easiest being installing drag bags inside the rear spring coils so that at the strip you can DRASTICALLY increase the spring rate and prevent the tires from loading the suspension and thus prevent the wheel hop.

If it works for you - great, but you look at the top 3 stock times (and possibly more), we ALL are running stock tire pressures with great results. I don't think that we all have it wrong.

 

If you like the stock psi(this might be a weird question) then how do you lauch off the line?

peak350 13.2
350zpk 13.8

Those I think are some good times for a stock 350 or for the most part stock. Teach this grasshopper how to drive on the track!

 

My launch typically (on a good run) is from 5,000 RPM's.

I fully stage as fast as possible, gives me more time to prepare so I can focus on the run and not feel rushed. Pick the RPM's up to around 5,000, hold the clutch so the car almost creeps forward.

When the third yellow hits let the clutch out, fast but smoothly. The key here is to let it out a lot slower at first and then add throttle and let it out faster. You want to get weight transfer and once you have that you can put down more power.

To get a feel for this do it at a MUCH lower RPM. Slip the clutch from 1,500 or 2,000 RPM's. Get used to the feeling of the front of the car seeming to lift up as you're moving away from the staging line. I can't always launch near this hard. When I had a 13.351 as my best it was partially becuase I bogged right after the 60' mark more than I would have liked to (slick track and I couldn't hold more than 4,000 RPM).

The clutch issue introduces a whole other aspect. You can't slip for too long or you'll have to go partial throttle the whole track and just lose the run.

 

Sorry to let you know this, but lowering the PSI does work on any car. FWD or RWD. IT allows the wheel hop to stop and also gets you a little bit better traction off the line. Don't forget once you take off the line, what does the tire do? It spreads out and puts all the rubber down when all the weight is transfered on to them.

 

Peak, did you leave all your stuff in the car or did you remove some items?