Launch Technique : wheel-hop

[belize1334]

So I managed to squeak in two autocross events this month. The car ran well and was competitive with STIs despite the meager 10psi I was running. But, every run was ushered in by wheel hop off the line. I'm running a 1G 1st gear and 4-spider diff so I'm not too worried about the transmission, but I'm thinking that for the sake of longevity this should be addressed. I'm not dumping it by any means but neither am I being overly gentle. So, I'd appreciate any comments on technique and or parts solutions for reducing hop. So far I'm thinking of some combination of the following...

1) Poly motor mounts. I currently have window-weld filled stock mounts... thinking maybe poly for front and both sides and leave the rear oem for a little less cab vibration.

2) Launch it harder. Maybe I could replace "wheel hop" with "wheel spin".

3) Launch it a little easier. Hard to do with a 6-puck but I'll give it a try.

4) Corner weight the suspension. I've got coil-overs that have never been "tuned" so it may be that my left-front is just unloaded and a bit more weight would keep it on the ground.

Feel free to comment on the above or offer your own alternatives.

 

[Street Surgeon]

What coilovers, what's your spring rates, do you have any other suspension mods (to include bushings etc) and whatnot? What RPM are you launching at, do you slip or sidestep, is your six puck sprung or solid hub and what clutch etc.

 

[belize1334]

BC Racing BR coilovers: 440:330 lb/in
Toe = 0*, Camber = -2*
WL front control arm bushings
~5k rpm (no launch control... just free rev)
Slipping the clutch. I never dump it.
Sprung 6-puck disk with Clutchnet Red 2x (~2200 lb)

It's also possible that it's actually clutch chatter and not wheel hop, but I doubt it. I've never had clutch chatter set off the windshield wipers before.

 

[3rdstrikedsm]

What rpm do you launch at?

The bushings in gthe rear diff and suspension play a major role in this.

 

[Street Surgeon]

Yes, yes interesting.... Tires?

 

dial in the suspension with dampening adjustments. do a cross wt if u can.

do you know which wheel your getting the hop from?

 

[belize1334]

Tires are Pilot Sport Cup. They're a track tire that never gets warm enough at autocross so they basically act like a high performance summer tire. 225/50/16 so kind-of a beefy sidewall.

For the back I'm running a 3-bolt LSD but it has a fairly loose pinion gear so I'll be swapping it out soon. Bushings are mostly stock in the back except for the upper ctrl-arm which as a WL camber bushing.

Subframe bushings are OEM all the way around.

Like I said I launch it at about 5k. That way the clutch hooks up at about 3k with full-boost and I'm off and running!

 

[3rdstrikedsm]

I always set my 2 step @ 6700 rpm to avoid wheel hop and the bog.

 

[belize1334]

So you think just launching it at a higher rpm will make it hook up at a higher speed and smooth it out?

 

[desant78]

I think a very important point is what tire is losing grip. I would assume its the front...and your losing grip as the weight pitches back. Perhaps increasing the rear dampening would offset this.

 

[belize1334]

Ultimately the damping will be set according to handling characteristics so for my purposes that nob is off limits. I agree that it could help. Maybe more front damping would keep the wheel from "bouncing" but I don't want to make sacrifices in that region. I suspect the front right wheel but I'll have to actually get somebody watching to be sure.

 

[desant78]

also has your suspension been tuned? Typically for a neutral car, the spring stiffness is opposite the weight bias of the car.

So doing a quick Google search a dsm has a 54/46...i don't know how accurate this is but well go with it. A neutral car tends to have a spring stiffness opposite of the weight bias of the car....so in this case it would be 46/54 ratio, or an example is 368/432, this improves handling by evenly distributing the car's lateral load transfer during cornering.

To have a spring stiffness like you have, 440/330....would result in a car that tends to under steer. This is just an fyi and you might already know this. I by no means know how your car should be set up...and I am not saying that your set up is wrong. Suspensions of course like everything else requires tuning since every car is different, and more important every driver is different.

sorry if I am going very off topic with this post..but looking at your spring rates the front being stiff compared the rear does not help if the front tires are hopping, looks like you could increase the stiffness in the rear to help with front wheel hop...assuming again the front is hopping.

 

[vr4play]

I used to get horrible wheel hop when I was auto-x launching but since I did all of the bushings and motor mounts I get none. I have poly control arm bushings, subframe bushings, window weld in the drive shaft bearing carriers and the rear active toe eliminator kit. Before the rear subframe bushings I used to get wheel hop out of the rear. The motor mounts and control arm bushings helped the most for the front. I launch at 6k briefly off the two step. If I let it stay on the two step to long it just smokes the tires /ubbthreads/images/graemlins/grin.gif. If I don't use the two step it kind of bogs.

 

[belize1334]

Quoting desant78:

So doing a quick Google search a dsm has a 54/46...i don't know how accurate this is but well go with it. A neutral car tends to have a spring stiffness opposite of the weight bias of the car....so in this case it would be 46/54 ratio, or an example is 368/432, this improves handling by evenly distributing the car's lateral load transfer during cornering.




I'm going to have to disagree here 100%. Consider the extreme case of a car with like 70/30 weight distribution. Your logic would suggest that the front springs should be less then half as stiff as the rear despite carrying more than twice the load. I challange you to find a high performance suspension system for a front engine car that has stiffer springs in the back. It's just not gonna happen. For a balanced car what you want is the same natural frequency front and back. That means that you're going to try to keep the ratio of weight to spring-rate fixed (modulo the motion ratio of the suspension geometry).

I will be getting the suspension tuned to try to even out the corner loads and hopefully that will help. I'll also be replacing the motor mounts with something a little stiffer and I'll try launching from a higher rpm and see if that smooths things out.

 

[vr4play]

I had to run stiffer springs in the back than in the front to get the car to handle properly. Its all about preference. I like a car that doesn't push. AWD cars naturally want to push so to compensate you run stiffer rear springs. BMW's also run stiffer strings in the rear than in the front. The stiffer strings also let you run a smaller rear bar which will help keep the inside rear tire on the ground. A lot of the BMW race cars that we do have like 800-1000 lb springs in the rear with 550-650 in the front and they handle like a MoFo.

 

[desant78]

Im heading to bed so I can't make this long. First off I want to stress that I like a healthy debate, I am no way telling you what to do or saying I know everything. Only trying to contribute to the forum.

Corvettes..vipers..etc all have much stiff springs in the rear. I found a few sources suggest a 600/900 split...but aren't so credible (forums) so that is not my main point.

In a turn..once in a steady state phase (to simplify the model) as you a front engine car will have the front tires do a lot of work to get the car to turn..the rear not so much due to lack of engine. The body wants to roll..putting more stress again on the front tires...known as lateral load transfer. This will cause a car to understeer. By having stiffer springs in the rear...the back tires now do more in the turn to keep the car flat...removing some load from the front. This evenly spreads on the front and rear tires...front. has less lateral load to deal with but more weight (engine) and rear has less weight to move..but more lot to handle.

If a large enough load is felt in front tires..they slip and underzteer will occur. The suspension is set up as I suggest to take the load.off the front to back...so the car tires up front can grip more...improving theoretical g's in the corner since you are ideally maxing out all four tires not just the.front.

All this on my phone sorry if it doesn't make sense tomorrpw I will edit if it is garbage.

Thanks and good luck with everything!

 

Roger,

I believe you are heading in the right direction with this. Mounts and bushings are going to be the main reason that a car will develop wheel hop under power. More specifically, excessive flex between certain components causes it.

Rubber is a spring. It has compression and rebound characteristics.

Lets look at a FWD car. When you launch it, most of the power will be transfered to the wheels, but some of that power will be applied to rolling the engine/trans opposite of the wheels. The rubber in the mounts will compress, and then rebound, and then compress, and the rebound. This causes the input of power to the wheels to flucuate, albeit only slightly, but at a high frequency. Which in turn causes the tire with the least amount of grip to spin momentarily. Then it will regrip, then spin again. The loading and unloading of that wheel gets transfered to the chassis, resulting in wheel hop. The same is applied through the bushings in the control arms and subframe, but the effect is less dramatic.

A good launching car doesn't have to have stiff springs, but it needs good solid bushings and mounts. Weight transfer is more a negative in regards to trap speeds and 60' times than it is to a shake free launch.

Hope that makes sense.

 

[Brianawd]

Higher Launch rpm.