Tyre Shake

[trackday addict]

After a touch of advice really please.
We decided to fit wheelie bars onto the skyline recently after getting to the point where we felt the car was just getting to lively off the line & twisting to much causing us to lose time.

Ran we them fitted at the weekend just gone & horrendous tyre shake issues all weekend, mountains of changes made in terms of the clutch settings between runs with mixed results. The track was nowwhere near as good as it was at the main event so also to be taken into account.
We actually broke the bars on the last run in the final due to shake. With the smaller engined turbo cars you just have to try & drive through it or as soon as you get off the power & lose boost then basically no power there etc.

here's a slow mo vid of 1 of the runs
[URL=http://s7.photobucket.com/albums/y288/041068/Project%20GTST/?action=view&current=CIMG3457JohnBradshaw.flv][/URL]

As daft as it sounds without bars to date we have never had any bad shake at all (similar clip below from the main event)
http://www.youtube.com/watch?v=vyGttGdaFwQ

Running basically same power, just wondering why?, have we not had issues to date before due to the car "standing up on the rears" due to the nose coming up etc?

Huge thanks
John

Edited by trackday addict on Monday 22 June 22:23

[BennettRacing]

How low are they set? Can't view video at work...

[Flying Toilet]

Here is a working Link John:
http://s7.photobucket.com/albums/y288/041068/Proje...

No idea on what to suggest though!

[v8 jago]

Ive been told that the center of the wheels on the wheelie bars should be 4" of the floor. Is that correct or is it different for every car ??

[BennettRacing]

v8 jago said:

Ive been told that the center of the wheels on the wheelie bars should be 4" of the floor. Is that correct or is it different for every car ??

Different for all cars.

[veryoldfart]

notice the different timing/actions of the two rear tyres at 00.36 on this one...

http://s7.photobucket.com/albums/y288/041068/Proje...

almost looks like the rear axle is moving fore/aft under power too

Edited by veryoldfart on Tuesday 23 June 10:13

[trackday addict]

It's a strange one & just needs plenty of runs to dial in i guess, came as a real surprise as to date without the bars one of the cars major strengths has been how it gets off the line - ave 60 foots at jap events 1.18 - 1.26 & mainstream 1.12 - 1.16 due to the better track surface. At the weekend 1.28 - 1.46 !!

The surface wasnt great & you could see tyre smoke on some runs leaving the line, wee put loads of clutch in on some runs & took it out on others etc etc. Problem we have is the next Jap comp is only a 1 dayer & no time to test beforehand so major decision to make ie/ do we take them off again & go back to the original spring rates we used before etc etc?

Just feels like the car is leaving nice & straight with teh bars on but the tyre is not spinning & growing as it would do when the car goes nose up & sits down hard on them without the bars etc?

[Lee@LA]

the videos do show hell of alot whats going on, you got to get to work on the suspension John.

[PhilSweeney]

can't see your video, but sometimes bars unload the tyres if you hit them too hard or if they're too stiff. Talk to Andy Robinson on the bars and rear end set up, you're headed Pro Mod so he's the man in the know.

[peterwalters]

Tyre shake is a bugger...

[anonymous-user]

Looks very much like you're unloading the rear axle to me John. just as the bars hit the track you can visibly see the axle move upwards. Raising the bars will help but I think you'll need to adjust the suspension to suit the new setup, a longer and lower IC should help to keep the back end planted but you'll need to speak to Andy Robinson for inspiration on that one (I assume he has your four link plotted on his computer?). Explain the fault to him and I'm sure he'll point you in the right direction. I think it just needs a bit of a tweak to keep the tyres pressed in to the pavement as the bars "hit". We had a similar problem with the Camaro when we went to a very long and stiff Pro Mod style set of wheelie bars.

[KP1]

You can tighten the front shocks on rebound and/or move weight forward in the car to calm it down from the line. Dont have the wheeliebars to low. Try to find a good balance in the car. You can´t fix everything in the clutch.

There is a very good slowmotion movie on youtube made with a highspeed camera on Patrick Wikstroms tireshake. I cant open youtube right now and get the link but search on youtube with wikstromracing

[trackday addict]

Thanks everyone,
I think for now we just have no choice but to take them off & revert back to previous settings without bars as our next comp is in 3 weeks & I just don't have time to test beforehand.

After that then i think it's time to get them back on & have a real look at what's going on, i suspect like most of you it's a number of things added together ie/ revised suspension settings, revised clutch settings needed with the change to bars, track surface on the day etc etc. Certainly by the looks of the last weekend just gone not an overnight fix & really need a whole weekend or 2 to try & get closer to the answer.

I've got no doubt we need to go to bars to help drop the cars times further & become more consistent off the line, just surprised at how badly we suffered at the weekend to get shake to the degree we did etc.

[bananaman1]

this 1 maybe
http://www.youtube.com/watch?v=7lJnZbTFrcI

[PhilSweeney]

trackday addict said:

Thanks everyone,
I think for now we just have no choice but to take them off & revert back to previous settings without bars as our next comp is in 3 weeks & I just don't have time to test beforehand.

After that then i think it's time to get them back on & have a real look at what's going on, i suspect like most of you it's a number of things added together ie/ revised suspension settings, revised clutch settings needed with the change to bars, track surface on the day etc etc. Certainly by the looks of the last weekend just gone not an overnight fix & really need a whole weekend or 2 to try & get closer to the answer.

I've got no doubt we need to go to bars to help drop the cars times further & become more consistent off the line, just surprised at how badly we suffered at the weekend to get shake to the degree we did etc.

it's all relative, the bars need to be tuned in line with everything else. The height of the bar alone doesn't determine how well it works.

[PhilSweeney]

peterwalters said:

Tyre shake is a bugger...

I'll bet your running a shorty box hung off a mid plate solid mounted with a coupler ? Any flex in a moly frame turns solid mounted itens into stressed members. Doesn't take a lot to crack a bell housing. Assuming it's a glide, most will go for a JW Ultrabell or add a urethanc mount if using a full length box.

[Time Machine]

PhilSweeney said:

I'll bet your running a shorty box hung off a mid plate solid mounted with a coupler ? Any flex in a moly frame turns solid mounted itens into stressed members. Doesn't take a lot to crack a bell housing. Assuming it's a glide, most will go for a JW Ultrabell or add a urethanc mount if using a full length box.

We used to have this problem in the old Mustang/Mondeo Pro ET Funny Car we ran and that only had an injected big block Ford in it.

[KP1]

Cracking the bellhouse is sometimes a problem on streetcars with high horsepower.
Put in solid mounts on the engine and let the transmission mounting float can solve the problem. Or a stiffer chassi...