Hi, recently I took the plunge and bought a 2nd hand 05 32/32 Phoenix Stealth with an 08 Rotax. Set it up exactly as per Phoenix web site (Rotax heavy, I'm 80kg, 178cm) and went to Todd Rd yesterday. Had v.old MG reds on (hard but plenty of tread depth) and I ran it without restrictor (still waiting for one). I've never raced sprint karts but done a fair bit of motor sport a few years back. After a few 10 minutes sessions I was running at mid 44's but after that (about 20 laps done) rear tyre grip started to drop off and the kart really started to hop and bounce mid corner. Couldn't drive around it no matter how smooth I tried to drive, it just slid/gripped at corner exit. My question is can this generally be tuned out (widen rear track?) even at a minor expense to lap times (ie for some more practice) or have the rear tyres had it and I should toss them? I lowered rear tyre pressure from 15 to 14psi but no real help. It was pretty violent - do kevlar or carbon "hard" rib protectors really help? Any advice appreciated - sore today.
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Most of the advice I've had in regards to hopping is that you have too much grip. So widening rear track would be a good start.
With ribvests I prefer the tillett wraparound to the carbon Kevlar ones as it seems to give you more feel. If you get a proper fitting seat you shouldn't have any troubles with cracking ribs though soreness will usually be around after a big meet with plenty of rubber down whether you have a ribvest or not.
s on July 7, 2009 at 23:01
Hi adrian
Bounce is caused by a lack of grip, which is different from what people will tell you. You need to stiffen the back of the kart, by tightening the third bearing, seats stays, harder axle, or longer hubs. Having said this 20 laps is a long session.
Mg red tyres, I normally run the first seesion started at 13psi cold, at the end of the session check all tyres hot and drop them to 15psi.
Cheers
do NOT widen the rear track... the reason it hops is because metal can only flex so far... as soon as it gets to its limit it returns and slams the inside tyre back down and the process starts again.. flexing, returning, flexing returning (the hop)... widening the bak makes it flex even more... put longer hubs on and seat stays if u havnt already it will stop some flex
another posibility is too much castor which causes immediate rear jack... great for wet weather but depending on the kart castor in the dry can be bad for the rear end setup.. narrow front wheel track could also be a problem... the narrower the front the more u have to turn the wheel, the more u turn the wheel the more it makes the kart flex... so try widening the front aswell...
just some of my thoughts.
Josh Willett said:Most of the advice I've had in regards to hopping is that you have too much grip. So widening rear track would be a good start.With ribvests I prefer the tillett wraparound to the carbon Kevlar ones as it seems to give you more feel. If you get a proper fitting seat you shouldn't have any troubles with cracking ribs though soreness will usually be around after a big meet with plenty of rubber down whether you have a ribvest or not.
The problems you are describing mirror what I went through. Sounds like your tyres are ready for the bin too. As a beginner, I recommend you do a driver training day. Best money I spent in karting so far and will help you with your setup. Its hard to know which way to go when your new, so someone experienced watching and teaching you goes a long way.
I have had my fair share of rib problems since i started in january. I bought a free m carbon ribvest which did wanders. However time in the seat will cure that pain. You body just needs conditioning.
Hope this helps
I suggest that you and your club put the Restrictor in and we all may be able to continue with affordable (and available) insurance cover.
Thanks for all of the information! This seems to confirm what I suspected - a lack of rear end tyre grip is the most likely problem. Narrowing the rear track does make sense and I will try it (as do the other suggestions) but I suspect the main issue is the rear tyres have had it - they are old. The rear end is set to pretty much max siffness. Note also I was doing about 7 lap sessions - not 20 in one go. If anyone in Melbourne has a set of decent second hand MG Reds for some practice I might be interested. I will get a new set to race on. I would also be interested in a M or L size hard rib protector. Thanks all!
Note also I didn't realise I shouldn't practice without a restrictor, I am in the process of getting one and don't plan to use the kart again until then.
Adrian I'll be down at Todd Rd this Saturday, Come see me and I'll give you a set of half decent Reds ;)
I'm in a Tony Kart/Leopard # 98
That would be great - I'll try and get down there late morning. I dont mind paying if they are good. Thanks!
Adam Willison said:Adrian I'll be down at Todd Rd this Saturday, Come see me and I'll give you a set of half decent Reds ;)I'm in a Tony Kart/Leopard # 98