13 replies to this topic

[joachimvanwing]

Dear all,

my problem is simple and conplex.

I'm driving in a Belgian kartingchampionship, "The new Freedom Challage" . We tuned our 4-stroke Honda GX200 up to 17BHP , and leading the championship after 6 of 14 races. Bridgestone YEQ are obligatory. And these tires are the [IMG]problem. They delivre so much grip. After an hour there is so much rubber on the track that the inside wheels come off the ground and we start taking corners on two wheels. We could go faster with less grip.

question: how can we deduce griplevels on my kart?

[MPea3]

i've never raced karts, but i raced quarter midgets with my older son for 8 years, and had to deal with ever-changing weights (at 5 his total weight including car, driver, etc, was 190 pounds, and by 12 it was about 260). we also had to deal with every conceivable surface, from concrete banked at 14 degrees to dead flat dirt. on concrete we often had the same problem you're having, the car would stick SO well it'd bind up in the corners on teh rubber that had been laid down. while quarter midgets have suspensions (i could dial out grip there), i also would spend a lot of time with tire settings. my guess is that you may want to try slightly higher air pressures, and a half an inch less on wheel rim width. not only might that de-grip you a little bit, it will also reduce your rolling resistance. i also assume you're balancing and shaving your tires round.... both of those actions can also help reduce rolling resistance.

*shrug* it's worth a try, anyway.

[GCrawshaw]

My suggestions are... when you come off the track take your tyre pressures first thing (IE while they are hot)... Then reduce them to the temp you think you really want at the end of the race. Its no good having your tyres coming "on" at the start of the race as they will go off when they heat up.

You should also take out any "4th" rail you have on the frame. Take the rear out 10mm at a time and maybe increase your front track. loosen your sidepod bolts until you can move them up and down (but won't come off!). also check with the chassis manufacturer about seat positioning.

Please note - go and do a test day and see how these affect your kart individually ie do one at a time and see how they help... no big changes unless you have heaps of patience and time! Try and test the day after a race meeting so you still have the rubber build up .... (unless it rains).

Hope that helps but you may have already done everything that I have suggested.

Good luck..

[Ross Stonefeld]

Id leave the tires alone and work on tube lengths and thickness. Or drive slower

[joachimvanwing]

dear GCrawshaw,

we have indeed tried all your suggestions already. Nevertheless, it's worth seeing these methodes confirmed. We lowered the seat, stiffened the rear-axle, increased the reartrack and reduced the front track (the wider, the less grip), so that the rear would slide more, but after some time we inside lifts.

We race on tarmac and concrete, and in fact, GCrawshaw, we learned that the stiffer the chassis is, the less grip it generates, that's why we placed a 4th and 5th rail, one at the front, one at rear.

Thank you for the tips.

Joachim

Ross, can you be more specific on 'tube lenght' and 'thickness', I don't understand what you're trying to say? tanks.

[100cc]

raise tyre pressures(also try decreasing and note the difference it makes),
widen out the rear track,
lower your ride-height/seat postition,
stiffen the kart,
loose weight ;) .

just some suggestions, they won't necessarily work=)

Do other karts run at the same track(with soft tyres)?

btw, the yeq is a hard tyre it shouldn't provide a lot of grip=). It has a shore rating of 56!!

Try something like YGA/YGB(or YEX , which is meant for cold temperatures and smooth surfaces, on high-power karts ) and you'll know what high-grip is.

I was running dunlop DAM's for the first time today(in a club race). Damn there was a lot of grip. I think these should be around the bridgestone YGB level, or a bit softer. Should've probably gone for the slightly harder DAH, because it was HOT.

[100cc]

Originally posted by joachimvanwing
we have indeed tried all your suggestions already. Nevertheless, it's worth seeing these methodes confirmed. We lowered the seat, stiffened the rear-axle, increased the reartrack and reduced the front track (the wider, the less grip), so that the rear would slide more, but after some time we inside lifts.
.

What kind of tyre pressures are you using??

[100cc]

oh and remember that the point is to lift the inside rear wheel. But not both inside wheels!

[joachimvanwing]

100cc,

we're driving fairly low tirepressures, around 900gr-1300gr, to decrease grip in a later stage of the race. In the first race half, only the rearinside come of the ground, once there is too much grip, we go around on two wheels.

joachim

[joachimvanwing]

100cc,

These Bridgestone YEQ are mandatory.

[100cc]

Originally posted by joachimvanwing
100cc,

These Bridgestone YEQ are mandatory.

yeah, I know=).

900-1300gr?? Is that like .9 - 1.3 bar??

if so, it sounds o-k, but you could still try going up dramatically(~1.6) and see the difference, also try going lower dramatically (~0.6), and see what it does. You'd think that if it happens only later in the race the tyres are overheating, so lower pressures should be right .

I only once had problems with two wheels coming off the ground, and that was in a narrow(small) kart(with a low-power engine), when I was already way too heavy for the kart. The problem was fixed with a small pressure change to the tyres.

One more tip, usually the only drivers I see going 2-wheel are either the very heavy drivers, or beginners, because their lines aren't really......that good.

Also on softer tyres a bounce is often cured by a small change to the driving line.

[100cc]

actually, your tyre pressures should be somewhere over 20psi(1.4bar)

[birdie]

Have you tried different seats? Try a plastic funkart seat, also the different Tilletts all have different properties.

I just had another bright idea. It might be rubbish though so be warned but do try it. Are your arms more or less stiff towards the end of the race? You might grip the wheel tighter at the start then relax a bit, or you might get tired and start to hang on for dear life. Try consciously keeping them the same stiffness if you can and seeing if that makes a difference. The main reason putting the seat back helps chassis tuning is that your arms will then be straighter and probably stiffer, and your body is in effect a torsion bar. If you think about it you probably weigh a large percentage of what your kart does so it stands to reason what you do has a large relationship with what your kart does. Terry Fullerton used to be able to steer by leaning for example.

We had this problem in a few weeks ago. Tom drives a Junior Rotax (or did, he's just moved to seniors) which has around 22hp and uses the same tyres as you. Chassis was a CRG Maximo. It did it all session, all day, so tyre pressures going up wasn't the problem. He is very fit, strong and experienced so it's 99% certain he was driving the same all session, and the only thing we can think of is that his arms were too relaxed. He's too tall so even with the seat back his arms are bent, and we can't put the seat back further without causing weight distribution problems. The only thing I can think of is getting him to keep his arms stiff while they are bent which might be harder.

I thought of this using a process of elimination. There's two things that change during the course of a race or test session (barring accidents) and those are the tyre pressures and the physical condition of the driver. If Tom's tyre pressures went up and it made no difference to his wheels lifting, that means it wasn't the tyre pressure being too high, and I already know it wasn't down to him getting tired. That suggests to me it's your fitness, and your arm strength in particular, that is causing the problem ;) If you are at Schumacher levels of fitness then I apologise, and do let me know so I can think of something else

So in summary, keep your arms straight and work on their strength. See if that does anything. If not, then it must be the tyres and you should fill them with either nitrogen or dry air (I heard NASCAR teams use Argon) so the water vapour doesn't expand and raise the pressures.

I was running dunlop DAM's for the first time today(in a club race). Damn there was a lot of grip. I think these should be around the bridgestone YGB level, or a bit softer. Should've probably gone for the slightly harder DAH, because it was HOT.

they are a bit softer. DAMs and YGAs are on the CIK FA tyre list, and YGBs are on the ICA list.

[Bex37]

Are your rear hubs long or short? If you are using short rear hubs (the end of the hub is almost level with the inside of the tyre), longer hubs will definitely reduce the tendancy to lift inside wheels. Also, if your kart has adjustable castor, try reducing the castor angle.

Having said that, I ran the YEQs for a few years and found that, on tracks with a lot of rubber down (at CIK rounds), the kart was actually fastest when the inside rear was lifting through corners. The trick is to have this lifting occur in a controlled fashion rather than a sloppy bouncing arrangement. Of course, if it lifts so far that you're worried about rolling its a bad situation as well.

If you actually get the kart to sit down on the track, you will probably find that as the grip increases during the day, your chassis will get slower off the turns. A lot of engine power gets consumed trying to fight rear axle twist whilst you're coming off the turn (i.e. still turning therefore inside wheel going slower than outside wheel but since you don't have a differential the tyres and axle are loaded up with torsional forces instead) instead of acceleration. Unloading the inside rear stops this from happening.

A lot depends on how stiff your chassis is. I am assuming above that you have got a chassis that's suited to your class and weight. What have you got? 30/32mm? 28/32mm? Whats the rear axle diameter? Is it soft, medium or hard? (and I am talking about the axle) Whats your class weight?