Rear axle ride height?

[Cujo]

Hi, I am looking for a bit of advice please.

I have just bought a spare Rotax chassis. When swapping my running gear onto it, I have noticed that the rear axle has been raised by about 10mm - lowering the ride height of the kart.

I'm keen to try out new set-up's, but will need to re drill stay bars etc to get everything to fit. So before I do, can anyone tell me the likely effect that a lower rear ride height will have on handling?

Cheers

[RUSHY44]

Bearing in mind I stopped racing karts 13 years ago, IIRC Lowering the rear would give more rear end grip which in turn may promote understeer. I'm sure that in the wet I used to lower the front to have the opposite effect (give more front end grip)

With regards your seat mounts and seat stays, I used to (manufacturer reccommended) mount the seat a specific distance from the rear axle, so if your axle is now lower, mount the seat accordingly. Having said that, unless you have a flat bottomed seat you might not be able to go much lower.

Like I said it's been a while since I was kart racing, but I hope I may have helped.

[bendy]

I have not had a Rotax before so I cant help you regarding the fittings etc but in terms of lowering ride height at the rear, firstly I didnt know it was possible on a kart like a Rotax, and secondly, I dont think it's worth it. Firstly obviously if it's too low the bottom of the kart will ground out and not ride kerbs as well, secondly and more importantly, any kart like a Rotax needs to be driven will little rear end grip anyway. too much grip at the rear will start to make the kart hop when going round corners (speaking from my TKM days) and actually a little power slide is faster i.e. requiring less rear end grip.

Hope this mumbled rubbish helps a bit!

[Cujo]

Many thanks both

The chassis I have is prone to excessive oversteer, which is possibly why the previous owner has tried this (bearing hanger brackets have been drilled to accept a higher position).

I think you are right Bendy about not having enough ride height, although there are no signs of kerb damage underneath. I am fitting a brake disc protector to be legal for 2010, I think it's going to ground out quite a lot so I'm going to raise the ride height.

Rushy, did you used to race Minis?

[RUSHY44]

Originally Posted by Cujo

Rushy, did you used to race Minis?

I did after karting. How did you know that?

[Cujo]

Originally Posted by RUSHY44

I did after karting. How did you know that?

I used to race against you in MM back in the day, are you still racing?

The Rotax is good for fitness, but still trying to get my head round driving it, compared to a Mini - I suspect weight isn't helping me

I bumped into your dad last year when I was forming up for a race at Brands, he looked like he was enjoying himself.

[RUSHY44]

Now I'm really confused

I still race, got an MG ZR in the MG Trophy.

Rotax was only just starting when I was at the end of my Karting days. I remember that driving a Rotax compared to the ICA (KF2 I think now) that I raced was totally different.

If you're worried about weight, I was 6' and 13 stone when I was 15 lost out in the dry, but was awsome in the wet

I wouldn't mind having a go in one again as like you say it's definately good for keeping fit.

Was that a Classic Sports Car Club meeting by any chance?

We're at Brands on the 10th/11th April.

[Casper]

Originally Posted by Cujo

Many thanks both

The chassis I have is prone to excessive oversteer, which is possibly why the previous owner has tried this (bearing hanger brackets have been drilled to accept a higher position).

Oversteer off the corner or entry into the corner? All karts that oversteer off the corner are understeering into it. Get rid of the entry US and the exit OS will not happen.

[Cujo]

I only ever get understeer in the wet

In the dry, turn-in is neutral, then I get oversteer at the apex and exit, requiring a lot of opposite lock and very delicate application of the throttle to get off the corner with any momentum.
Lots of fun, but not particularly quick!
Fresh rubber on the rear helps reduce it a bit, as does widening the rear track. So I guess it sound more like a traction issue than balance?

[Gilles lives!]

Hi Cujo, you want to move the C of G to the rear of the kart to remove that o/steer. Do this by raising the front RH (stub axles down) and keeping the back low (axle up). We had a MS chassis that had terrible o/steer and this totally transformed it.
Try this and adjust rear grip by moving the rear hubs in and out, 3mm at a time until you find the perfect set up. You can also play around with the rear torsion bar too to add/remove grip, horizontal-less, vertical-more. Or move the seat back a bit, you shouldn't have any problems damaging the underneath unless you really do like those kerbs !

Don't forget that in the wet you will want high RH front and back and caster adjusted fully too, naturally rears in and fronts out.

[Cujo]

Originally Posted by Gilles lives!

Hi Cujo, you want to move the C of G to the rear of the kart to remove that o/steer. Do this by raising the front RH (stub axles down) and keeping the back low (axle up). We had a MS chassis that had terrible o/steer and this totally transformed it.
Try this and adjust rear grip by moving the rear hubs in and out, 3mm at a time until you find the perfect set up. You can also play around with the rear torsion bar too to add/remove grip, horizontal-less, vertical-more. Or move the seat back a bit, you shouldn't have any problems damaging the underneath unless you really do like those kerbs !

Don't forget that in the wet you will want high RH front and back and caster adjusted fully too, naturally rears in and fronts out.

That's very helpful, many thanks. I will certainly try all of this

[Gilles lives!]

Hopefully it will help you like it did us, we wasted nearly a whole season before altering the CofG and it transformed the kart totally.

I forgot to mention adding between 4-7mm tow out in the wet too, to give a little more front end bite

Good luck with your set up.

[bendy]

Can I just interrupt, when doing all this to remove the oversteer, be careful not to go too far, too much rear-end grip is the worst possible thing, especially in a kart like that. It'll start hopping at the rear losing way more time than a bit of power oversteer. A bit of controllable oversteer is the perfect balance that youre looking for. Moving the rears in and out is the best method I found with my TKM that I had. Also, what tyre pressures do you normally run?

[Gilles lives!]

Feel free to interrupt anytime bendy .

The chassis that we had the inherent oversteer with was a MS TKM, for this year we have a TALKO TKM, both are completely opposite to each other in their handling characteristics ! With the TALKO we have had to lose rear grip, due to it hopping and gripping up as you described, with the MS we NEVER had any rear grip until we adjusted front and rear ride heights as above. I was just explaining what did the trick for us.

Cujo needs a good days testing where he can try all these set ups to suit his kart, whatever his Rotax chassis maybe ?

[bendy]

I had a TALKO as well, awesome in the wet, but did offer too much grip in the dry, especially at the rear. But as I said before, having too much grip is far worse than having a little too little!

[Cujo]

Thanks both, yes I understand that it's important to test these tweaks out in one at a time. I'll post how I get on

The chassis is a Deavinson, who I think are no longer in business - from what I can find out, they are great at Rye House, but need a lot of tuning for other tracks