Proper tyres on stock rims?

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petew
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Proper tyres on stock rims?

Post by petew » Mon Oct 23, 2017 9:02 pm

Ok, so I know this is a tough one, but there's plenty of wisdom here.

Saturday's motorkhana was frought with constant understeer. We race on a wet skidpan. Even with 48/25psi front/rear, the car wasn't great. What I'd like to do is get some better tyres for the front at least. At the moment I'm using a chinese 165/80R15 on the stock 4.5" rim. If I went to a semi-slick of sorts would that help?

I found 165 semi-slicks here...
https://nankangtyres.com.au/tyre/ns-2r/

Like to hear people's thoughts. Yes I know I could go to a larger rim, but then I think I'd have clearance issues.

Thoughts?

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ChadH
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Re: Proper tyres on stock rims?

Post by ChadH » Tue Oct 24, 2017 10:32 am

I'm not totally familiar with motorkhana. I take it it's sort of like autocross, but a little slower and much tighter? Pivot cones and such ? If I'm understanding correctly, you want the car on the ragged edge of stability (barely streetable) to be able to throw it around the corners.

Not knowing what's already been done to the car, I'm just going to throw out some BS ideas.

It seems like staying with narrow tires is a good plan. They'll heat up a little faster, and I think cut through the wet track better. I don't think it's a good idea to mix and match tire types (not size) front to rear, since differences in available grip might make the car completely unmanageable.

If stockish, definitely remove the front swaybar. Run soft plain ol' oil shocks in front, and max out your front camber. Caster shims would help too. If the car is lowered, be sure that you have enough travel that you are not "bottoming out" the suspension as the car leans in corners. A little toe out on the rear too.

Look at your driving technique too. If you're still hard on the brakes during turn-in, the car will under steer. Get your speed down before turn in, but get the car turning while the front end is still loaded from braking.

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petew
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Re: Proper tyres on stock rims?

Post by petew » Tue Oct 24, 2017 3:25 pm

You've described pretty well
A. what motorkhana is
B. my setup

And a few years ago with the old wheezy motor...
http://www.youtube.com/watch?v=OAD675yTDlQ

Here's the car in action on Saturday...
http://www.youtube.com/watch?v=kGxbAzL-6ks

What I have now is a camber compensator hooked onto the axle tubes with U bolts, Spax adjustable rear shocks wound up reasonably hard, stock (for oval) 24mm rear torsion bars and everything in the back end of the car triangulated ala kafer bars. The rear is quite firm. The front is stock and quite soft. I have some plans for a rose jointed front end, but I'm not there yet. I'd also LOVE a Quaife torsen LSD for the rear, but my wife seems unimpressed with the AU$1500 price-tag for some reason...

https://shop.quaife.co.uk/volkswagen-be ... fferential

:shock:

What I'm experiencing is the front washing out pretty easily. It is slightly better than it was before, but even so. The "oh what a feeling" test is a good example of that. I get about half way up 2nd gear and it just runs out of front grip. Then is dances backwards and forwards between understeer and oversteer. However much steering input I add, it makes little difference. As I back off, it will suddenly grip.

Most of the rest of the field are using semi-slicks of some description.

Hence my desire for some new tyres if possible.

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ChadH
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Re: Proper tyres on stock rims?

Post by ChadH » Tue Oct 24, 2017 4:09 pm

That looks like fun. It's different enough from autocross, that I don't have any really useful advice. Especially with regard to tire selection in the wet. I almost wonder if the speed sensitive under-steer is from actual hydroplaning. It doesn't seem deep enough for that though.

The only other thing I can think it might be helpful if you can get some additional weight up front to load the tires and suspension better. I'm not sure how the rules are structured, but any kind of ballast that's allowable might help. I think the added front grip would save you more time than lost acceleration from the weight.

I'm not sure if this is an issue your having, but once the car begins to plow, continued steering input will only make things worse. It's hard to learn instinctively, but once you sense understeer, backing off on the steering input will help you regain front grip and actually turn better.

I found this on Youtube. Short car with lots of weight on the front. He's pivoting just inches from the cone. The stoppie at the end is the best part.

https://www.youtube.com/watch?v=QcglWsEco4g

So, yah, I'm eyeballing the quaife for my build too. It's probably not going to happen. :cry:

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petew
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Re: Proper tyres on stock rims?

Post by petew » Tue Oct 24, 2017 5:09 pm

Yeah, I was backing out steering input once it started understeering. I could add balast. One guy at the event on Saturday said he used to put a bag of cement in the front of his beetle. LOL :lol:

...Which is not a bad idea really. It also wasn't full of fuel, so I should probably do that next time too.

The UK pound is coming down in value. I should really petition the local authority again for a few years of Christmas and birthday. It was AU$2000 a while back, which shows how much their currency has dropped. Ironicly, the LSD is pretty well the same price as a good set of tyres ($1000) and rims ($450). Funny how we wouldn't think twice about tyres and rims but always baulk at a large 1 off thing that will last forever. :roll:

P.s. that mini is amazing, and represents the sort of competition I'm racing against in some ways. FWD, skilled, lots of grip.

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petew
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Re: Proper tyres on stock rims?

Post by petew » Tue Oct 24, 2017 8:15 pm

Just had an interesting chat with my wife...

Apparently I might be able to swap 5 years of bday and Christmas presents for the quaife... when we get the money. Which might be a better solution than tyres.

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ChadH
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Re: Proper tyres on stock rims?

Post by ChadH » Wed Oct 25, 2017 7:30 am

It's hard to see in the videos..are you not able to get power down to kick the back end out - just the inside wheel spinning? I'll bet the LSD would definitely help.

Years ago, I raced a '68 beetle in autocross. Even with a camber compensator + shimmed up z-bar, the inside tire would lift in corners. I couldn't get any power down until the car straightened out. I actually learned to break up the corners, and would straighten the car out mid corner. This would allow me to get some power down for an instant, so I wouldn't loose all speed. That probably wouldn't work on the super tight corners your dealing with though.

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Re: Proper tyres on stock rims?

Post by Piledriver » Wed Oct 25, 2017 12:11 pm

A long, long time ago...
It used to be common for AX cars to have custom 13" wheels or run the wider 14" Bus rims (still 5x205) with racing rubber.
This lowers the unsprung weight/rotating mass and the car and gearing for free, and you can still get superb used 13" racing rubber at least.
(formula fords etc---not so sure about 14")

This would be doable and highly effective without pawning all potential future goodies.
I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

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