Sportsman kart hopping issue

patrick slattery · Posted: Tue Jul 03, 2012 12:38 pm Post subject: Sportsman kart hopping issue

Kart handles fine, than late in day starts to hop in fast tight turns.

PKT 30mm 21/1060 axle with shortest hub all the way to the axle. Full caster with bottom pills, and 2mm total negative. 15 lbs of air all day long with the YHC tires, everything seemed good but the last time out started the hopping. What do you think, harder axle, wider/narrow in the front. Open to suggestions please. This is a Yamaha Sportsman, 250 lbs and a fairly fast driver.

Thanks

Pat

TJ Koyen · Joined: 03 Oct 2004 Posts: 1425

I know sportsman karts sometimes setup different than typical karts, but I'd start by taking some of that caster out and widening the rear.

If you've got full caster and the rear all narrowed up, it's going to hop, on almost any kart.
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JT Miles · Posted: Tue Jul 03, 2012 3:19 pm Post subject:

TJ is correct...Start with widen the rear then tune from there...
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patrick slattery · Posted: Tue Jul 03, 2012 6:45 pm Post subject:

Maybe I didn't state it correctly, the kart is as wide in the rear as the axle will allow with the short hubs. 1060 axle PKT A-21, also have the Douglas spun 5.5 on the rear

Gary Lawson · Joined: 21 Aug 2001 Posts: 542

less caster and less air. closer to 10

JIM SILVERHEELS · Posted: Wed Jul 04, 2012 6:24 pm Post subject: WHY

Why would taking caster out making the inside rear tire sit down faster loosen the rear? Wouldn't less pressure would mean more grip to a point? thanks
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Gary Lawson · Joined: 21 Aug 2001 Posts: 542

Less air is less grip unless you run on dirt.

Low hp sportsman karts do not like running a lot of caster typically. Less caster will decrease weight transfer and settle the kart down.

TJ Koyen · Joined: 03 Oct 2004 Posts: 1425

The hopping is probably from overloading the outside rear, to fix that, you take caster out.
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Dave Boles · Posted: Mon Jul 09, 2012 12:16 pm Post subject:

If you have side seat mounts loosen them. It will free the kart up in tight turns, giving the frame more flex.

Ihor Bilyy · Posted: Mon Jul 09, 2012 5:03 pm Post subject:

I'm with TJ, more caster - more load on outside rear initially. Soft axle will bend back and force to accommodate load, producing bouncing.
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Rick Sweeting · Posted: Tue Jul 10, 2012 5:10 pm Post subject:

What chassis? How much air pressure build up?
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Anthony Burke · Joined: 19 Mar 2011 Posts: 22

Patrick, as the other guys have already mentioned, the hopping most probably comes from a tyre that's overloaded.
Is it possible that the seat needs to be lowered a little? We might be talking as little as a 4mm change. If you're already making "too much grip" at the widest setting you may need to think about lowering it. This would allow you to run the rear track slightly narrower and still have the option of "removing grip" when the track gets crazy sticky. Of course, you could just lower the chassis instead, although this might produce other issues.

Mike Unger · Posted: Thu Jul 19, 2012 4:44 pm Post subject:

When Noah ran sportsman we ran softer axles in the hot sticky summer races. Remove the seat struts if you have them on too.