Kart Hopping on Exit after changing from 6" to 7.1" rims -

I’m currently running a emmick cobra. It’s a 98 but it was only slightly used back then. It’s a really fast chassis still. Still can put down fast lap times that the leaders are running(with harder compound tires). But now they’re switching to the really grippy Hoosier.

That’s now where my issue lays. With the 6’s I can get my kart to roll through the corners nice and fast. Just a little tweeking on wheelbase widths. I scrubbed out the side wall on one of my 6’s now I’m stuck with 7.10’s in the rear. I can book it through banked fast corners but when it comes to flat fast, slow and flat slow. My rear end starts to hop exiting the corner. I can enter corners no problem. But mid to exit. I’ll hop or bind up and push out of the corner.

I’ve tried narrowing the rear, and widening the front. Widening the front helped a bit, just not to the extent of solving the issue(can’t really go any wider in the front, I only have one more spacer.) with my harder compounds that I used to always run, I’ve never had to run any seat struts back to my bearing hangers.

My question is, if I add the seat struts. Will I get the lift on inside rear tire at corner exit as I need now? Also if there’s anything else that could help would be awesome. I don’t want to go buy a new set of 6’s until I burn these 7.10s. Also being able to find a way to run both and have a base chassis set up with both is what I’m shooting for.

What tire pressures? Have you tried higher? And yes, struts should help. They keep a more consistent pressure on the left and right rear in the corners. Widen the front, narrow the rear, does the same thing if you can get away with it. You don’t want to 2 wheel. Have you tried moving the seat forward? A little goes a long way! Have you scaled the kart?

Yes, I went up in air pressure also. Haven’t scaled the kart in awhile though. But I do need to. I’ll install the seat struts then scale the kart again. my chassis runs the best with 43% front and 57% rear as told by an old emmick engineer. I could try moving my seat up, I’m away from home and my log book is there so I don’t know off the top of my head the height and angle I set the seat at. I’m really wanting to try to find 3 different chassis set ups. Base line in the middle of both sets. Then a set up for the 6.0 and another for 7.10. Last night was the first night I ran the 7.10’s soooo much tire. Just need a rough idea on which direction to go towards for the 7.10’s. I’m pretty good with harder compound set ups. Just these super sticky tires are a whole new ball game to me.

I’ve heard that number of 43 – 57 before. This driver told me (1998) the kart, an Emmick, was pushing and the weight was set to Emmick preferred specifications. It might be a good place to start, but you adjust to where it will handle the best. Old karts, on the new tires, sometimes have problems. Tires today have a lot more traction than they did when I was racing. I had a similar problem with my Mayko Shark in 1979. I knew what to do to fix it, but I was no longer building karts, and soon, not racing them. I did race one time after that, on somebody else’s kart, and I still go to the races and consult whoever will listen to me. I’m apparently thought to be an ass hole. I’m also homely and funny looking so that doesn’t help.

Are you able to adjust rear ride height on the Emmick? If so, lowering could help you center off. If widening front helped, adding jacking, have you tried more caster?

Just a couple quick thoughts that may help

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Can’t later rear ride height. And the chassis come stock with 10 degrees of caster.

Hmmm, yeah the move to that rim changes things a lot. Track width and perhaps most importantly spring rate of the tire/sidewall, perhaps even ride height too.

It’s an easy thing to try and given that you have a hop, it’s worth trying to stabilize those oscillations whether they are coming from the chassis flexing more than before (perhaps from more overall grip) or from the characteristics of the tires on those rims.

I think I’d attack it by widening the rear until it stops hopping on exit. Then see what I can do to make the front work in harmony with that.
To tack on to what Don said about ride height…lowering the seat slightly would have a similar effect. Both widening the rear and/or lowering the center of gravity have similar effects in that they decrease lateral load transfer.

Okay cool. I just put a nek modular seat in yesterday so now I sit lower then before since my old one wasn’t a flat bottom and added a seat strut to each bearing hanger. Everyone at the track kept telling me to decrease rear track width. I’m going to go to the track this weekend and put some tuning time in. So I’ll try to work in the opposite direction if the new seat and struts didn’t change much.