OTK Kart hopping / bouncing rear

OTK 2018 MODEL CHASSIS HAS DONE ABOUT 10 - 12 CLUB MEETS PLUS PRACTISE IN BETWEEN
Running on Dunlop DFMs, Hard Axle (OTK H) 1400mm width. Perhaps I could have tried HD Axle in those conditions.

Yesterday at the track we had a lot of classes / karts running and as a result the track gripped up. Air temp was also quite warm, about 28 - 30 celcius.

I had issues with the rear of the kart hopping on a couple of corners when I tried to get on the gas.
Recently the kart has been really good on this setup, then all of a sudden in the 2 afternoon races I lost 3 tenths compared to my competitors.

Any advice on whether this could be setup, driving style in those conditions or chassis getting a bit on the old side?
I also trialled running the flat front bar in the vertical position in the afternoon sessions to stiffen the kart up.

Hey Daniel,

Combination of driving (can affect the bouncing) and setup.

Try a shorter rear axle that should help with release, and squeeze the power on later. So carry more speed on entry so you’re not vulnerable.

That should help.

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I’m no expert in set up but as far as I know it’s normal when the track grips up. You need to flatten the rear (less lifting of inside wheel) to make maximum use of the grip pier surface

Thanks Dave, this was at Todd rd yesterday, mainly noticed the hopping through the double right. Just reviewed my data and it certainly could have been my driving in the final.

With the shorter axle, do you mean 1000mm instead of 1030mm?

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Too much caster in low speed turns can cause hopping .

Higher tire pressures!! Better Seat bracing! Wider rear track.

In that order!

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On the non-tuning front, hopping is from asking too much of the kart laterally at turn in. It’s particularly pronounced in heavy, caged rental karts. The lighter the kart, the less obvious.

The answer to hop in a rental at least is to make the corner rounder. Less V into apex, more C.

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Echoing some of the suggestions here with regards to having too much caster and possibly going to higher rear pressures. Sounds like you’re already at max rear width, otherwise that would be my first shot.

Basically what’s happening is you are loading up the chassis too much at max load, like Dom said. That means too much weight transfer is happening. To reduce weight transfer, I would start by reducing caster, as caster can have the double-benefit of also reducing front scrub and helping the kart roll off the corner better.

I would also take a look at either lowering the seat or lowering rear ride height to get your center of gravity lower. The ride height is something I play with pretty often on the OTK, though I’m usually raising it because of my size.

The H axle is the right direction. I don’t find much difference between the H and the HD or HH. Those are worth a shot if you want to go stiffer, but I don’t think it will make as big of a difference.

The front bar vertical is a mistake in my opinion. You’re correct in your thinking that you are stiffening the kart and perhaps limiting flex in the rear, but you’re also adding a bunch of front weight-jacking into the kart. Always try and picture the chassis as a spring. When you have a hop, you are overloading that spring, and it’s snapping back and oscillating, causing it to load up, release, load up, release, repeat. When you stiffen the front bar, what you’re really doing is stiffening the “spring” of the frame and then, with the additional weight-jacking, increasing the force with with you load that spring, so it could actually make the kart hop worse if you stiffen the front bar.

Reduce caster would be my first shot. You can go negative on the top pill and keep the bottom the neutral pill, but I probably wouldn’t go full negative, with an eccentric in the bottom too. If you go to the half-negative and it’s still a little hoppy, I would tweak something else too to fine tune it. I don’t like to max out one adjustment in search of a fix if I can avoid it, because once you get to an extreme like that, the possible side effects of that adjustment can start to rear their heads.

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Thanks for your detailed reply TJ!! Fortunately the track isn’t always that grippy, but when it is like that again I will try your suggestions.

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Moving seat is a big one and can fix issue if caster isn’t doing the trick.

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What is your Body type? Height and Weight? I am tall and relatively heavy compared to most drivers. I have broad shoulders, so a lot of my upper body weight is above the seat. This adds a lot of weight transfer when cornering. I used to get a really bad hop at mid-corner to exit. Like TJ said, a stiffer axle will help and then play with caster. Taking caster out (top forward) will reduce the jacking affect. Combined with the softer front end (bar horizontal) it will reduce inside rear lift and let is set down slower. That should improve the hop. Also, like Dom said, rounding the corner (more C less V) will help with preventing an overloading of the outside rear causing it to break traction. Watch TJ’s video on Staying Under the Rubber. Could be as the track grips up, you are getting outside of the traction zone and the rear doesn’t stick as well.

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We had the same issue on same corner during Saturday practice there. Hard axle and slightly wider rear solved it but your front bar run vertical will make it worse as TJ mentioned.

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