Hop: could someone explain what's happening

So I encountered my first severe hop in the heavy standard rentals I drove the other day. This was completely new to me in that the hop was severe enough to be painful.

The hop would occur if I tried to push the limits of the kart on corner exit only. If I asked a bit less, no problemo.

I haven’t felt this in any of the myriad other rental karts I have driven. The guys there said that it had to do with the tubing being so thick. It’s an unusual brand that isn’t widely used.

Wierd thing is that I wasn’t sliding at all. Just straight to what felt liked gripped up hopping.

What exactly is going on here? Feels like lateral load causes the tire to grip, then release, kart buckles, and this repeats until you have kart straight. Once I knew the limit, I could work around it. Just seemed wierd.

I was reading Greg’s complaint and it made me wonder if we are feeling the same thing. His caused by above average height and therefore weight. Mine caused by a really heavy kart with thick tubing and no flex?

Other than the hop, the karts handled well. You could drive around it. My pace was apparently just fine as I came in 38.5 which is a decent rental time. So I don’t think the kart was broken in that they all exhibited this.
If not mistaken, it was right turns only where you’d get hop, which coincides with the engine on right side of kart.

From https://kart-start.com/206-articles : Wheel Hoping Dynamics.pdf (344.6 KB)

Thx Morgan,

“In extreme cases of too much wheel lift, a wheel hop will occur. In this case the dynamic weight of the kart has transfer too much that it overcomes the grip of the outside rear wheel. This causes it to break traction. This forces the inside wheel to return to the ground. The outside wheel then regains traction and lifts the wheel back up in a hopping fashion. This problem can often occur in high grip situations and also with tall/heavy drivers that have considerable dynamic weight transfer. Ultimately, what you are looking for is the minimum unloading of the inside tire as possible that allows the kart to turn. Obtaining the proper unloading of the inside wheel is a balancing act between jacking obtained through steering geometry, chassis flex, and center of gravity.”

Ok so basically extreme weight (heavy kart, relatively light driver). Since it can’t be tuned on rentals, the next time this happens, I should try moving seat back a bit and shifting to outside of kart to turn.

I’m guessing that what they said about the frame being inflexible is the primary issue.

I see as a weight transfer issue along with a dampening issue, since it is a rapid cycle.
Since you can’t tune in a rental, I’d say moving the seat back should address both.
I would also think as to how to drive around the issue. Try to get the kart to do more before apex than after?? More steering earlier and Less steering later??
I’m still trying to figure out rentals - so grains of salt there. Next series has less practice (4 laps) and more laps in the final. I feel that helps vets more than new guys/ladies. But, the final is now a combo of heats 1 & 2 average finish.

Mike,Thanks for your response.

So I experienced the severe hop in first couple laps on the tighter right handers. It was pretty easy to drive around, however. In the case of these karts, rather than sliding a bit on right turns if you overdid entry, hop.

The answer was to be less aggressive at turn in. I didn’t have to go slower, really, just less demanding of the kart at apex. Different line, sorta.

I didn’t record my first session so the one that hurt my lower back was NOT subsequently repeated.

Yep. It’s not a light and flexible chassis so asking a ton at turn in like our normal karts was a non starter. There was a limit and one could not pass it. But, laptimes were fine.

@Mike_Clark Here’s a lap where I do 1 with no hop. I’ll see if I can find hop one so we can see how line differed.

Here’s a decent hop. I didn’t get any of the severe ones on film. But, it does seem that if I tried to ask too much at entry, at exit, it rebels.

Hey Dom. I think you are on the right track. Being Tall & Heavy, I found my hop was reduced by moving the seat forward. I think it helped shift some of the weight transferred to the front wheels reducing the inside rear lift on tight fast corners. In your case, being lighter I think moving the seat back a little can add some more bite to the outside rear wheel and keep it pinned to the ground. Rentals can be tricky and like Forest Gump says “You never know what you’re going to get”. Could be the kart you had for that run too. If the frame was tweaked, it may be more susceptible to turns in one direction compared to another.

Generally stiffer tubing will flex less making it harder to lift the inside rear causing a flat slide. I found shifting my weight around by leaning back under braking if locking up, forward on turn in if pushing and outside during cornering to increase grip if sliding or inside to reduce grip if hopping could compensate for lack of tuning available for just a plain beaten up rental. Seat position is always critical for getting a kart to work right. I would play with that first if this is common among all the rentals. If you can get the kart to work through most corners, then try shifting your weight for the ones it doesn’t.

It’s funny that you mention this moving around in the seat thing. You and @Bobby both brought this up. I just logged out of Sim karting having realized “Well, Duh. There’s no way to influence the kart with your body. That’s what’s missing.”

All those laps in the rental kart had me moving around in my seat, as usual, and to be honest, it hadn’t even occurred to me that we all do use our weight, sometimes poorly, to influence the karts ability to be productive.

So what you say makes great sense. And I will definitely be paying attention to weight transfer in the upcoming rental series. Thanks for the great suggestions.

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I’m far from an expert in kart fine tuning and am just a beginner when it comes to driving. But, I’m inclined to think that finding a driving solution would end up being a better option. Just going by how much understeer rental karts have, especially if the seat is not all the way forward.

Last year I was doing some rental kart racing. Moving the seat forward decreased hoping, but made laps slower. Lifting for a moment and getting the tight part of the turn done early, while leaving seat back, made the lap faster

That’s a good point. Lifting to rotate helps in the heavy karts. They don’t like being asked to do anything in a hurry. Very easy to overload. I did do some lifting, mainly partial, combined with making the turns a bit rounder.

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I run rentals a few times a week. Indoor in sodi rt8s on concrete. Kart setup is always a surprise. For tight karts that try to buck usually the answer is seat forward and body weight over the outside front tire from turn in to apex.

Late apexes help, especially delaying/slowly going to power to prevent hop. Reducing side load on the exit phase makes all the difference. A rental is very stiff. Lighter drivers experience hop opposite to a comp kart. Instead of too much lift, usually you get almost none. It’s a struggle to get them to jack properly and keep them up and balanced. Usually committing to power early just flattens the kart and leads to heavy bind/hop on exit unless the kart is headed mostly straight.

Thanks Zak. Makes sense. I was watching a pal race rentals at his track and I was noticing that he was getting lateral hop as well. Seems to be a feature of the heavier karts.