I may have missed it above but you can also try short rear hubs. I’ve used this with good results where some of the usual changes haven’t been enough.
Do short rear hubs stiffen the rear? I’d have assumed it was the opposite?
I expect it allows more axle flex and can’t really explain why this works. All I can say is I have used this with both hard and medium axles when the other tools mentioned above have not fully resolved the issue. FYI I have an OTK kart. Maybe someone else can shed more light on the theory.
I try to think in terms of resonance and damping. On one hand a shorter hub might seem counterintuitive, but because of how it changes the damping properties it might work.
One way to think about it is whether the bouncing is happening because the rear is being lifted up and dropped down too harshly due to caster.
It’s hard to say without being there, but I think I’d dial some caster out, see how it responds. Then decide to dial more out, or leave it as is and look at other things next.
No you want longer hubs. That will have the same effect as a stiffer axle.
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Thanks for that Dennis! 
Hey Liam, I have run this chassis (see name tag photo) and will let you in on what I know.
There are 2 modes of releasing a kart in a corner. One is mechanical jack the other is centrifugal (dynamic) load in the seat. Both can give you the objective of releasing the kart. This is why some say go soft and some say go stiff when the grip comes up.
The kart you have is very torsionally stiff and is prone for the rear gripping up. Adding third bearing will make the kart more reactive on entry and mid corner, but as TJ has alluded it will come up faster and go down faster, which can manifest as a hop.
If you go the stiff route you will have a kart that is heavy in the steering on entry and mid corner as you will be holding the inside wheel up mechanically with the steering wheel. This requires very smooth inputs and no wheel correction mid corner to be effective, as any steering wheel unloading will cause the wheel to drop and kart to bog.
If you go the full soft route you are relying on the kart to twist in the waist to release the rear wheel and the side load in the seat to hold it up. This method is easier to drive as it is less sensitive to wheel inputs. And of course you can have any permutation in between.
I would try
short hubs = better mid corner and exit release
short axle = 1010mm same as above.
remove the front bar (may be too much, but will be easier to steer and less sensitive to inputs)
rear tyre pressures are big on the DFH tyre. Run 9psi cold at this time of year (summer here) don’t worry as the side wall will do more work it will still get to the 14psi optimum.
just remember everyone’s points here are valid and they will argue in the results. But always break changes down in to mechanical and dynamic as this is what confuses people the most. Then you will see a pattern of advice from people “change X and Y will happen” and then you will be able to group them into the 2 categories and make more sense of them.
sorry for the rambling.
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Thanks so much Marin!
Can I also PM you with further questions?
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Sure thing…
Some follow-up questions for you all. Please excuse the length of this post…trying to cover all questions at once:
Would there ever be situations in which a softer axle on one brand of kart would have the opposite handling effect of a harder axle on a different brand of kart? Seems like it would require a lot of trial/error and working with one particular chassis brand in order to figure this out. Most recently I’ve been racing a brand called “Solo Kart” that seems relatively new.
Most of my recent tips are coming from the team at Oakland Valley Race Park (“OVRP”) (in upstate, NY if you are familiar), and specifically Tim Hannen, Joe Fredericks, and Chris Rock. All have decades of experience tuning karts, so I generally trust their advice when I’m stumped myself. Most recently, I asked Tim how he went about “freeing up” the rear end, to cure my persistent hopping condition. His advice included a number of the things you mentioned (e.g. lower rear ride height, less front-end caster), but notably he also had some conflicting advice, such as using a softer rear axle, unbolting the third bearing (using zip ties to hold it in place), and “widening” the front end. I simply took his word for it, since that weekend I was scratching my head trying to decide what to adjust. We never ended up changing the axle, but made virtually every other change Tim suggested.
Also what helps is not applying power while the steering wheel is turned. This is especially helpful with shifters.
Do you care to elaborate? In the past I’ve had lots of success with applying power while still cornering, especially when trying to get a fast exit down the next straightaway. There must be a way to setup the kart so that power + cornering doesn’t cause it to hop, but rather just break traction if you give TOO much gas (similar to what would happen in a racecar). Also, in some corners like OVRP’s “box” turn, a little maintenance power throughout the corner feels like it helps balance the kart.
I can’t remember ever having too much grip. All I can remember is wanting more.
I can definitely relate to this! The question is, how can we make a kart go around corners faster, but without the darn hopping? For reference, I’ve been racing in Tag Masters (running a Rotax 125cc), and using MG Red tires. But MG Yellows are also permitted for this class, and rumor has it they are a LOT faster. I’ve often seen the Senior (younger kids) class drivers pass me on MG Yellows, and they are SO much quicker in the corners that I can’t see any reason why you wouldn’t run MG Yellows, assuming you could afford the more frequent tire changes…
What tire pressures? Have you tried more pressure? There have been a couple of occasions where we were getting that bounce and raising the pressure cured it.
Here’s another situation where I get conflicting advice. Chris Rock at OVRP told me to aim for 10 psi “hot” pressures on all four tires, meaning you should set them to whatever cold pressure gets you to 10 psi hot (that could be 9, 8, or even asymmetrical depending on the track). I almost never mess with tire pressures, because I agree that tires probably have an “optimal” pressure based on the amount of flex the manufacturer intended the tire to have. That being said, Chris and others told me to go lower on the tire pressures to fix my hopping condition. The logic here was that lower tire pressures would heat up the tires more slowly throughout a session, and so the hopping might start around lap 10-12 instead of lap 6-8. This didn’t cure my problem, but it definitely delayed it and made the kart faster earlier in the run. I’m curious to know why higher pressures could also cure a hop…
… bolting in my third bearing would be a good idea to help tune out some bounce in my kart.
Here is another area where the guys at OVRP have different opinions. When I kept coming into the garage trying to fix my hopping condition, one of the changes suggested to me (among other things, like a softer axle) was actually to unbolt the third bearing and use zip ties instead on it. We tried this, and it did seem to help, but the difference was barely noticeable.
LASTLY…
On the topic of seat struts , what experience have people had using these? Adding seat struts (I was told that weekend at OVRP) should cure my hopping condition. Indeed, after we added one seat strut on each side, that single change made the biggest difference in my kart’s handling (for the better). But I wasn’t sure WHY. I would think seat struts should stiffen the rear end, producing opposite effects from other changes we made (incl. unbolting the third bearing). But for some reason, they worked well, and by the feature race on Sunday I had one of the quickest karts in the corners. Additionally, I should mention that other drivers in my class actually ran FOUR seat struts that day (two on each side). Why would they do this?
Overall, I seem to have a hopping problem more often than not, even on colder weekends. On the relatively hot weekend I describe above, and before using seat struts, the hop was so bad that my front wheels would actually hop too, preventing me from getting on the gas early in corners and getting a clean exit. What can I do in the future to prevent this, and ideally find my setup sooner?
@Marin_Vujcich You sound very experienced setting up karts as well. Can you weigh in any ideas?
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Hey Darrick, where in the corner is your hop?
Mid corner or exit?
Yes. A different kart may have a different frame material or design, so might require a different axle to fix the problem. As I noted in the first post as well, one brand’s “medium” axle might also be totally different than another brand’s too, so it might not be an apples to apple comparison when talking two different chassis. The guys you listed know that track better than I do, so I would listen to them if they say soften the rear. I can only speak from my own experience. The only way to really figure this stuff out is to experiment and test things.
For seat struts, you basically are going to want to run at least one per side almost always. Two per side doesn’t make a huge difference, but can help increase the weight transfer from the seat.
Seat struts make the transfer of weight from your upper body to the axle more direct. They help pull up on the inside tire and push down on the outside tire.
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It happens mid-corner through the exit (but can also happen on entry if I try to carry lots of speed). It prevents me from getting the power down because the hopping jostles my body around so much that it’s a physical challenge to keep the wheel straight and the power application smooth. I can also get the sense that the hopping bogs the engine down significantly, even if I do manage to get the power on.
Interesting to note about the seat struts increasing weight transfer from one rear tire to another. This makes sense to me. Perhaps my original hopping problem was caused by not enough rear weight transfer rather than too much?
Also on that note, I wonder if there are ever situations where seat struts wouldn’t be desired at all? Otherwise I could see kart manufacturers building every chassis with seat struts by default.
Kind of tough with so little information. Weights, tires, pressures etc. etc.
Not enough weight transfer almost always results in the kart being totally flat and not lifting at all, or lifting very little on turn-in but immediately settling down, not inducing a hop.
Breaking it down to its simplest form, you’re trying to get enough inside rear wheel lift to get the kart to rotate when you turn the wheel. Too little, and the kart won’t lift and will understeer. Too much, and the kart overloads the outside rear tire and hops or slides. All you’re doing is tuning the stiffness of the rear and the weight jacking capability of the front to tune the rate and amount of inside rear wheel lift you are getting. You’re looking for that sweet spot.
Without seat struts, you’ll have a softer relationship between the seat and the bearing hangers, effectively having a more flexible center of the rear of the kart. This additional flex can help the inside rear come up easier sometimes, but it also reduces how much force you put on the outside tire, so that tire’s traction limit changes a bit. What we’ve seen without seat struts is a kart that lifts, but can’t hold the lift due to lack of outside rear tire grip, so it slides or hops depending on track grip.
Hard to build a kart with seat struts already on, as they need to be removable so you can place them where they fit for different seat positions etc. Plus the exhaust configurations for different motors would require a different strut to fit around the pipe.
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Why I ask, as hopping in mid corner is exactly as TJ has described. But underpowered karts can manifest a hopping on exit due to no differential (I coin in bucking to differentiate when talking about it) this is when the kart drops too quickly loads the inside tyre that grips then releases and grips and releases. This can also happen in high powered karts when you don’t carry enough corner speed or don’t roll enough corner and gas up too early. Just rationalise in your mind which one it is. the mid corner hop can set up the exit hop(buck)
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Thanks to all who’ve weighed in on this - Already an awesome thread. Please keep going!
“It start to kinda hop on entry.
First the rear will slide and suddenly catch grip and that lasts until the apex”
I’m getting this occasionally in hire karts. With no power and no tuning available, my reflex is to drive through it. Can it be done?
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Just another thought, and I’ve read through 90% of this so hopefully I didn’t miss it. Is the track surface all the same. My track is mostly asphalt but does have one concrete horseshoe. The concrete has less grip so I have to drive it differently and can’t tune for that corner or I’m off on all other parts of the track. That’s not the case here right?
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Assume you’re addressing the OP Bryan.
Anyway, in my case, no.
Lee,
Your options are limited to body posture and driving technique. Try leaning different ways in the corner. Lean to the outside to try and plant the outside tire harder, lean to the inside to keep the kart flatter.
Experimenting with different lines can help alleviate it as well. Trying an earlier or later apex can change how the kart loads up and could keep it from overloading as aggressively.