I found that when it’s really busy and hot at the track I start to get too much grip.
I have tried stiffening everything but it still isn’t enough.
What do you guys reccomend I should do when this happens? (I drive otk rotax senior with mojo d5)
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Is the track super rubbered up?
What have you tried?
widen the rear, lower the rear, remove seat struts. More - camber, less caster, narrow the front, lower the front, move seat forward, lower seat, lower lead
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When you say too much grip… how is the kart behaving… and where in the turns.
I would guess one of two things if it’s got too much grip either the kart would be scrubbing too much speed mid corner/ bog down trying to accelerate out of the corner. Or maybe he feels like you aren’t getting the weight transfer you want out of the kart?
In high-grip situations for the OTK kart, you should be going stiffer on the rear axle, which it sounds like you’re doing maybe.
@Jim_Maier’s tips are pretty spot on for most cases.
As @KartingIsLife said, what is the kart actually doing in this situation? How does it react on turn-in, apex, and exit?
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It start to kinda hop on entry.
First the rear will slide and suddenly catch grip and that lasts until the apex
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Remove front grip by reducing front caster, track width, or softening the front bar. Stiffen the rear by going to a stiffer axle.
The kart is reacting too quickly with all the rubber on the track, causing the rear to lift suddenly and overload the outside tire too quickly. This causes a lose of traction, and the rear starts to hop and oscillate. A harder axle will smooth out the lifting motion, reduce overall flex, and keep the rear more stable on turn-in and apex.
The soften front end will keep the kart from reacting so dramatically on turn-in and reduce initial weight jacking.
Always make one adjustment at a time, in small increments.
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Some follow-up questions for you (in bold):
Widen the rear, lower the rear, remove seat struts (I’ve been told by a number of reputable drivers that I should ADD seat struts to alleviate a hopping condition…which may be partly caused by the fact that I’m 6’3” and 188lbs, so on the bigger side for a kart driver. Is this accurate? What exactly do seat struts do anyway?). More - camber (How does this help exactly? Any relevant links/articles you can provide?), less caster (Same question), narrow the front (Same question), lower the front, move seat forward (same question), lower seat, lower lead
I’ve also been told that when the kart is hopping (I.e. you have too much rear grip, relative to the height of the kart+driver Center of Gravity) then you can fix this by moving to a different axle stiffness. If that’s true, would a softer axle help? Or would a stiffer one?
Just trying to learn as much as I can here, since the original poster seems to have a similar handling problem to what I often experience, especially in higher power/high grip classes, like Rotax
A harder axle will smooth out the lifting motion, reduce overall flex, and keep the rear more stable on turn-in and apex. If someone else told me to use a softer axle to fix hopping, then I’m officially confused…also if you suggest that a stiffer axle will do all these things, then would a softer axle do the opposite? (I.e. less smooth lifting motion, more overall flex, less stable rear end on turn-in up to the apex). I welcome your thoughts.
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Some of this comes down to the kart in particular you are working with. For example, the OTK brand karts we are referring to here might tune differently than another brand. Some karts prefer a stiffer axle to remove “grip”, some prefer a softer axle. Of course, we don’t know the relative stiffness between two different brands of axles either. For example, a CRG “medium” axle might be equivalent to an OTK “hard” or vice versa. Also, a CRG frame might be stiffer than an OTK frame, so the axle is going to affect things differently on one kart vs. the other.
Second, we need to determine why your kart is hopping. A hopping condition can be caused by multiple setup issues or driving issues. In most cases, a kart is hopping because there is too much weight jacking. On a track with low grip, this will manifest as a slide, because there isn’t rubber on the track to allow the kart to regain traction once it breaks loose. On a grippy track, the kart hops. The kart jacks too much weight initially, overloads the outside rear tire, the tire breaks traction, slides momentarily, then once enough speed is scrubbed off, it grabs again and continues this oscillation. It’s like rubbing a pencil eraser across a sheet of paper at an angle. It’ll catch and release and ‘scoot’ across the paper.
To counteract that, you need to get the kart to transfer less weight on initial turn-in. This can be achieved by reducing the front end geometry (removing caster or softening the front) so that it jacks less weight toward the outside rear or jacks weight more slowly and progressively. It can also be achieved by (in many instances including this thread) stiffening the axle. The stiffer axle will limit some of the flex in the rear end, thereby not allowing the kart to ‘wind up’ as hard on that outside tire, keeping the tire within it’s traction limit and reducing any slip.
You can also widen the rear track to lower the relative CoG with the driver (i.e. make the ‘triangle’ from the driver’s head to the outside rear wheels wider) or lower the seat or rear ride height.
A softer axle in this case will allow for more flex in the chassis and get that inside rear wheel hiked up more easily. That’s why on the OTK kart we use the soft axle in the rain or very low grip situations. Because there’s no grip on the track surface, you can’t generate the kind of forces necessary to flex the standard axle enough to get the inside rear unloaded. Going to a softer axle requires less force to flex and lift the inside rear, so on a wet track you can still unload the kart properly and get it to rotate. On a track with grip, the soft axle flexes too easily and gives out when cornering hard, resulting in hopping/sliding/all sorts of bad handling things.
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Also what helps is not applying power while the steering wheel is turned. This is especially helpful with shifters.
I can’t remember ever having too much grip. All I can remember is wanting more.
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.
Tire pressures where in the sweet spot
From where do you receive that information? What is the “sweet spot”?
I went to the track with a friend of mine, he was having the same problem as you, maybe a little more severe, I suggested raising the tire pressure he told me the same thing as you’re telling me. A friend of his came by, apparently a person he trusted more than me, and his friend told him the same thing. He did as his friend and I suggested, no more hop.
Seems like a really easy thing to do, and undo if I’m wrong!
To tack on to what Al mentioned, remember that tire pressures greatly affect the spring rate of a kart tires which of course will change how the chassis will respond when the outside wheel is getting loaded up like that.
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If we go any higher the tire pressure will rise too much. Our goal is to have the best pressure within short distance/laps.
We have tried a lot with pressure but we always come back to the same amount because it works best for us.
In my opinion, using tire pressure to drastically change the handling characteristics isn’t the right approach. A tire has a “happy spot”, we’ll say. They’re designed to be used in a specific pressure and temperature window. Getting outside of that window would just be a bandaid for a bigger issue somewhere else.
Some solutions for too much grip in OTK karts have already been suggested in this thread. OTK’s really don’t need to have the kitchen sink thrown at them to get them to work. A good starting point for when a track is gripped up would be 2 small spacers in front, flat bar, caster rolled out of it, cut N, 55 1/8" rear track, bolted third bearing. I’ve also seen success with keeping OEM caster but lowering the whole kart. But that doesn’t really work if you’re at a bumpy track because you’ll skateboard the chassis over bumps and you’ll run into issues with losing grip mid corner. If the track is SUPER grippy, you can run lowered chassis, hard(er) axle, and no caster.
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Awesome information Zach.
I’m curious, although I don’t have an OTK I imagine a lot of your points would be universal? It has only just occurred to me (after reading your post) that bolting in my third bearing would be a good idea to help tune out some bounce in my kart. I’m already at max rear width but my kart tends towards loose with some hopping in slower corners. Would the third bearing in make it too even harder to lift my inside rear in the higher speed corners?
Liam, some of the changes could carry over. But, for the most part, different chassis’ will react differently than others. Simply because they are made up differently. Chassis material/diameter, wheel construction, axle stiffness, etc are manufacturer dependent.
The best thing I can suggest is to try them out. Have a full day at a track to just try things one at a time.
Bolting the third hearing will help take flex out of the center of the axle. It can be a quick reference change to make if you want to see what a harder axle might do. Go one session with it unbolted, bolt it, then do another session and see what changed.
You could be having front end issues too, like Tj mentioned earlier. In a slow corner you could be transferring too much weight too early and that could be causing your hop.
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Thanks heaps Zach. I’ve just gone and gotten my bolts etc ready for the third bearing to try it out. It is currently just sitting there held loosely by a cable tie.
It will be interesting to see if I can tell the difference. I am also wondering if I need to be lower in the rear or as you say changing something at the front.
I’m in an Arrow X3 if that makes a difference.
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