Understeer with fresh tyres on rubbered track (race condition)

Hi all,

Just finished a race weekend.
I am having a serious issue with new tyres - I can do laps with 0.1-0.2 diff to top national drivers in practice sessions with old tyres on a not that rubbered track. However the diff would just expand to 0.6-0.7 on a race day with new tyres on a highly rubbered track.

I took a look at my video in the 2 given conditions and found one small difference - I need to steer “twice” with new tyres on a rubbered track. I can’t get enough rotation with one steering input on race day.

Could you give my some tips to tackle this issue without changing the setup? Just turn the steering wheel a bit longer? Going under the rubber?

Thank you very much!

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Are you having trouble with understeer during the initial turn-in phase of the turn, during the rotation energy build phase (between turn-in and apex), or during the final rotation phase at the the apex?

Why would you not want to change the setup? Going from old tires on a greener track to a rubbered track on new tires will require setup changes to get the most speed and consistency.

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Like Ricky says, if the grip level changes that much, you will likely need to make some chassis adjustments.

Could you share the video so we can see?

I had a very similar problem; Practice I was fast but in qualifying only so-so most of the time. My problem was the front end had too much grip and kart reacted too quickly for my liking. I had to take grip out of the front by softening the front bar and put on OTK MXC wheels. These changes helped but I still didn’t do well in qualifying.

Practicing on new tires might help but that get expensive.

You have to change the setup.

Thank you very much everyone!
Not wanting to change the setup is because I want to see if I can improve something and hone my skills better

First clip :
Normal practice day, old tyre with perhaps green track (not a race day). I am 0.1-0.2 to top drivers

Second clip:
New tyre after 1 qualifying session & 2 heats on a race day. I am 0.6-0.7 to top drivers.

What I can see is I couldn’t turn the car enough after the steering input, hence I would need to turn the steering wheel twice

Also - I felt if I carry too much speed into the corner, the kart just start hopping. But if I slow down a bit - the engine start bogging. Probably an indicator that the kart is rolling too slow and the kart just doesn’t turn…

It’s a KZ2 on Tony Kart 401RR with N rear axle
Maxxis crappy tyre

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For me it looks like after you rotate steering wheel past a point, the back end steps out and hence the need to steer twice. In other words as you apply more and more mechanical lift by turning the steering wheel - the inside rear lifts too much, breaks traction from the outside rear. Looks like you want to drive a tighter line what the kart can do with the current set up

If I was driving I would take out rear lift - caster to be exact - until you can steer into the corner without back stepping out

Yeah I agree there’s a point where the rear breaks traction. Is it possible that I turn the wheel too much to break the traction because I could not turn the kart like I used to do?

Thanks for the replies !

For me the situation looks the same on both new and used tyres. So I would take caster out and go faster on either case. You should have a set up that allows you to drive a fast line and steer as tight as its necessary. I wouldnt steer less. I would change the set up. With a shifter a more rounded line usually causes slower lap times

Used tyres usually dont generate the same g forces as the new tyres. So the kart might feel better as it doesnt lift the rear due to g forces but by mechanical lift. It might be in better envelope

As you have new tyres - you are propably going initially faster to the corners = generating more g force and therefore rear lift by itself. But now ad to this mechanical lift and the kart oversteers. Therefore the lap times ares slower

Reducing mechanical lift isnt something you do once and forget about it. If your lap times improve, you will generate more lift in the corners by g forces. You need to reduce mechanical lift even more. Drive fast enough and you need mechanical jacking very little


Now knowing you are driving a KZ, definitely agree with @Mikkometalli on the driving. He may have a point with caster also, something to try anyways. With a KZ you must take full advantage of the grip you have in the tires, but they can only take so much force before giving. With four wheel brakes and lots of power you must concentrate on utilizing the tires for those two things primarily. Obviously turning is part of the game also, but mid corner speed is not so important as in a single speed kart. Braking straight and hard allows deceleration to happen in the shortest distance, then you rotate the kart, unwind the wheel and power out as straight as possible to maximize acceleration. Also be careful of downshifting too early and “engine braking”, I heard some early downshift and rev spikes in your video. Downshift quickly after most of the braking is done, and avoid spiking the revs too high to avoid catastrophic failure.

There is some to be said for braking technique also, which I’ve realized over the last couple years. I did not realize it, but I tend to trail brake into the corner further than I thought I did. Only did I find this out after reviewing my own video while trying to help coach another driver. Many corners I am not fully off the brake until just before apex. It all made sense when I figured this out because I can usually get away with less front grip in the kart than some others (ie. front bar removed) by keeping load on the front tires with the trail braking. I also run a good percentage of front brake bias and load the front of the kart hard under initial braking before trailing off into the corner. Definitely not the only way to make a KZ fast, but it is one way and it is interesting to study the different methods drivers utilize to achieve the same thing. More of less just mentioning all this as food for thought.


Huge thanks to @Rdub3 & @Mikkometalli for the comments.
I also experienced hopping in few tight corners (hairpins).
So I just found a setup guide by @tjkoyen
Do all these things (hopping & mid corner understeer) point to “too much caster”?
Interestingly, I asked for a bit more caster right before the final session - and I became ~0.2 slower after the caster setup.

here’s the full final video for the race day

I see a couple things in your driving, one is leaving room on the outside of the track prior to entry. Looks like half a meter to a meter at times, try to utilize that space on entry and exit. I will caveat that sometimes doing so isn’t always faster, as sometimes the shortest line can be the fastest in a KZ. Plus in a race you sometimes will run lower than the ideal line to protect your position. Also really focus on pointing and shooting the corners more versus trying to arc through them, this will change how the kart works so you may have to make some adjustments but definitely should make you faster in the long run. Basically, get it into the corner as fast as possible, rotate quick, straighten out your exit as much as possible and get the power down.


For me too much caster causes hopping and specially in the tight corners. Too much caster also causes mid corner oversteer in high speed. If experiencing hopping I start taking caster away. If taking ALL the caster away hasnt cured it, I reduce the front width until Im all in, if these dont help I start adding more negative camber. These all reduce mechanical lift. Negative camber is the last resort - I try to do everything else first

While trying different set ups with the shifter - adding caster (mechanical lift ) has not worked for me. The lap times go worse. I dont race in the wet. Two rear tyres are better at pushing forward than one

@Rdub3 made a good observation how he uses front brakes to tie the front to the ground. I also tie and trail brake late in to the corner. A technique I learnt while driving rallycars on snow/gravel. Ads ton of front feel/grip. But with a flexible kart I have had to tone down my tie down tactics. Its challenge to brake deep into the corner but release in time so that front brakes do not drag and bend the flimsy chassis

If the track has tight corners - These corners will be super tight for a shifter. Adding more ackerman will release the front coming out of the apex and the front drags less