Weight balance and grip balance

Chassis Setup, Tires & Handling Forum
1

A bit of an old topic, but hear me out as I have some controversial observation. Here’s some background to start:

  • I have a 2024 Tony that I used for 1.5 seasons and have counted 60 hours on this chassis so far
  • Rotax Senior engine, 365 lbs class weight
  • Bit of a tall driver (187 cm, 180lbs)
  • My local track has a good amount of grip and we use Levanto KRT tires (similar to LeCont whites). I run into similar issues on other tracks too though, but less prominent.

This is my first OTK kart. First ~20 hours of use, I had no issues - good balance always - race day or green track. Then, it started to understeer when the track is gripped up. Understeer happens after initial turn in and to the apex (from 15% to 50% of the corner, if it 's clearer).

On a race weekend I usually run full caster, stiffer front bar, the rest is mostly standard setup. Full rear width, 1 large spacer on the front, ~1mm toe out, 0 camber. Also I run a harder axle (type A or H).

I tried different setup changes but nothing really helps. So having exhausted normal options I decided to experiment with weight balance. I don’t have much lead (4 lbs) but I also moved the seat 1-2 inches. When I moved weight forward, that didn’t help (although I expected it to help). And then, when I moved it back to where it was, and even a little further back - suddenly it seemed to help. It’s hard to say for sure because the 2 sessions were more than an hour apart and the track could fall off a bit, removing the condition for understeer.

That’s one thing. The other thing, with driving, I noticed that if I add more trail braking, it makes more understeer. When I add throttle at apex, it adds a little more rotation (not to be confused with rear sliding).

So, with both static weight distribution and dynamic (brake/throttle), it seems like adding weight to the front creates understeer, and adding weight to the rear helps with it. Which is opposite to what we usually expect.

Wanted to hear everyone’s thoughts on this, and whether you ever experienced something similar.

I will try more testing myself, but unfortunately this has to wait until the next race as the problem never surfaces on a green track.

Also, not sure how much can be perceived from the video, but here are 2 videos.
The first is on an odd day with green track, kart feels balanced:

Second is from a race weekend session (some lack of front grip or overgripped rear):

2

Most of what you’ve said is pretty normal and expected. The weight balance thing can be a little more nuanced. Have you scaled the kart to see what the weight distribution is? Depending on where you are in that range, it is entirely possible to get better front responsiveness by increasing rear weight. More weight on the rear of the kart (typically on the seat) can help the kart ‘tip’ and flex the rear of the chassis more, allowing more inside rear wheel lift and better rotation. You’re adding weight to the pendulum of the seat and creating more inertia on the rear end of the kart.

3

With the seat installed by my dealer last year, I scaled the kart and it was at 39:61. This is probably mostly due to the fact that I have chicken legs but significant weight in the upper body. This season I put in a new seat and positioned it further to the front (convinced that my problems must be due to heavy rear). Scaled at 41:59. This was the starting point for experiments I made last weekend, during which I didn’t scale, but by math I think I went roughly to 44:56 and then to 40:60, and liked the latter more.

You’re right about the weight nuances - it adds “downforce” on one hand but adds inertia on the other, cancelling each other out - but that’s just school physics, I know reality is more nuanced.

Also, when I’m understeering, I feel like my inside rear is lifted, so I don’t think I have a problem there. That makes me think, maybe it’s just overall too much work for the front outer tire, and moving some weight off it helped?..

The other thing to note is that due to my body build and a fair level of grip, I’m reaching pretty extreme amounts of weight transfer. On gripped track I would regularly bicycle and almost flipped a couple times. That was mostly solved recently after they raised my class weight from 360 to 365.

4

I recommend putting the seat where the manufacturer recommends it. Typically that is going to put you somewhere between 58 and 60% rear weight.

If the kart feels nice and balanced like it does in your first video when the track is less grippy, I would work to slowly free up the rear of the kart as the grip came in through the day. On my 401 RR that process mainly involved removing seat struts and going to a stiffer front bar. I have also found that adding a little bit of rear tire pressure can help.

As you have noted, you can move the seat around and have a pretty decent effect on the way the kart behaves, however, the kart is not primarily designed to be tuned that way. The kart is designed to be tuned with the seat at the recommended location. Extreme mounting positions that deviate from the manufacturers recommendation by a large margin often result in a non-favorable trade-off in performance in my experience.

Typically (this is not a dig at you because I am as guilty of it as anyone) if you follow the manufacturer recommendations for the positioning of the seat and you are not able to get the kart to handle correctly, the issue is often driver technique related.