Vibrating at high speed

My son races KAjr on an OTK kart. Recently he has mentioned that it feels like the kart is vibrating at high speed 60+ mph. He doesn’t often give any feedback, so this was interesting.

The wheels are balanced. The kart is set up pretty neutral.

Any thoughts on what it could be?

I had my rear rotor grind in to the hub making it out of round and cause it to vibrate. I would look in to that since its a big mass rotating at fast speeds

Possible causes off the top of my head…

Bent rear axle.
Loose steering components, kingpins, tie rod ends etc.

Did he say he can feel the vibration through the steering wheel (Hands), or through the seat (Butt\chest)?

Steering wheel. I check the kingpins regularly, and they seem tight.

I’ll check the axle. New one installed a few months back and no on track issues I can recall since.

If haven’t already check everything in the steering. Joints for play and tightness, steering wheel, uniball steering column bearing on the chassis, steering column bushing etc etc.

My first thought was a very slight axle bend. But, is it doing it on straights, when turning, both?

Straights and only at high speed. 60+

I missed where you said the wheels were balanced, but double check it?

It could be the hub bearings.

Does it start lap one? Is it towards the end of a run?

I’ve had times where I pick up a vibration towards the end of a race.

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The one thing I would check that wasn’t mentioned is the rims. A bent rim would cause this issue. Easiest to check this without tires on the rims. Rears are the most likely to bend.


I had a similar situation earlier this year, thinking it was something in the front end. Changed wheels, bearings, spindle, kingpin, etc. Ended up being a slightly bent axle which solved the issue.

I think I’ve narrowed it down to the axle or wheel. I did the marker test as I spun the rear axle, and I got a mark on a small part of the tire. I had to get the marker about 1-2mm from the tire to get a mark, so I don’t think it’s too bad.

He’s had his fastest times ever in the last few weeks, even finishing in the top 10 at rte66, so I don’t believe the issue is overly affecting performance.

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What is the marker test?

I use a black sharply and hold it as close as possible to the tire without touching it. I was holding the marker against the bumper to keep it firm. Then you spin the tire. If the marker touches the tire, you know you have a bent axle. And wherever the marker hits the tire determines which way the axle is bent.


Ha, I did the exact same thing (but with a screwdriver and looking for a gap) after an incident on track. Funny enough, I also set a PR with about a mm of runout. Maybe it is the latest speed secret? I can see all the fast guys stocking their trailers with axles in increments of 0.010" runout :joy:

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