Kart jerking / bouncing

Chassis Setup, Tires & Handling Forum
1

Hello fellow drivers,

In the video, my kart kept jerking and causing the steering to fight back at me (mostly)mid corner, having me to fight the kart and not be able to make steering input smoothly, and this jerking motion would amplify towards corner exit as i throttle so i had to lift and slow down and lose time, sometimes so bad that i feel like i’m about to get thrown out of my kart. it has got worse this season as the tarmac is getting worn down and track surface has gotten grippier.

Initially i thought it was my very aged chassis that has been welded multiple times that had inconsistent flex that caused the problem. I have then swapped to a brand new 401RR chassis and a new hard real axle, with all the bearings replaced and a lot more, the only remaining parts that i haven’t swap out on the kart are basically the engine / braking system / steering shaft, yet the problem remains. my steering still has a little bit of play and the kart doesn’t feel very tight. I am unsure whether the problem was caused by hardware or setup issue, its getting exhausting :neutral_face: hopefully i can find some professional insights here on forum, thanks in advance!
*All the other OTK karts were driving smoothly that day.

Update 2023/04/04:

Hello guys,
Thanks for all the inputs earlier, i went on and tried different setting (i think lowering the Ackerman helped the most!) and adjusted my driving & line and finally the hopping/bogging has gone away,
but the jerking motion from steering is still there and i wonder if that could be some fundamental issues with seating/steering position?
(Weight Ballast: One 5kg bolt on left side of seat & One 7kg bolt on bottom of the seat)
I see some drivers stretch their hands fully to combat the bounce from steering wheel, wondering if i should try that too since my kart has so much excessive motions.

Yesterday we went on and did some rolling starts. In the video the drivers in front are turning in later than me, but i am still battling some twitchiness in my steering (mostly rear end of the kart). on higher speed corners i just feel the back end stepping out on me and i had to make several steering inputs, and i have yet to find a solution to this…

Here is my current setup:
Camber F: Slightly Positive
Caster:Neutral
Front Toe: 1mm each side
Front Bar: Round Silver
Front Width: One Large + One Medium Ring
Rear Width: 1400mm
Front Hub: Standard
Rear Hub: Short Hub 70mm
Front Height: Middle Position
Rear Height: Middle Position
Rear Axle: Type C
Steering shaft: Upper hole (Less Ackerman)
Seat: Tillet T11 Fairly tight but running as low as it gets
Rear bumper: Loose
Tires: Maxxis Orange at around 40% life

Thanks for helping out! you guys are mega!

2

How many cm from the back of the seat to rear axle?

Have u tried narrowing the front? Remove caster and make the lift slow down, you can also add some negative camber to calm the steering.

140cm rear width?

Seat type?

3

Looks like hopping, too much grip on the outside rear, basically.

Loosen the rear a bit, depending on your chassis either widen rear track/change to a softer axle or vice versa (some chassis weirdly react opposite to each other :joy:)

Others will pitch in I’m sure

Edit sorry just read OTK do I’d try a stiffer axle

4

its a Tillet T11 seat, im not sure how many cm from back of seat to rear axle, i’ll check when i get to the kart!
Yes i played around with front track but it didn’t make a difference, my current setup is running a mild caster already. im running 139-140cm in rear width

5

im already running a Type C which is the stiffer model, but its still more or less the same as the track started gripping up

6

You are running neutral caster? Otk has a lot built in already. I would try a zero pill at the top for sure.

You can also lower the ackermann to calm it even more, put the tie rods at the upper holes on the steering shaft :grin: You will need to do a new alignment, 2mm total toe out

And if you put the front bar to vertical position it will affect the lift at the rear too.

1 Like
7

Couple of questions:

  • are those standard front hubs or long? Maybe check overall front track width compared to another OTK not having an issue.

  • what is your toe setting?

  • center ride height front and rear?

  • what wheel are you running?

  • you said the front has play? If so, where is the play?

  • OTK H axle and do you have the third bearing in? For rear width is it 139 or 140? There is a huge difference. I wouldn’t run a Briggs 206 kart at 1390 if there is any grip in the track so it would drastically affect higher horse power.

  • when you initiate a turn do you primarily pull the inside hand (what I’m seeing I believe) or push with the outside hand?

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8

Not sure how different it may be with OTK, but he appears to be running a DD2 package @KartwerksDan

9

Agree with Dan’s recommendations, and did you try all the things I mentioned in the other previous post you made?

Copied for clarity:

Hopping usually comes from overloading the chassis and making it jack weight too aggressively. This can be a setup problem, but if you are driving the kart too aggressively into the braking zone like at 0:21, you can also make the kart hop. You’re loading the kart up sideways on entry, then you have to put a bunch of steering input in after braking to get it to rotate back the other direction, which flexes the chassis too much and causes it to rebound and hop.

It’s okay to be aggressive in a braking zone, but we want to make sure we get the chassis settled back down before we initiate turn-in, otherwise you are compounding a bunch of inputs and forces through the kart, and when you go to turn-in, the kart will be unstable.

First, try and be less aggressive in that brake zone and see if you can get it to settle down. If that doesn’t work and it still hops, then you can try to tune it out.

Since hopping comes from too much or too quick weight-jacking, the first thing that stands out to me is that you have full caster in the kart. The Tony Kart has a lot of front geometry to start, so there is rarely a need for full caster. I would reduce the front caster to start. If you have a neutral pill where you can go to half-caster, I’d do that first, but if you only have the stock pills, I would set them to full-neutral (both arrows on the caster pills pointing forward or backward). The full-caster on the front end is jacking a LOT of weight. Probably also why the kart feels twitchy and nervous on harder tires. When you put the harder tire on, the hopping disappears because you aren’t gripping up when the weight transfers, and instead you just overload the less grippy tire and cause it to slide immediately with all that front-end in the kart.

On the OTK chassis line, you go stiffer on the axle to combat hopping. Generally a softer axle will rebound and spring more, so it increases rear traction and can induce hopping. But you shouldn’t be looking at axle changes yet. The N axle is the correct axle for 95% of conditions and should be the starting point all the time. Tune all the other easier options first and if you can’t get it to stop hopping, then you can look at starting at a new baseline with a different axle. If you want to eliminate hopping you would go to a stiffer axle (H, HD, HH etc.)

Given that it looks like it’s only hopping in one corner and nowhere else, my guess is that this is driving related, specifically to your braking technique for that corner. Remember that the kart is a constant, it doesn’t change throughout the lap, so if there is an issue in one corner, it’s likely not the kart causing it. If it was, the issue would appear in multiple corners per lap.

Narrow the front up, you can go as narrow as one small spacer and still be relatively normal. This removes weight jacking. Baseline setup is just to have the one 15mm spacer in, so you’re wider than normal.
Widen the rear to 1400mm. This slows down weight jacking. Dan is right, 1390 is very different than 1400. The 10mm makes a big difference.
Go negative half on the caster. This slows down weight jacking. Heck, even go max negative caster if you have to.
Lower the seat. This removes weight jacking.
Lower the rear ride height. Like lowering the seat.

If you do one or several of these adjustments, it should get better. If you do a few of these, it will for SURE stop hopping.

The kart is transferring too much weight too quickly.

Is the DD2 on a 40mm axle? C axle is only available in 40mm I think.

10

Thanks for the insight TJ! i will test out several possible changes in the next few days, currently it feels to me like i’m just jacking too much weight, will try to reduce Ackerman and caster! currently running the rear at 1400mm but i will also try to narrow the fronts a little bit more as you mentioned. my seat position is already at the lowest (the ballast attached at the bottom of my seat is already scraping :upside_down_face:) so i probably don’t have much room in terms of rear height. axle wise, i’m not sure about the size, i only know its C axle but i will check and get back!

Driving wise, i am also adjusting my braking as you previously mentioned that i might have braked too late / aggressive and loaded up too much force on the chassis into the corner and had the spring effect as soon as i throttle out, but so far it has only gotten worse as the track is getting grippier this year!

Thanks!

1 Like
11

i would def try lowering ackermann next, thanks for the tips! and also try a 2mm toe out! im going to play around with the bar first, i read on the forum saying the gold round bar is the most desirable option but it acts around the same as a flat bar in vertical position. but wouldn’t that make the turn in sharper too?

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12

I think running the flat bar at vertical position is slightly stiffer than the gold bar. The gold bar is not common to use, otk often don’t like to be running way beyond stock setup. But it is worth a shot!

It will make to front stiffer and sharper yes, but it will also affect rear lift since the bar could hinder the rear to flex as much from my experience😁

I would just start with less caster as the chassis overall seems to be good but just with an allot of jacking. Then some Ackerman and last bars :call_me_hand:t2:

And with 2mm total toe out I meant 1mm on either side

13

Leave the flat bar in horizontal. Any stiffer and it probably will make the problem worse, as it makes the whole kart twitchier and springier.

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14

i just checked, and it was 21cm from the tip of the seat to the rear axle. which is very close to the 20cm that was recommended from OTK manual

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15

I ran a 30mm chassis OTK shifter and once the grip came up it hopped when power was applied while the steering wheel was turned excessively. Try modifying your line so that power is applied with minimal steering angle. Maybe think; Brake hard in straight line, get the kart rotated, then straightened and then apply power.

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Have you tried any off the suggested setup changes?:grin:

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yes, i did try adjust my driving accordingly too and it definetly helped!

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i did, i found dialing back the ackerman on steering shaft position the most effective, then i actually ran a stiffer bar and it helped my turn in without springing the kart even harder, which was nice! i didn’t play with the caster as my friend (also my main rival) says he’s fine with running neutral caster on his OTK so i left it as is. and then also changed my driving which definetly helped too! Thanks for the tips! now i am just a little bothered with my back end stepping out on corner exits, but i suppose a new set of tires in grippier race weekend would actually dial that out automatically?

Thanks!

2 Likes
19

Sweet stuff! Do you have any seat stays added? It would help you at the exits

20

i could only fit in one seat stay on the left side as DD2 package is a little restrictive with engine on the left side. Also, i’m thinking maybe the rear stepping out was partly from swapping to a hard axle, so im curious wouldn’t adding extra seat stay make it even harder? i’m considering maybe lowering rear tire pressure as an alternative for the race. Thanks!