OTK 401r for LO206 Snr, go softer to stabilise rear?

Chassis Setup, Tires & Handling Forum
1

**OTK 401R, completely standard in terms of front torsion bar, rear axle etc. **
Running in LO206 Senior at Whiteland Raceway park (365lbs weight), Bridgestone YLC tyres.
OTK Mag wheels (almost certain they are MXC) 130mm front, 212mm rear on 4.50 and 7.1 tyres. (Tyre type and width is mandated)

Looking for a bit of advice as while I am hugely experienced with 2 strokes, mainly shifters, I have recently added an LO206 to the stable which is completely new territory for me as, except for some Le Mans 24hr races 10-15 years ago in twin engined Honda Prokarts, 4 stroke karts are alien to me.

I have an OTK 401R and have set it up from a chassis baseline sheet I got from the forum here.
http://www.tonykartaustralia.com.au/tonykart/set-ups/Downloads.asp?AreaID=16&sortorder=Alpha

However:

  1. The setup sheet is for Mojo D2 tyres (the above setup sheet says they like up to 6 deg neg camber), I’m using Bridgestone YLC.
  2. I believe the setup sheet is aimed for 2 stroke engines, as from what I have read the rule of thumb is 4 stroke/206 karts need to be setup softer. Something I haven’t experienced since my Formula TKM days.

Original setup:
1mm toe out
2mm neg camber per side
Standard caster (Both pills with arrows facing forward) which I believe is 10.2 degrees
Front torsion bar flat
No 3rd bearing
1 additional seat stay per side.
Front and rear track width as per above setup guide
Seat is set 5mm below frame rail as suggested.

I have done one event in the kart and it felt good, laptimes were competitive. However I did feel the rear needed to be more planted. Some of this is needing to adjust my style to 4 cycle but I’m experienced enough to know it wasn’t all this. The front was amazing, maybe too good and causing the rear to break away.

So I’ve started with softening the rear by removing the additional seat stays per side (yet to test it) but just wanted some tips on if my train of thought is good for next steps if removing the bars doesn’t help:

  1. Reducing rear track
  2. Move ballast up higher on rear seat
  3. Increasing front camber
  4. Reducing front Ackerman
  5. Reducing front caster

I also have to run insane amounts of lead (about 55lbs) as I am about 145lbs soaking wet so have SOME opportunity to move weight around, but with that amount of ballast most places to mount it are already taken.

I have managed to find some snippets here from LO206 drivers who have used an OTK at WRP and the consensus seems to be go softer, which is why as I said above the first thing I am trying is to remove the additional seat bars, but I just wanted to double check my train of thought above is consistent with what needs to be done for a 4 stroke setup given the chassis is essentially a 2 stroke chassis.

Thanks in advance.

2

Where is your seat placed? There’s a blurb in one of the setup guides about moving the seat back up to 10mm for “hard” tires…

3

Hi Andy,

5mm below the frame rail (bottom). I’m planning on mounting more weight higher up the seat than moving it up 5mm if it comes to that.

4

sorry…I meant more front to back.

1 Like
5

Everything seems like you’re heading in the right direction. My only note would be that having the seat 5mm below the rails seems low. I am 135lbs and I run the seat either even with the bottom of the rail or in between the rails most of the time. Moving weight up on the seat or raising rear ride height will both accomplish similar things.

If you’re close on lap times and feel the kart is pretty good, I would start with some really small adjustments. Biggest mistake I see is people trying too hard and making too big of adjustments, especially on the OTK which tends to respond pretty nicely to small things.

If I have slight oversteer, my first adjustment is to narrow the rear track by 5mm. If you’re at neutral caster right now, I probably wouldn’t go negative just yet. I’d be worried at your weight you’d have a tough time unloading the inside rear at all if you take the caster out. I can count on one hand how many times I’ve gone to negative caster in my life.

6

Thanks TJ,

Interesting, didn’t realise there was a senior driver out there lighter than I! :wink: I’m going to start with putting ballast high up the rear seat, as I’m personally not a fan of having swiss cheese for a seat to move it up 5mm or so. In my shifter my seat is higher, also an OTK (801 Krypton), thanks to stuffing lead under it and the rear is more planted.

I actually moved the rear hubs about about 5-10mm to meet the setup sheet above so as I say I will see what effect removing the bars have, followed by the ballast then move the rear wheels in.

Interestingly, I’ve read on the OTK setup sheet that LOWERING the rear ride height increases grip. I know OTK tend to be oddball, but I can’t really get my head around that.

Finally, totally agree with you on adjustments. I’ve been in the game long enough to not make that mistake. Thanks again.

7

Lowering the rear ride height will help the rear squat and keep the kart flatter off the corner. This can “increase grip” in the sense that both tires are getting traction, since the inside rear isn’t unloaded as much. The negative could be flat sliding if you can’t get enough sidebite or overall chassis binding if there isn’t enough power to keep the scrub from bogging down the motor. I would be concerned about that in a 206.

Raising the rear ride height adds sidebite to the kart, as it helps the kart tip and dig on the outside rear tire, forcing that tire into the track surface more. The negatives to this would be hopping in high grip situations or overloading that outside rear too quickly and causing traction loss at max load in lower grip situations.

Really depends on the kind of “more grip” you’re looking for. I prefer to think of it in terms of “inside wheel load” rather than “grip”. A kart that has too little “grip” or too much “grip” can actually feel and behave similarly in some cases, so it’s easier to conceptualize what’s happening when you start dialing in on what the inside rear wheel load is doing.

8

Makes complete sense. I think too many people generalise about grip and like you say, lowering the rear end helps plant the rear on traction but on an application like 206 that is low power I honestly can’t see that being an issue.

I’m glad you agree with my thought process. I remember my early years in Formula TKM with the rock hard Maxxis SLC tyre and you had to set those karts up like an arrow (narrow front, wider rear track) to allow the kart to get the weight transfer to unload the inside rear, otherwise they understeered like hell.

I’m suffering on corner entry at the moment, more so on high speed corners and of course that means you are ‘holding’ the rear all the way around then. Some of that is changes I can make to my driving style (I’m a fairly smooth driver but am a 2 stroke veteran) but I’m confident the above changes will help. I’ll post my updates on here sometime along with my fleet.

9

One thing I’ve discovered is that it’s important to separate understeer/oversteer balance from if the IR is lifting enough. If it’s feeling free enough, I’ve had some luck with either moving the seat or individual weights forward to cure oversteer. It sounds a little backwards at first, but the way I understand it, you want to get the amount of force you’re asking of each end of the kart to be as equal as possible to the amount of tire available to provide that force. The set of tires that can provide the most grip is the one most equally loaded, since the friction coefficient actually becomes lower at greater loads for most tires (check out tire load sensitivity in a vehicle dynamics book if you’re intersted, its a good piece of info to have for racing). If you picture a kart lifting the IR perfectly, you probably have an equal or greater amount of tire contact at the front compared to the rear, so it might be working more “efficiently” than the rear in your kart. This past weekend, I also found a couple bolts in the 3rd bearing help with this too (for an x30 anyway, and for a different reason than moving weight).

10

I have a couple very fast OTK karts running 206 at Pitt Race - YLC last year and Mojo D2 this year. What seems to work well for me so far is softening the kart up a bit through the middle and running the front a little narrower than typical. Other than that I’m running essentially the same setup as a TaG or 100cc baseline with the exception of an Energy 1000mm medium axle. And that is strictly because I had no OTK axles with inboard keyways otherwise I would have had 1000mm N or H in the kart.

Softening the kart through the middle reduced some of the weight transfer and narrowing the front reduced the front of the kart wanting to drive the rear around from mid corner to off.