Need to understand OTK 4 cycle setup

Hi everyone, my son races a Kosmic 4 cycle chassis in LO206 Jr here in Canada. I’ve been trying to understand chassis setup better, and we’ve made some progress improving the stability of the kart this season, but I’d like to know more.

We run vega blue tires here (Vega SL3 I believe), and up here we have a weird rule where we have a max rear track of 50".

Im currently running the N axle, with stock medium length hubs, but our track isnt right to the max of 50" yet. Our track is extremely low grip, I’m looking for ways to improve overall grip.

Should i bump it straight out to the 50" max at the rear? I have a softer Q axle i could try. Im thinking the narrower rear track isnt letting the axle flex like a wider track would, so maybe a softer axle would help?

Anything else to consider?

So weird hearing there are still places using the 50" rule, I thought most everyone had adopted the CIK 55 1/8" rule by now.

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Its only for 4 cycle, 2 cycle up here is 55 1/8".

Yeah, still I feel pretty uncommon with the downfall of the “gold cup” style bodywork and narrow karts like that.


Q axle will give more rear traction for sure.

Are you noticing specific handling issues? What is the kart doing on entry, apex, and exit?

Well we started out with a lot of mid corner understeer, we seem to have ironed most of that out with some front end changes, and overall he seems happy with the balance, he just seems to be complaining about overall grip level. I know this isnt exactly scientific, but he’ll go out in practice with the class points leader and come in and say he just cant corner as fast as that other kid. I’m completely willing to believe its driving issues, but id like to at least try to give him a bit more traction in general.

Since this wouldn’t be the first time I’ve seen it - I’m wondering if the lack of speed is mistakenly thinking you have “not enough grip”, when you’re actually “overstuck”? If the balance of the kart is good, and you’re struggling for pace, it might be a fair question to ask, based on our experience with the kart and those regs.

I assume you’re on the STV450 4-cycle chassis?

I find it interesting that you’re lacking grip, especially with that tire and with our Canadian regs - although the low grip track might be your reason.

In Edmonton, where we’re running Junior Briggs to the same regs, but on the Mojo D2 tire (I think the Vega is a softer tire from the one time we went East and ran it - albeit in Briggs Cadet), we have the opposite problem. Instead of lacking grip, we struggle to free the rear of my daughter’s kart up enough at our track (higher grip track) on the STV - especially when it gets warmer.

We’ve always run to the max 50" width, and we run the only set of 180mm MXC’s I know of in Western Canada… obvious things you’d do to free a kart up on a track with lots of grip.

In our case, we found that the narrow rear width with the Canadian regs, combined with a small driver, changed some of the “general” approach to OTK axle tuning.

Last year, when she was smaller, the Q axle with neutral caster worked best for us, despite a high grip track. We found we had to go to a softer axle to get the inside rear to lift - basically getting the axle to flex in the short distance between the rear cassette and hub - and we found it could maintain that lift and set back down well when coming off the corner. If we ran the N with neutral caster, the rear would just lay too flat. Adding caster would lift the inside rear, but it would set back down too hard and kill exit speed. The other thing we found last year was running no seat struts was better. Basically, with a small driver, we had to make the rear of the kart soft.

This year (granted very early in the season with not a lot of testing yet), after a winter growth spurt (5" taller and 25 lbs heavier), last year’s setup was garbage. We’ve now gone back to the N with neutral caster and 1 seat strut per side. I think there’s now enough weight being transferred to lift the inside rear and maintain it, unlike last year where the N axle would leave the rear fully planted on the track… fun thing with this sport is there are so many variables, that what works for one person on a given track doesn’t always translate to someone else… or even a growing version of the same person.

One thing I’ve found with this chassis with these regs, is it is VERY sensitive to front-to-rear weight balance. I’ve heard that the STV tends to be biased towards lots of rear grip (at least compared to the standard OTK 30mm 2-cycle karts - I’m guessing a combo of the 17mm front end and the different rear cross bar location), and I can confirm that moving weight forward on the kart has been the biggest improvement we’ve made to free up the rear of the kart - a 5 lb weight moved from the bottom front of the seat to the front of the kart made an unusually big difference in lap times.

One other change that helped us this past weekend was running the front bar in a “stiffer” setting (not flat). My guess of why this helped is that while it may have added grip to the entire kart (maybe not the perfect solution - because we aren’t looking for more rear grip), the impact has been to add more relative front grip than rear, to offset the natural rear grip bias the kart has.

One other thing we haven’t tried that I’d love to do is run the eccentric OTK rear cassettes to shorten the wheelbase. I’ve heard rumors that this can really help offset the natural rear grip bias in the chassis, but I’ve never been able to test it.

Prior to this year, it was us on the STV against a ton of Birel AM29’s, which is a great chassis with these Canadian regs. This year, we’ve seen a bunch of people show up with the standard OTK 2-cycle chassis, and they seem to work real well, with neutral setups, no matter the size of driver, with these rules. I like the STV, but I think it takes a bit more tuning to get it to work in some conditions than the other chassis.

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Yeah when I’ve tuned for guys on the STV in the states it was a nightmare with stickier tires. I don’t know anyone in the US at the bigger races running the STV anymore but it can definitely still work if u can tune it right.

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Cannot speak to the STV, but can speak to the 401R and 401RR in 4 stroke racing.

We always ran maximum width, 55-1/8” unless in very cold or wet conditions when we would run 53”. Our baseline setup was 55” every time out. Our regional specs are all SKUSA based and limit rear width to equal or wider than rear bumper or 53”, whichever is greater.

What we found was that many many drivers were running H, HH, or HD axles cut down from stock 1030mm. I saw 1000mm often and even down to 980. It makes the axle stiffer, but the hub axle interface flex more. We tended to fight a front push, when we added caster then we would fight a rear hop and slide. If he slowed his hands down it bound and bogged badly. When we went harder axle it helped. We did not cut down and learned we were being beat by cut down axles when we moved up to KA and had a former 206 competitor on our team for KA.