Tony Kart Weight Bias / Understeer

Hi all i have a 2017 Tony 401s running x30 on MG yellows and i have been having pretty bad understeer and corner exit binding issues ever since it hit the track. Im 5’11 and have set my seat to where the factory recommends which is 640mm from front rail (pretty well as far back as it can go). Everything else is neutral straight out of the box

Ive seen a lot of people saying to try and get 43/57% weight bias for optimal handling but even if i move my seat all the way forward to 610mm i still can only get it to around 60/40

My question to those more knowledgeable than me would be, is it worth adding lead to the front to get to this magic 43/57 ratio or should i look at major setup changes ie axles, seat change?

Look forward to your thoughts


Bit of a strange one. We’ve had quite a few taller guys on the kart with no issues at factory seat settings. It seems strange that your weight distribution would be so far off at that setting.

How far underweight are you for the class? If you are already at weight, I wouldn’t recommend putting any more weight on than you need to.

How much pace do you need to find?

TJ, on new rubber im about 2 tenths off the pace but the kart is chewing the tyres really quickly so by the end of the day im over a second off the pace.

Im just over weight so i cant really afford to add any more but i also cant go any further forward as it will start getting uncomfortable to turn the wheel. FYI this is what i have calculated so far

Seat in OTK recommended position for my height (640mm) minimal fuel
Front - 39.6%
Rear - 60.4%

Seat at 610mm with battery relocated to front foot rest and minimal fuel
Front - 41.9%
Rear - 58.1%

If i fill the tank it then goes to 44/56. I haven’t tried this new setup yet but im just scratching my head as to why everyone else seems to be able to get the chassis working at factory settings?

Tell us more about your understeer…

What kinds of turns?
Where in the turns is it happening?

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Have you had anyone else drive the kart to see if they get the understeer?

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We need to know more. Could be driving style. Like Davin’s idea, but the driver should be both accomplished and similar build as yours.

James, Understeer is evident on all corners but more evident on high speed corners and happening at corner entry. Rear end binding is noticable coming out of both fast and slow corners, other karts are pulling away a lot quicker.

Davin, No I haven’t had anyone else drive the kart.

31colm, pretty confident its not my driving style, reason being i have driven other OTK karts and they felt really balanced, super easy to drive and very responsive to setup changes.

I get the feeling that the kart needs a quicker rate of lift to get the inside wheel up and also hold it unloaded until corner exit to prevent the binding.

I tried lowering the seat, this made the kart looser mid corner but didn’t help the corner entry issue. Im hesitant to raise the seat as won’t this give more side bite / mid corner rear grip?

It’ll help initiate the lift quicker as well, but I still am struggling to see why with your height you are having this bad understeer problem.

Kart is straight? Just because it’s new doesn’t mean it’s straight… I saw a brand new FA come out of the box a few months ago and the waist was literally dragging on the ground…

If the kart is straight, start increasing the front grip the usual ways; wider front, more caster, stiffer bar.

I don’t know if this has been addressed or not, however I haven’t seen OTK ever suggest 64cm for seat position. (With the exception that Shifters run further back than that at ~66 in the last few years)

Start at 62, 62.5, and the flat bottom parallel and flush with the bottom of the frame rail. That’ll more than likely solve your Understeer problem entirely.

Get the seat in the right place, set everything standard and tune from there, make one change at a time and make sure it’s back to standard at the beginning of each weekend or you’ll have yourself tuned out in no time.

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This was emailed to me direct from OTK. It clearly does not work for me but i assume there are other variables such as tyre compounds / track surfaces etc between what i’m using and what the factory teams use.

I like your advice though Ryon, im going to try all the way forward untill it i get it oversteering then work back from there.

Interesting, the old numbers were 61/61.5. I know that some of the teams in Europe had been using the longer measurements at Euromax with the Mojo, but I know they also weren’t using the standard seat, Axle or ride heights.

My last conversation about seat position with Robazzi (the owner of OTK) he had basically said that the taller drivers can get away with seats being further back with front brakes, but “don’t go past 64cm”. That’s why I’m surprised to see that.

When we won Pro Tour with Craig and Pagano 2014/2015 we used the numbers I sent you. So I’m reasonably confident in them. :stuck_out_tongue:

Ken, that cut sheet says 1040 wheel base. The Racer 401S has a wheel base of 1045. The 1040 wheel base I assume is their STVK 401 - 4 Cycle Chassis. OTK has sent you the wrong cut sheet.
My notes say 610mm and 616mm for drivers over 5’ 8" for 1045mm Racer 401 (2016 model), and I assume they would be the same for 1045mm 401S (2017 model). My notes came from a respected OTK tuner that I trust.


The Racer Chassis homologation calls for a 1050 mm wheelbase +/- 10 mm. The chassis has been produced from 1040 mm to now 1045 most recently.

He has the correct document, it’s just become massively out dated and is a bit of a crude oversimplification considering the differences weight, engine and tire type introduce.

I once came across an article by Col Fink from KRacer Australia. I cannot find the link but I kept the content. He mentions that you often have to go the opposite way regarding seat position. Below some quotes:

Most people think that moving the weight in a kart backwards gives you more rear grip (and forwards gives more front grip). Most people are totally wrong. Let’s learn why:

We already know that the tyres provide the lateral (sideways) force that turns the kart around the corner. If a kart with driver weighs 100kg, then the tyres need to generate 200kg of sideways force in order to pull “2G’s” around a corner.
We also know that the weight distribution of the average kart is roughly 40/60, front and rear (most karts are actually further forward than that, but we’ll use those numbers to keep the maths simple).

What most people don’t realise – but which is of absolutely critical importance – is that while the rear tyres are carrying 60% of the vertical (gravitational) force, they must also generate 60% of the lateral (sideways) force to send the kart around the corner. The rear tyres (or really, the outside rear tyre by itself) need to generate 120kg of lateral force to keep our 100kg kart pulling 2G’s around the corner.

If our kart is constantly sliding out in the rear, and we move the seat back… we’re asking the rear tyres to do even more work! If it can’t cope with having to create 120kg of lateral force, how is it going to handle 122, or 125? Similarly, if a kart is constantly understeering (pushing) a common mistake people make is to move the seat forward, which asks the front tyres to do more work than they can do!

Any comment on this from the pundits?

Agree with him, to an extent.

It comes down to determining why either end of the kart is struggling for grip. If the kart is breaking traction from overloading the tire, any additional force you try to put on that tire will make the problem WORSE because you’re already over that tire’s traction limit.

It’s so common to hear from drivers that “the front end doesn’t have enough grip”. So we keep dumping caster in and widening the front track and stiffening the bar. Still not enough, still not enough… Well then we can determine that the reason it doesn’t have enough front grip is because either
A. The driver is just over-driving the corner entry and asking too much of the tires.
B. There was already too much front weight/grip to begin with, and all we are doing is making the kart approach that limit quicker and quicker.

As a tuner or coach, you can see the driver’s hands, read the tire wear, check the tire pressures and make reactionary adjustments based on that. But only the driver can tell you WHY the kart is doing those things. You need to keep asking your driver “why” to get to the bottom of the problem.

As a junior I did was weight distribution testing. What we found is that if we were struggling with front grip for example, we could move the seat forward and get the front to bite a little better. But if we got out of the 43-40% front weight distribution, the kart started to respond poorly to changes. So while we might have been able to fix our temporary front grip issue for a session, we struggled to adjust the kart from that point on when the track changed, because our adjustments weren’t working or were very unpredictable.

As with anything tuning related, keeping the kart within the “tuning window” will yield the most predictable chassis response, however, sometimes you stumble onto something that’s way out of the box, that might work great for a session or two, but it’s a dead end in terms of continuing to refine the setup after that.

I kind of went on a tangent there, hopefully something useful is buried in that novel!

The generation of lateral force (which only exists because of “friction” between the tire and road) of tires is strongly dependent on vertical load, but also on tire width, size and pressure, which clearly are related to the contact area between tire tread and the road. The real definers of grip are adhesion and hysteresis.

Tire modelling is one of the more difficult aspects of vehicle modelling and it is really easy to oversimplify it.

I have a stupid question, was it a new kart? If not is it possible the previous owner had a wacky set up (minimum caster, odd ride heights).

Thanks Nik and TJ for the replys. Nik, it was not something I experienced firsthand but I thought of this specific hint when I read this topic. Or were you replying to the topic starter?

Maybe off topic, but how would TJ’s experiences relate to rental karts? I ask this because normally you can adjust the seat in a rental kart within seconds so it’s very easy to test seat position. Although I know that rental karts (low on power, heavy and with little grip) are in many ways the complete opposite of a high performance kart.

The hard tires, heavy karts, minimal front end geometry and slippery track surfaces all mean that you’re pretty much never going to get a rental to lift and rotate like a proper kart should, so while your body posture and weight bias will have some effect, the kart isn’t going to flex like a race kart will.

The part about vertical load was in response to your posting. I didn’t intend to come across so mean though :smiley:

Hi all, thought i’d give you all an update as to where im at now with this setup.

@Matthijs_Hofman This was my initial thought and there is a video where Col talks about a feathering wear pattern on the front tyres from overloading which is what i was getting, but it wasn’t the case as the further i moved my seat back the worse the issue got (couldn’t move it back any further).

The problem was not enough weight over the front tyres. I moved my seat forward until i had 42/58 weight bias and the problem was fixed. I tried 43/57 but the rear became too loose under brakes.

I found this fixed the understeer issue but i then found i was binding in the rear on corner exit. I switched from a N axle to a HH and the kart felt a lot more alive but the binding is still evident. My seat is flush with the bottom of the chassis at the moment, would lowering the seat solve this issue?

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