A front brake kart will allow for more weight to rotate forward under hard braking compared to a rear brake kart. It will corner differently. The scaling process is to attain a 43-44% front to 57-56% rear weight distribution and 50-50% side to side. These weight distributions can only be achieved with the driver (fully dressed) seated in the kart. The purpose is to provide the best position for the center of gravity of the kart.
An image you can use is that of a pyramid where the corners touch the center of the tire contact patches on the floor and the peak is located just in front of the driverās stomach which is off the ground approx. 30 cm. Ideally this will allow you to make symmetrical adjustments both at the front and back of the kart.
The assumption here is that the seat is located according to the chassis manufacturerās recommendations. Some folks may move the seat around however thatās a huge change of the Center of Gravity in the kart so I would re-scale the kart after a seat position change. For me that means itās a shop change only.
There are exceptions when scaling: If you have some weight to be added then using an app like kart balance it does allow you to move weight around and achieve the right ratios. If youāre overweight then all you can do is move the seat to get as close as you can to the right ratios.
There is a lot to tuning a chassis and the best descriptions I found were in a book titled āThe Lāil Green Book of Kartingā available on Amazon.
Generally, yes. They can act as a very fine adjustment in terms of front end flex, but basically just leave them tight.
Yes. Mostly use it as a quick tool to test if the rear stiffness is correct. If you have a hunch you need a stiffer axle, tightening the third bearing can provide some stiffness to prove or disprove that hunch. So in that respect can also be used as a fine tuning adjustment for axle stiffness. 90% of the time we run it loose.
Iām looking to do some testing at RA the first few weekends of April since Badgerās opening is delayed. With the lack of grip and high-speed corners, what set-up suggestions do you have for the OTK?
Tire pressures to 50 psi, rear track to 50ā, seat 2ā above the frame rails, 150mm hubs front and rear, fourth bearing tight. She still gon be like driving on ice
so finally got this kart on the scales at GFC shop and my previous seat setup was 61% rear and 39% front, which explains why my front end bite and turn in was so bad beforeā¦
we moved the seat forward just over an inch and got it to ~59% rear on the scales, and the seat was also a touch below the rails, so we moved the seat to be even with the rails now, so bit higher seat position.
This was a āsupernatsā special used for KZ 125 shifter, and I had never moved the seat, so not shocked it was too far backwards for big power 125 shifter with front brakes on sticky tires, vs our very low grip dusty tracks where I race 80 shifter and rear brake only and on the harder evinco blue tires.
so we moved the seat forward, honestly I probably would have prefered more front end weight to get it to 43% and then play with the 15lbs i have on this kart on back vs front of seat, but we ran out of time in the shop. Gary Carlton also recommended I raise rear ride height to max and added a touch of more toe out so we did that also. took kart out once and OMG such a change in the handling, almost feels like driving someone elseās kart. the kart handles a lot different with lots of fuel vs much lower fuel so I wonder if we still donāt have the seat far enough forward tbh but handles way better.
I will check my front end width but I have more than once spacer on there with max caster so that is interesting.
current setup:
-normal flat OTK front bar - horizontal orientation
-two seat struts on radiator side one on motor side
-seat flat with rails
-59/41 rear/front weight
-double bolt longer rear hubs (not the stock normal OEM ones, will measure how long they are)
-aiming for 15.5-16psi hot on the evinco blue tires - which with nitrogen has me starting out with rears about 0.50 psi lower than fronts cold and usually around 13.5-14psi cold rear and 14-14.5psi fronts cold
-have ~15lbs of weight on the bottom front of the seat
-rear bumper loose
-third bearing zip tied in zero bolts loose.
-mid front ride height, max rear ride height
-I believe I am at 2mm total toe out static on the stand.
-max caster
-have been playing with OTK MXC rims vs MXJ not sure which one like more yet but thinking for shifter on low grip tracks probably MXC will work better
-will circle back and edit with the front/rear widths also once i get home
-may also try one step stiffer front OTK bar (the round silver one that is not chrome)
The rear ride height completely changes the feeling of the kart on itās own, so I am not surprised to hear all those big changes resulted in something totally new feeling.
GFC knows his stuff and that setup seems pretty standard so I would say youāre on the right path now.
Would suggest keeping the MXC wheels. Even disregarding the heat exchange properties, the MXC is just a better overall handling wheel and gives better feel than the Js.
thanks TJ! good tip on the MXC and appreciate all your help. next time I am out will take some onboard and share here and would be really curious to get any feedback from the community here.
yeah gary/GFC is an absolute black belt, incredibly competent and experienced and also just a crazy nice guy epic customer service.
On a green track I think your numbers are in the ball park. Typically I would run a little lower on something with some grip, but for what youāre doing thatās probably alright.
Maybe it depends on the tire and track, because every photo Iāve seen in my quick Google of OTK KZs shows them on MXCs or MXLs. Ardigo, Luyet, Formal, Puhakka etc. so a variety of track conditions and driver types.
I overheat the Js on my KA in about 4 laps, so I donāt know how they would last on a KZ. I tried to run the MXJs at Road America at the USPKS race and even on that super low-grip surface they didnāt work. The MXC was still faster even though itās a āless grippyā wheel.
Maybe there are conditions where they would be better, like if it were really cold or damp. Badger got repaved this off-season, so Iāll try and do a back-to-back Cs to Js there on a totally fresh surface and see what happens.
I know the J is a very common shifter wheel, at least for the rears. Maybe more so at temporary circuits. Rok Vegas had quite a few guys with Jās on the rear. I bought a set at Rok Vegas searching for more grip. OTK specific handling Iāll leave to someone else as Iām allergic to them karts .
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