Hello, I’m looking at purchasing a Compkart (secondhand), and would like to know how stiff the chassis is and if I should use a softer or stiffer seat? Thanks.
Go soft seat. The standard soft seat it comes with to me has no loss in performance over a more expensive Tillet VG. But I do like you can get a more tailored seat shape in the Tillet. Won races this year in both.
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which year and model ?
Its a 2020 covert 4r
The 4r is their low HP chassis suited for the LO206 up to a KA100. I believe it is a mix of 30 and 28mm tubing so it is on the more flexible end of chassis flex. The soft Compkart seat is likely the way to go.
What motor are you planning to use it for?
I’ll add that I’ve also not noticed any difference in performance between the standard soft CompKart Seat and the Tillett VG, but I have noticed a difference in durability with the Tillett holding up better in the long term. Worth noting that my rear ride height is always low and I do probably bottom it out more than what’s typical so it may not apply to you, but if you’re in a similar position where the seat bottoms out with any sort of regularity it may be worth springing for the Tillett. If not, the regular CompKart branded seat performs just as well.
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In my shifter i run a soft seat. I really like it as opposed to the med/hard seat i was running before.
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Buying second hand and its a rotax evo fr125, what are the complications for using something like that instead of a ka100
The 4r isn’t really designed to run 125cc engines. I believe the components are a bit de-rated. Smaller spindles, brakes, and frame. I think you will be asking a lot of the chassis as a whole to be running the Rotax engine. A KA100 would be fine and I know some smaller guys that run the 4R with that engine package.
If you are just doing laps for fun then it’s okay. What’s your goal with the kart?
I would agree with Clayton. While it may work, not sure if it will be the most effective. The brakes especially are not going to be as strong as you may want. This chassis doesn’t have the option of a center (3rd bearing). I would suspect the chassis might flex too much as well.
Don’t have much of a goal really, its my first year of karting and kind of just want to get out there
My only concerns would be overwhelming the brakes. If they look like the style in the video they aren’t made to bring an adult weight kart with a 125cc to a stop. More for 4 strokes which normally aren’t braking hard or much during a lap.
If they are this style then no worries.
I’m pretty sure those lower spec brakes are just the brakes Birel used to use on all their TAG karts for like 15 years.
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I was thinking they look like the “banana” brakes I remember from Birel too.
100% the same brakes they used to run on 2 cycle stuff but with a slightly wider vented rotor iirc.
Yeah I ran that same brake on an RY-31/Rotax in 2009. Worked fine!
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Good to know. Just seems a thin non vented rotor with such low thermal headroom would get overworked quickly.
Depending on the track I can overwork the modern stuff at Masters weight.
Simple solution, stop braking so much. 
That is a good point though. I’ve known drivers who burn up sets of pads in a day while mine last for half a season, so it is a little technique dependent on how hard you work the braking system.