Axle recommendation - Trying to resolve a hopping issue

Birel CQ31, 2004, Yamaha KT100, 190 lb driver, 365 lb race weight, Bridgestone YLC, low to medium grip track

Problem: Hopping all the time, in almost every corner, fairly severe hopping

What I’ve tried so far
Move sear forward, change the lean angle on the seat to lower center of gravity, lower chassis in the rear, remove seat stays, Tried both lose and tight rear bumper and 3rd bearing.

The kart is setup with a 40mm axle as it was from the factory on that model. Changing to a 50mm axle is not financially in the cards right now. The 40mm axle that the car has in it is NOT a freeline part and we have been unable to identify any markings on the axle that would distinguish it and tell us anything about the hardness of the axle. It is 3mm thick and 1040 long is about all that we know.

We’ve changed just about everything except the axle at this point trying to make the least expensive changes possible that were within our control. We believe the issue is axle related and after contact in a race yesterday the axle in the kart now is bent and needs to be replaced anyhow (probably a blessing).

Curious what axle would be recommended for this situation. Everything I’ve read would indicate go to a softer axle to get rid of the hop…but softer than the unknown that’s in there now is obviously going to be difficult to pin down.

While freeline axles are obviously the OEM equipment for this kart I’d like to stick with a more affordable alternative if possible. Any opinions on Swift vs OTK vs. PTK or any others brands as they all seem to be about $100 less than the Freeline alternatives.

Thanks in advance

What is the rear width? I’ve found in the past if my kart was hopping, going wider in the rear helped. Obviously while not going over the 55in limit.

It’s already 55" wide. I should have put that earlier sorry I forgot.

Has the kart always done this?

Yes, but we have only owned it for a short period of time.

We fully inspected the chassis for cracking today and found no cracks or cracked welds. Bottoms of the tubes show very little scraping / flattening.

What is your caster at? Take out caster if you can. The chassis is a little older and most older chassis were stiff. Meaning they had a lot of mechanical grip in them. Is the chassis 30 or 32 mm? Lower seat to get center of gravity down. Other than the hopping, how does the kart handle?

I’m sensing you have a more fundamental problem deeper than axle choice here. Do you have any video we can examine?

40mm and 50mm appear to be equally competitive across the nation with 206, so you’re good running 40mm… But you may do well putting a 40 freeline axle in there if you are stuck for ideas. Perhaps even a used one.

This is not something I have observerved as a trend. Can you give some specifics on years for what’s considered older? Are you talking specifically about Birel chassis?

Stiff and soft are super subjective terms when it comes to chassis themselves. I know we use them a lot in the sport, but I think it’s usefulness is limited. Using it to describe the feel of a specific chassis when comparing setups on it, I can understand. But the idea of saying x chassis is stiff and y is soft without any way to measure it is just guesswork especially when there’s a dozen other factors that will affect how the chassis feels.

I owned and raced a 2008 Birel R31 and loved it! Mine (eventually) handled well, was competitive and even won some races in KPV’s. One key thing I did to make that chassis work well on low/medium grip tracks was to go with an axle slightly softer than the Freeline F (Medium). My recommendation would be to stick with a Freeline axle. Also, what type of seat are you using?

I’ve found the opposite to be true. I’ve driven/raced a 1997 Tony Kart, a 2008 Birel, as well as brand new stuff. Both those older karts were free as hell; way more free than all the new stuff that’s out right now. However, the frames were still stiff. I think that frame stiffness doesn’t always correlate with more/less mechanical grip.

1 Like

My apologies, I meant is the older chassis that I have tested. 2000 tonykart, 2003 birel, And 1994 tony kart. They felt stiff to me, but that is subjective and could be biased due to kart setup. So my apologies for the confusion.

Yeah it’s super subjective. From memory a 1994 could be an Esprit or Racer which are pretty different from each other. In 2000 you had varying versions of the Extreme knocking around from the 97-99 era in conjuction with the Mitox and the Rekord. Again, pretty different frames and setups even changing from 120mm rail spacing and variable tube sizes from front to rear.

(if we had a way to measure) I would imagine you could have a kart that’s “stiff” laterally but more flexible IMO longitudinally and diagonally with every combination in between. At the minimum you have to be on the same tires (Sidewall stiffness) and pressures to compare, afterall that’s the most “compressible” part of the vehicle.

All stuff I’d love to geek out on and get some data, gotta build a cool website for karting first though.