If someone had the answer to this question, they would make a lot of money
Burpo is right. Its about what you want the chassis to do. Its about tuning style, tire choice, grip levels, track layout, chassis design, driver size, class weight, (and probably 50 other factors).
The two fastest MGM drivers I know:
One of them switched to 25/50 a few years ago and never looked back. He wins at tough clubs and runs up front at national races.
The 2nd one runs a 17/40 winning championships at the club level and cannot get a 25/50 set up further up than mid pack.
Totally depends on how your kart is designed to work. A softer front end will reduce scrub and not work the tire as hard, but at the potential expense of front grip. But your specific kart may work best with a stiffer front end, so there is no general answer. Most karts have a specific philosophy in how they work best and how to tune them, and this can vary a lot brand-to-brand.
What tracks are you running? What kart have you been on previously? Are you a big guy? Maybe someone here can at least point you in the right direction.
For what it’s worth the kart I’m building at the moment is going to be a 17/40. If by chance the 17 spindle gets bent I will replace that with a 25mm stub.
I think 40mm is a good size for the rear and if I bent an axle I would replace it with another 40mm.
Main reason I see karts going 25/50 is when they pump out the power. Like a 30HP Rotax.
i was under the assumption of the bigger stuff for more power too, but I see so many lo206 chassis going to 25/50. for what its worth, our FS4 is 25/50 and running in Junior 2 and we are really liking it. May move to a soft axle next season to see how that changes it up, but its been a good kart and pretty east to tune
Question is are the replacing 17/40 to go to 25/50 or is it the natural progression of 99% of karts are now 25/50 from the factory? So any new chassis will be 25/50.
If someone is buying new stubs, new axle, carriers, brake and sprocket, plus wheels to go 25/50 I’d be interested to find out.
A bunch of the MGM guys bought all new 25/50 stuff to replace 17/40 setups 2-3 years and now they are shipping the majority of them with that setup.
Overall, you’ll see 90+% of a field on 25/50 in 206 now. The few exceptions are some leftover MGM guys who prefer the feel, Comet Eagles, Birel AM29, and VLR.
I’ve not driven a proper race kart before. Just heavy rentals. Can you tell me what they feel like and how they compare between 17/40 and 25/50. I’m assuming the 17/40 feels more mushy?
And I just spent a few hours swapping parts from 25/50 to 17/40 chassis! lol
This year we’ve traded in our Birel AM-29 with the 28/30mm chassis and 17/40mm axles for the 30mm OTK with 25/50mm axles.
For me the deciding factor was two fold.
The quality of parts on the 4-stroke specific karts is bad. The 2-stroke chassis might be $1000 more but I spent that just rebuilding the brakes this last year and I still got the same bad brakes.
The minimum weight for adult classes is a lot for the smaller diameter equipment. When we raced Junior2 things were good. As we went to senior and added 60lbs of lead the kart needed stiffness. Each time we stiffened things (seat, struts, torsion bar, etc.) handling improved.
that’s the main reason we have stuck with it. Plus the philosophy of if you can get it right then your working with something different then anyone else making it easier to pass potentially.
the main thing ive started is once everyone starts gravitating to the same philosophy on setup they start to have a hard time passing especially in 206 racing with such equal power
What do people think about the handling characteristics of a 40mm vs a 50mm? I started on an old Intrepid with a 50, then got a Compkart 4R with a 50, and moved onto a Birel AM-29 with a 40 last year. I am finding it hard to be consistent on the Birel and started thinking that may be a symptomatic of switching to a 40 after my first two karts which had 50s. I find it difficult to feel the limit of grip when I am driving, especially on corner entry. I am also finding that the 40mm has a narrower tuning window. Is there anything to the idea that a 40 has a narrower window or requires a much different driving style?
