Looking to tap into the vast expertise on this forum, I hope you can hang there with me on this post. We run a Birel AM-29 in our local region’s LO206 Jr. Class. This class allows the black slide for the engine, and a spec tire: Maxxis Victor. From what I can see online, this tire is a rental kart tire, which leads me to my request for help/feedback. I’m sure not many here have experience with this tire, so for the sake of the discussion we’ll assume my problem is low tire grip on asphalt.
The kart is setup as follows:
Max ride height (reason for this is the asphalt we run on has several seams where the surface bows in for water runoff)
Two seat struts on the left, one on the right
No front torsion bar
Tires at 10psi front/12 psi rear HOT
Rear track: 1076mm
Front track: 1016mm
Wheelbase (middle setting on rear axle): 1040mm
Rear axle: UNKNOWN. It’s got no markings on it, it’s not a Freeline, so I don’t know the hardness of it.
Alignment: 0deg toe, 0deg camber (this has resulted in nice, even wear across the tire without excessive sidewall wear on the outside). I haven’t measured camber, though - it’s resultant based on the camber setting.
Corner weights: we have the kart weight distribution at 49F/51R with pretty even cross weights. I’ll have to dig that info up.
Club spec’s a min. weight at 290lbs, my driver/kart is currently at 300lbs off the racetrack. Kart has five 5lbs lead weights to ballast up (I can likely lose one based on scaled weight): two up front and three on the left side of the seat
My problem on these tires is and has always been lack of rear grip. Kart has a good amount steering (I say this because my driver has been unable to push the kart fast enough to really test front grip, but is easy to oversteer. It’s worth noting that until we get to the hotter months, the kart struggles to lift in the inside tire when turning. The kart can also be a bit unpredictable - as my driver starts to really push, we’ll have an oversteer incident or two.
Things I’ve tried:
No seat struts and one strut on each side: no discernible difference
We did try the front torsion bar installed when we initially got the kart, but I removed it and haven’t tried putting it back on. I’m thinking to try it to take some of the front steering out?
Tried playing with tire pressures: no discernible difference. These tires aren’t sensitive to pressure changes as much as softer tires are, I’ve found.
That’s about it. The kart is very stable on straights and can carve up esses very well - it’s usually hairpin turns or the turn at the end of a straight where oversteer rears its ugly head. The kart will oversteer on medium-speed hairpins, and will drift (meaning the rear will come out and my drive will hold a small slide) on medium-speed sweepers - which I can’t understand.
It may also be worth noting that my driver is a regular podium finisher, but has lost out on 1st and 2nd because of a spinout. I’m confident my driver isn’t making mistakes in their input (sharp sheering inputs, stabbing the brakes) based on watching them on track and onboard video. Comparing times to the podium finishers in the LO206 Sr class, we’re 3 seconds off pace and trying to close that gap for when my driver make the jump to that class.
Any help/suggestions/critique is greatly appreciated.
A lot of info to discuss. But first I’d like to confirm that you are essentially running a senior engine setup at junior weight, yes? If we are 3 seconds off and significantly lighter than the seniors, there is more work to do on driving, and setup should be your secondary focus. But let’s dive into the setup.
I’ll preface this by saying I’ve only worked on AM-29s a few times and I have never run the Maxxis Victor tire, but the general theories of chassis setup should still apply.
I think the first step is getting the kart back to a more baseline setup. What you’ve got here sounds waaaaay out of the ballpark.
Unless the kart is scraping significantly, I would run the ride heights medium to start.
I wouldn’t run seat struts on 206. Not enough power to flex the kart and on a very hard tire, you will never get the kart to dig into the track anyway, so all the seat struts are doing is making the kart snappy and stiff.
No torsion bar in front is going to remove front grip on turn-in, so if oversteer is an issue, having it out is fine. Unless a BirelART guru here says otherwise.
This seems super low for a ‘rental kart’ tire. On very hard tires, you need to pump up the air pressure more to build temperature. I would not be afraid to go significantly higher on pressure if this is truly a rock hard tire. On MG Oranges, it is not uncommon to run 25 psi+ in 206.
42" on the rear… typo? Is this even possible? Is this to the inside of the rear tires or outside? Typically we talk in overall track widths outside-to-outside tire. Euro karts in 206 seem to like a bit wider, but your overall width is still going to be narrower than a more powerful class.
Same as above.
Normal, no problem.
This is a big baseline item for the setup. I would try and figure it out or get a known quantity axle. The axle defines the base from which a lot of your tuning comes from, so having this be unknown is tough to work around.
Wear is not the primary concern with front alignment; speed is. Usually we are running a bit of toe-out and a bit of negative camber. The kart will be more direct with some negative camber in, and toe-out will help stability on turn-in and also help the kart respond better to steering input.
44% front is the highest I ever go for weight distribution. 206 experts can jump in here and correct me if I’m wrong, but 49% seems waaaaay high on the front.
I can understand why the kart would be snappy. It’s raised up in the air so the center of gravity is high, the nose weight is very high, the seat is very stiff from the struts, and the tire pressure isn’t high enough to build heat. It sounds like you’ve got most adjustments on the kart set for a very twitchy and unstable platform.
But again, back to the beginning of my post. If you are that far off on pace, you need to set the kart back to neutral and let your driver pound laps. Chassis setup is worth maybe 1 second.
If you’re correct on these weights, then these 2 things are definitely related. I’m not a 4-cycle expert either, but in the 2-stroke world we aim for around 42% front weight.
Thank you so much for your feedback - I’ve added my responses to yours in bold:
Unless the kart is scraping significantly, I would run the ride heights medium to start. Noted
I wouldn’t run seat struts on 206. Not enough power to flex the kart and on a very hard tire, you will never get the kart to dig into the track anyway, so all the seat struts are doing is making the kart snappy and stiff. Noted, will remove
No torsion bar in front is going to remove front grip on turn-in, so if oversteer is an issue, having it out is fine. Unless a BirelART guru here says otherwise. 10-4
This seems super low for a ‘rental kart’ tire. On very hard tires, you need to pump up the air pressure more to build temperature. I would not be afraid to go significantly higher on pressure if this is truly a rock hard tire. On MG Oranges, it is not uncommon to run 25 psi+ in 206. Noted. We’ll do some testing with higher temps.
42" on the rear… typo? Is this even possible? Is this to the inside of the rear tires or outside? Typically we talk in overall track widths outside-to-outside tire. Euro karts in 206 seem to like a bit wider, but your overall width is still going to be narrower than a more powerful class. I was measuring to tire center, so I measured outside to outside: 40.5" (1029mm) front track, 42.5" (1080mm) rear track.
Same as above.
Normal, no problem.
This is a big baseline item for the setup. I would try and figure it out or get a known quantity axle. The axle defines the base from which a lot of your tuning comes from, so having this be unknown is tough to work around. Noted. I have a buddy with a mid-harness Freeline axle, I’ll run it and see how the kart behaves.
Wear is not the primary concern with front alignment; speed is. Usually we are running a bit of toe-out and a bit of negative camber. The kart will be more direct with some negative camber in, and toe-out will help stability on turn-in and also help the kart respond better to steering input. Noted, we’ll experiment with some toe out & negative camber.
44% front is the highest I ever go for weight distribution. 206 experts can jump in here and correct me if I’m wrong, but 49% seems waaaaay high on the front. I went back and dug up the weights, we’re at 43/57 on the nose.
I would get rid of one of the weights. 5 lbs over the minimum is enough. And mount all the weights on the seat. Start with them on the back of the seat but don’t be afraid to move them around and test. You may find that mounting the weight up higher than expected helps.
Some axles are marked underneath the keys. It is always worth pulling the hubs to check. I would look around online to see if you can find the Briel setup specs. It never hurts to set the kart up completely neutral/ baseline and work from there. Things like seat placement can make a big difference.
Too be honest we like to only make change at time. Put some laps and get a feel for it. Did it help or did it hurt? If you change more than thing you will never know what worked. Personally I would move the weight off the front end before moving the seat.
As the Indiana BirelART dealer that has front running karts on all tire compounds, I disagree with @tjkoyen on the seat struts. Always seat struts, all the time on the AM29. One per side.
Always front bar, all the time.
Add positive 2mm camber. -2 total toe out. Front bar in. 2 10mm spacers on front. 51.5-52" rear track width.
Tire pressures way too low.
Get a 1000mm Freeline B axle.
Get the seat in the right position if it isn’t. ~102cm LF kingpin to top back of seat.
One thing I forgot to ask: where should caster be? If the kart isn’t jacking the inside tire, could that cause oversteer, and should I add positive caster?
It’s very possible that on a very hard tire that provides minimal lateral grip that you won’t actually see the inside rear unload. There simply isn’t enough force going through the frame to flex it. When we used to run super hard tires at club level, we typically aimed to have the kart freed up by letting slide the rear just slightly. On a hard tire this scrubs minimal speed since there is no grip to bog you down.
Even on softer tires, if the track is very slick we find that trying to force that tire into the track just doesn’t work because there simply is no grip to give. At SuperNats this year I spent a day loading up all the caster, max front width, putting all the weight jacking into the kart, and it would not dig the outside rear into the track, because there was no rubber on the track surface. Going the other way helped the kart rotate better and work with what the track and tire would provide in terms of grip.
Follow Matts advice on chassis. He knows his stuff.
Only additional detail I can add and it’s not 100% apples to apples, is tire pressure. We’ve never ran on the tire you are using but looking it up, the durometer is 65-68 depending on the site listing the tire
When we ran the Bridgestone YDS, which was similar duro, we were well over 30 PSI. Times over 40 just to get heat in the tire.