7.1 rear tire is what we run.
Here a picture of how I am sitting in the kart.
My seat was at 38 degree angle and I changed it to 33 degree.
The front height is set at medium height, the rear at high height.
Front width at 25mm spacers. camber +2 degree, toe each side 1mm out.
Silver freeline torsion bar, 1040mm Righetti Ridolfo soft axle.
The rear width I started at the above recommended 50.75inch and super short 50mm hubs, which worked well in the morning and at the first 3 laps. Afterwards the kart was hopping and binding up, the lap times got worse after 3-4 laps.
The next day I went back to my long 105mm hubs and set it to 1355mm (sorry for changing units). That day I set my personal best and was able to keep up with the others. I think the changes with the seat and rear right height are positive, the kart felt better at turn in.
Thanks for all the help and recommendations.
PS: Still, a Top Kart might be on my wish list with an Iame 100, after my driving improves.
I think the rear axle might be a problem. With the RR axle you never know what stiffness you get compared to the Freeline one. And i would also suggest to try a step harder one. 1355 is way to narrow. And as you say you put on longer hubs and went narrower and it got better, its defenitely that the soft axle is too soft. With this one you just dont get enough lift. With narrower rear and longer hub you help to lift the inner tyre. So i think a harder will help you
Hi Dimitry, I tried a medium RR axle and everything else the same (hubs, axle width, tires and so on) and that made the hopping worse. It was not just a little worse but quiet a bit. The soft Freeline and RR axle worked very similar. The advantage of the RR- axle is that I have 1040mm instead of 1000mm and that allows me to try shorter hubs with the wide >1350mm setup width.
When you state ā1355mm is way too narrowā I have to state that I am willing to try to go wider but 1,400mm is the max allowed. In the past I tried about 1380mm but I could not find much or any lap time reduction compared to 1,350mm. But its worth a try again with my latest setup changes.
Thanks.
We normally never go under 1390cm. Maybe its for the LO the right width. But for higher hp engines its too low
Iām just going to chime in on a suggestion given to me this past weekend regarding the tires. The used kart I just bought came with 7.1ā rears and I was told itās not an ideal width for our 206ās due to the sidewall being squared off on a 180mm rim. I was told to try a 6ā tire. I have a set of 210mm Freeline wheels that came with the package and I had a severe push when it tried those at the recommended 1395 chassis setup guide I put the 180mm wheels on this weekend and narrowed the track to 1330 and it helped, however I felt like it was binding up as the track gripped up. So Iām going to try the 6ā tires on the narrower rims and take it back up to 1395 and go from thereā¦perhaps that may help you as well. Take what I say with a grain of salt however, Iām just passing along info from a longtime karter thatās been really helpful so far. Hereās a picture of the tires on both wheels and what he says theoretically makes sense to me that the rounded profile will allow the kart to roll onto the edge and bite better in my case and help with jacking as well. I run at Orlando where itās a tighter technical track so ymmv.
Just keep in mind that a 30mm adjustment on rear track width is a massive swing and itās pretty easy to go the opposite way on your handling issues by making that big of an adjustment. For example this past weekend I changed the rear track from 1390 to 1380 and it completely changed the attitude of the rear of the kart. It went from slight oversteer at 1390 to completely overworking the rear tire and creating too much weight transfer at 1380.
Definitely donāt run 7.10s on 180s
7.10s on 212s are fine.
Agreed. So far what heās been suggesting has helped me tremendously and has made a positive impact on where the kart is versus how I got it. It was set up very narrow with a small seat so I guess it was being driven by a junior.
Where I race, there are no options. It is 7.1 tires MG red, 212mm wheels on the rear (dry).
I am not seeing a chance to change the local reglement for the whole race series.
I think it is similar at many other tracks, there are not any choices unless you run outlaw.
Digging this up from the dead. I could have attempted to revive a few different threads about the am29 and/or MG reds (SH2ās now). If this makes more sense somewhere else let me know.
With the new SH2 compound this year and our birel am29-s12(2021) chassis we are LOCKED down on exit. -Bogging and understeer- on exit is killing our pace. Chassis will carve into any corner though. We race at Stockholm Karting Center in MN with the originator of this thread. We are limited to the 7.1 rear tire in the Sr. Lo206 class (360lbs.)
Hoping someone here has found success with this chassis/tire combo recently and would be willing to share how they keep that IR light/jacked thru to exit? Because we havent stumbled across it yet.
Here is what we consider our closest/fastest setup so far this year:
weather: 75Ā°F air temp / 105+Ā° track temp:
Front bumper: Loose
Floor pan: tight
Front bar: stock Birel round (chrome)
Front spacers on the inside of hub: 3 thick 1 narrow 35mm (44 1/2")
Front ride height: Low
Ackermann: Top hole steering shaft / Inside hole on spindle
Toe: 2mm out each side
Camber: 0 - 1 notch (Neg 1mm static on snipers)
Caster: 0.5Ā° stock pills - 1 notch
Axle type: M2 Medium Righetti (cut to 1000mm)
Rear wheel size: 210 swift mags
Rear hub: 100mm aftermarket
Rear track width: 53 1/2". (outside to outside on 7.1 tires)
Rear ride height: low, top hole
Wheel base: Middle
Seat struts on?: disconnected
Seat: IMAF standard / size:2Ā° / mod: F6 / COD: F62
Measurements are very, very, very close to birel factory spec now however i will have to remeasure to verify exactly.
A big concern is the weather and track are only going to get warmer/worse for these tires and I feel like ive got the setup pretty drastically far from baseline already.
Any reason to not try the birel M or K (hard) axles on these karts?
Remove more caster?
Super soft seat? which one?
Float the seat? best method?
Seat placement? ā We moved it back (from where it was mounted when we bought it new) 4cm late last year(2023) per a fellow racers recommendation after he drove our chassis. We kept the weight bias the same (42% front) by moving lead however it helped tremendously in freeing up the chassis. It is extremely close to birel spec now.
Reading plenty of threads here I keep getting the impression our rear width may be way out of whack? I will say we ran 53" wide our most competitive year with SH1ās on this track when it didnāt take any rubber. 53" - 2.2"= 50.8" Thats my rationale for why our 53" rear was so happy then compared to the 6" tire guys I read on here. haha.
Any am29 guys living this same struggle? Thanks in advance to anyone that can help unscramble my mess.
Howād you settle on this? I found that I had similar behavior with my Praga Dragon / 206. It flat refused to turn without massive steering input (which got the rear to jack). It scrubbed a ton that way though. Raising the rear axle, seat, and lead really woke it up and got the rear weight to actual transfer to the outside tire instead of sitting flat.
With our understeer/bog on exit problem since these new SH2ās went on, we started down the path of trying to slow or delay the jacking effect.
Friday practice season opener:
*tight bog on exit= widening rear width seemed to help
1st race day starting off pace in practice, then worse by .5 off in qualifying with no changes but the track temp rising.
- Medium axle change for prefinal and we set fastest lap of that heat coming from mid pack. (draft helps obviously)
- Took seat struts off kart felt slightly better on exit.
Next day track was equally as warm with more rubber down.
- our next change was lowering the ride height in the rear. This seemed to delay what lap the understeer showed up on. But hasnāt seemed to make a noticeable difference in pace relative to the front runners each day.
@dodo Were your issues on entry of the corner then? Ours seems to turn in great, almost too good. Makes me think I need to take caster out but at what point will the rear width need to also be narrowed back up or not?
Then you want more weight transfer across the rear, not less. When tuning caster, remember to look at what your driverās hands are doing. Midcorner may have neutral steering if you had good rotation going in, so changing caster will do exactly nothing. On exit, even with mild understeer, the driver might have minimal steering input, so you arenāt generating much mechanical jacking. Depends on how much rotation they got going through the turn.
My issue was everywhere - turn-in, midcorner, and exit. The kart was just dead until I raised it, and I understand that is unique to me, but still, you basically want as much lateral transfer (at the back) as you can get without hopping. The inside rear tire is just dead weight for 206 since we donāt need it to help put power down (what power? haha). With understeer, more mechanical jacking and more centrifugal jacking were both good things. I think this is why moving the seat rearward helped you last year. The CG at the rear went up because the driver sits high (relative to the rest of the kart mass) and was moved rearward. Total CG did not change, but the lever arm lifting the inside rear went up. This also makes me wonder if overall CG is too rearward. Moving it forward will take some of the effort out of trying to jack up the inside rear, and itāll help the front grip.
Edit: to be clear, I am open to these points being picked apart. I am not an authority, but I tend to use an authoritative tone. Poke all the holes you can, haha
One thing I am experiencing in real time this weekend, that we havenāt dealt with in recent times, is overworking and binding the outside rear tire in the corner. With the softer tire you need to find that window where you are using all that grip and getting the inside rear to unload. But you donāt want to work it too hard or the tire will literally bind and twist and bog the kart down. Itās a balancing act.
For example I want to raise rear ride height to get the kart to transfer more but Iām concerned about crushing that outside rear into the track so hard that it binds. So Iām considering raising rear ride height to get that transfer but also going to a softer axle to reduce the pressure on the outside tire.
Is that with 206? At least with 206 and SH2s, Iāve not felt the rears do that aside from when my air gauge shat itself and I went out at 5psi. Definitely got some soft sidewall then!
Not a 206 but more of a problem with tracks that have some real rubber down. Not so much an issue on a low grip track.
But these tires do lay rubber down unlike the old ones so itās possible that some tracks will build rubber they havenāt had before.
@dodo I believe you may be on to something there with your theory. Itās definitely the widest and laziest we have tuned the kart. We widened the front a bunch (swinging for the fences) for one heat and it seemed to just understeer everywhere so it was narrowed back up. This seemed to enforce my theory of picking up the IR and setting it down too early but there is no doubt I am lost on setup and definitely in my own head. It could have been a false flag like the EOL for the tires too? They looked worse than ever. You could tell they were sliding/chattering.
I ran out to the track today and took exact measurement on the seat.
Our IMAF (F6) standard seat is set at A:102cm. B:21cm. C:14.5cm. (A little back and down it seems from factory spec).
My next question would be what seat and where should my measurements move to? I have in my possession the stock free line (F6) standard seat that has been patched along the bottom. A standard IMAF (stock free line replacement F6) And a Tillet T11 (standard, feels stiffer than the other two). Or do I need a t11vg?
If we were to move the seat first what should the next tuning steps look like? Or is there some tuning I can do to prove that the seat needs to be moved?
@tjkoyen Iām not 100% sure we can get the same twist on the sidewall you were experiencing with these 206ās. But, I will note for one cooler morning practice we went out at 18psi and were just as quick (relatively) as previous 5 lap practice sessions. We always seem to be. 2nd-4th fastest in morning practice usually at 13psi. Temps rise and we end up 5th-9th it seems. There is this voice in my head trying to convince me to run super high psi in an attempt to stiffen the sidewall. Never thought Iād ask for the SH1ās back. In 2022 we did experiment with 18-20psi on SH1ās and it wasnāt horrible.
Not an expert by any means since Iāve never ran the chassis myself but when Iāve tuned for guys on the am29 weāve tried a couple things to free up the kart with sticky tires. 100mm rear hubs seem too wide to me, you could try narrower ones, on my kart we run narrower hubs to free it up as well as taking as much caster out as possible. Also have you tried the long wheelbase? It might not be what your looking for but the wheelbase changes have always made a big difference when Iāve tuned for that chassis. Side note as well the driver I work with set up his steering column wrong once and had it in the top hole and he hated it, more of a preference thing but Iād run it in the middle hole.
Seems like itās an unwritten rule with the birel that you never run without the front bar or seat struts but thatās just from what Iāve seen. Everyone Iāve seen run a birel has either used the silver freeline seat or the tillett vg to free it up as well. Another drastic option would be to try the dm wheels, some guys like those in high grip but they definitely arenāt cheap.
Just some ideas, to be honest you should talk to either Marc Stehle (Stehle Motorsports) or @CrocIndy, they are Birel experts and they have had a lot of success at CKNA with the birels which is an even sticker tire than the sh2.
Thank you for the response. It may be worth another attempt to reach out to some of those regional/national level Birel teams.
But thank you to everyone here that is willing to share any advice or knowledge!
Good news, we found some pace this last race weekend. Taking the @dodo approach, and some very similar advice from the local tuning guru, we went down the path of trying to get more weight transfer across the rear. Narrowed the rear width until the ālittle birel that couldā woke right up. We experimented with more caster at our narrow rear width, but it began hopping on entry. We are also running the Douglas M series wheels now.
The chassis is very free and very fast in the first 5-6 laps. However, it did start to hop at apex during the last two laps of our 14 lap finals.
My current understanding of this chassis is stiffening the rear to āfreeā the chassis up. and we narrowed the rear to get the weight transfer. Moving the wheel-base back? would that aide in this theory or be an adjustment in the other direction?
Also, Is the tillet t11vg softer or harder than the standard freeline seat? I have a T11 that feels quite a bit stiffer. I am wondering if that would act similar to a harder axle on this chassis?
Thanks again.
Curious what lap they came in on. Could you just lower pressure a touch and see if kart still hops last laps?