2017 Praga cadet LO206.
Kart slows as grip increases.
I’ve been told we need to free the kart up as grip increases. Not sure what that means exactly. Reduce traction overall or just in the rear?
I’ve tried everything I can think of. Short hubs, wide rear, lower ride height, loose bumper, less caster, etc. The kart scales at 43 percent front weight bias. We have been running a medium axle, and seat is 10mm below frame rails.
I just ordered a soft axle and thinking about moving the seat forward. We are at the weight limit with no added weight.
Lap times were competitive with the front runners up until recently, but last race dropped off by .7 second as more rubber was laid down. We also have tried mag and spun aluminum wheels with no difference in lap time. We did 5 sessions last weekend in testing and made changes every time and best lap was within .02 in each session. Suggestions?
In the lower horsepower classes especially, it’s important to keep the kart from binding when the grip starts to ramp up. You need to either take “grip” away from the rear, or increase how much inside rear wheel lift you’re getting. By going softer, reducing caster, shortening hubs, and widening the rear track, you’re removing “grip” from the rear because you’re softening the rear of the kart up so it absorbs the weight transfer and starts to slide a bit. This can work on an especially hard tire or on a track that doesn’t build much rubber. However, this won’t work on a track that builds rubber or a tire that can actually generate some rubber build-up. The kart simply won’t slide, leaving the whole chassis flat from apex off, binding the rear tires and slowing the kart down.
My suggestion would be going back to your baseline setup and then working the opposite direction. If the kart isn’t lifting, add a little caster, narrow the rear track up 5-10mm, raise the rear ride height etc. But do it all in small increments.
It’s tough because soemtimes the 206 doesn’t generate enough power to really get a kart to flex, so it can be hard to keep it free.
Thanks TJ. So when I’m told I need to free up the kart, it’s not the same as making it loose? Meaning I’m trying to loosen it up at corner exit or reduce traction overall at both ends? This is only our 2nd season so I’m still learning. Tires are Evinco blue. Track is Kinsmen in Dixon.
It’s likely binding like I think you and TJ suspect…
Are you doing any datalogging? If yes, have a look at the data (Overlay two clean lap examples from the start and end of the race) to compare and see (Well confirm, in data) where you’re losing time in the latter part of the race.
Also, since you have tried so many things… I wonder if you need to evaluate your starting tire pressures? Are you setting them as late as possible before leaving for the grid?
I’ll take a look at the mychron data. I suspect I’ll see the problem is in the long sweeper coming onto the straight. He holds his own everywhere else but really gets pulled in that section of the track and then loses more all the way down the straight. 3rd place caught and passed him from 4 kart lengths back.
I check the pressures right before we go out on the grid. Was running 13 cold which put us at 15 hot, but lowered them to 11 cold for the main on suggestions from other racers and didn’t seem to help.
“Loose” generally means you’re getting oversteer. “Free” can mean “loose”, it’s just different people’s terminology preferences.
In karting, it’s a bit different as well, because unlike cars (where all this terminology originates from), you need to hike the inside rear wheel to keep the kart rolling through the corner. In a car, typically either the rear is sliding or the front is sliding, and those are your two options. In a kart, the rear can be “loose” aka oversteer, “free” aka rotating properly with no binding, or “tight” where both rear tires are on the ground and the inside wheel is scrubbing/binding off the corner.
You aren’t trying to reduce traction, you’re trying to get the inside rear wheel to lighten so the kart rotates without scrubbing. That’s why talking about adding or taking away “grip” in a kart can be really misleading. I always try to refer to the inside rear wheel lift rather than the “grip”. You might actually be trying to increase mechanical jacking (adding “grip” or “sidebite”) to the chassis so that the outside tire can dig in harder and allow the inside wheel to lift.
I understand this can be super confusing trying to decipher all these terms…
But your goal here in “freeing” up the kart is to get the inside rear wheel to lighten or lift. Generally, to do this, you need to increase the amount of weight jacking that’s occurring through the frame. This can be done by adding caster, widening the front track width, or stiffening the front bar. Those are quick and easy changes to try out. Those adjustments are going to causing the frame to flex more as more weight jacking is pushed through the frame while the wheel is turned, hiking the inside rear wheel more and letting the kart roll through the corner without scrubbing.
Would the size of the driver affect the direction we go when tuning? My son is chunky and carries a lot of weight in his upper body. My thinking was to lower the kart to get the cg down. If he barely clips the inside curb in certain corners the kart bicycles and has almost gone over a couple of times.
That’s definitely a variable to consider. If he’s already having a hard time with bicycling, it sounds like he’s already getting enough weight transfer, in which case I would ignore most of what I posted.
Would be helpful to see some video to get an idea of how the kart is looking through the corner. Also, @KartwerksDan or @PosiMo_Andy might have some insight on cadet tuning.
A ton of great advice up there from TJ - really gives you a lot of things to work with.
A few quick questions for you as it’s been a few years but we spent a lot of time on the Praga Piccolo and had a lot of success, primarily 2 cycle though. A lot of things still apply the same though:
Is this a single bend Piccolo? Meaning do the main rails have one bend by the gas tank or does the rail bend once and then run straight for awhile (perpendicular to the axle) before bending back out again? I’m going to assume that it is a single bend as that is the most prevalent. It is also the most amazing green, cold, wet, damp track kart you’ll ever run but it does struggle some in the grip.
When you say you are using a Medium axle and have ordered a Soft - are you referring to the IPK/Praga factory 5mm thick axles? If not, which axles are you running?
What seat are you running, factory IMaF?
Are you running a MyChron 5?
When you were quick what was your baseline setup?
Rear width compared to axle, length of axle, front spindle spacing, caster, camber, toe, ride height, hub size front & rear?
I believe that you are having a typical high grip issue that the Praga needs to be tuned around. I think axle
And front width will answer a lot of questions.
Here is a pic of the frame. We have a 1/4 pad tillet t8 in it medium flex.
Running ipk 5mm medium axle. 1.5mm camber, half caster, 2 spacers on front, 2mm toe out, factory front hubs, short rear hubs full wide. Medium ride height. Yes we have a mychron 5.
Cool - that is the traditional Praga single bend kart. I still get the warm fuzzies from seeing one - what a badass little kart, particularly for low horsepower applications. The general shape of that kart with the single bend lets it twist and flex fairly easily even without a ton of horsepower or weight transfer.
As for axles, we traditionally found them to be most responsive to the thinner wall axles in the shorter format of 960-1000mm. With a bigger driver, if you are running without any weight, the 2.1-3.0mm axles will save you overall weight and I think the kart will react a little better. If you are looking to pick one of those up, let me know. We have a pile of them here pretty cheap.
As flexible as the kart is already, I still run them very loose through the center of the kart. I loosen the two bolts on the upper front nerf bar, make sure the lower nerf has play, loosen all pan bolts and make sure the side nerfs are loose & not bound up.
I’m not a big fan of the T8 seat in general but it is really a bit stiff for a Cadet kart. If you don’t have an IMAF composite or a VG/VTI T5 Tillett available you can try running the lower seat bolts a bit loose and/or cut a slit in the top, center of the seat at the back. The kart flexes and works so well, you don’t want a rigid seat taking away what makes the kart so good.
In regard to the MyChron 5 do you download the data and compare sessions or days? Just curious what it shows comparing your laps now to when you were quicker. The data will tell you if it is entry, mid-corner, exit or straight before you dump money in to chasing things. It’ll also show overdriving really easily!
Just a few thoughts to try.
Interesting last sentence on the last paragraph.
Good topic header (James can you split?)
Can you ellaborate on how to indentify overdriving with data. Obviously massive overdriving is straight forward, but what about when you start to get really picky?
In my opinion, once you are confident in kart this is the biggest killer of speed.
Thanks in advance