Cross weight and F/R question

roybiggens

New member
Current chassis is a Praga Dragon EVO2. My local track is a Praga dealer, so when I got into karting about a year ago, I went with the local brand for parts support etc.

I'm 6'1" 170lbs, and I have long legs and arms for that height, this has made seat positioning a challenge, especially as it relates to front/rear weight % and left/right weight %.

We run 365lbs in the 206 Senior class. I have to add about 15lbs to the kart to get it up to weight, with the weight concentrated on the left side of the seat as far back as I can mount it.
I'm at about 10lbs difference across the rear axle, light on the left rear and heavy on the right, and I'm also about 5lbs different across the front but in the other direction, left front is heavy and right front is light. I've moved the seat as far left as I can, and I've got the motor mounted as far left as possible on the Odenthal mount (running clutch in-board too), so I'm kind of out of options at this point.

I'm also at about a 41.5% Front to 58.5% rear split, and that's with me about as close to the steering wheel as I can get, and the steering wheel in the most forward position it will go.

It could be a frame problem, as I've not had the opportunity to put it on a table, but I'm also concerned that it's related to the fact that the Praga is really built for the lighter two-cycle engines, and if that's the case I'm just chasing my tail.

My question is this, how do the purpose built four cycle chassis' (MGM, Margay, Comet, etc) help in this area? Do they somehow offset the motor more to the left?

A more general question is, given my dimensions, can I expect to ever get a more ideal weight distribution if I'm limited to adding only 15 - 20lbs to the kart to make weight?

Local track is AMP near Atlanta if it matters. It's .85mi in length and it's very heavy on left hand corners, really only one right hand corner of any consequence.

Thanks
 
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How does the kart handle?

You can tweak the frame to get the weight even on the front. Put a fuel can (or whatever) under the heavy wheel, have someone stand on the opposite rear wheel, jump on the light wheel in the front. Continue until the weight is even across the front.

Does it push? Narrow the front a little or widen the rear a little.

Is it loose? Widen the front a little or narrow the rear a little.

The front to rear weight differential seems a little much. If needed, it doesn't take much moving of the seat to cure a push or a loose condition. 1 inch is a bunch.

Adding weight is the last thing that I would do, if at all.

A small rear wheel weight differential is less consequential than the front is with a weight differential.
 
Common problem. Every driver is different size and causes weights to be slightly different. Assuming your chassis is straight, You might want to look into something like this:

https://cometkartsales.com/40mm-50mm-Margay-Adjustable-Cross-Cassette.html
It can also be done by slotting the holes in your bearing hangers but is a little harder to get perfect.

This will fix your cross problem and allow you to move your weight to the front of the seat (even put 5 between your ankles on the bottom steering support) to fix your front percentage problem.
 
You want more front weight I normally run about 42 to 44% if not a little more. I also use the adjustable left rear bearing cassette like is listed above. That way you don't have to split the spindle heights and a sniper laser reads correct. But if you don't have one of the bearing adjusters you'll need to move the front spindle shims. You need to raise the front spindles up to get more front weight. At first I was just going to say move the weights to the front of the frame, but you said the left rear is light anyways. So just leave them there. Once you get the front weight around 44% try and equal the front split. You said your front is only about 5lbs different and the rears are 10lb. That is about where I like to keep split differences at most. Now if the front ends up being any more off you would just need to drop the light side of the spindle down 1 spacer. Keep the cross close to 50% if you can. But since you said AMP is more left turns, more left side cross weight would be ok.

I use the phone app Iraceweight to get the percentages if the scales I'm using don't automatically read them for me.
 
Thanks guys.

I’ll look into the bearing carrier. The chassis already has rear axle height adjustment capability built into the current bearing hangars, and I hadn’t thought about dropping one side or the other to be honest.

I realize the bearings “float” to a degree in the carriers, but it seems like it would create binding if one side was set lower, or am I completely missing what’s being suggested? I’m picturing that the axle would not be parallel to the ground, correct?

Again, thanks for replies.
 
Well, there is stagger, most of the time, so the axle is not parallel to the ground, but you're right about the frame going up and down and not the axle.

If I worried about anything it would be the misalignment of the brake disc To the caliper and the misalignment of the sprocket.
 
Well, there is stagger, most of the time, so the axle is not parallel to the ground, but you're right about the frame going up and down and not the axle.

If I worried about anything it would be the misalignment of the brake disc To the caliper and the misalignment of the sprocket.

Believe it or not, it only takes a few mm up and down on the cassette to move percentage points of weight. The minimal movement doesn't usually effect brake rotor or gear hub alignment (especially compared to the movement that happens on the track due to chassis flex). If you were feeling super spendy, you could splurge for a floating brake rotor and floating gear hub.
 
Believe it or not, it only takes a few mm up and down on the cassette to move percentage points of weight. The minimal movement doesn't usually effect brake rotor or gear hub alignment (especially compared to the movement that happens on the track due to chassis flex). If you were feeling super spendy, you could splurge for a floating brake rotor and floating gear hub.
I'm not exactly sure what you mean by "floating".

I picture 3 or 4 holes in the bearing hanger for adjusting up and down. I'm wondering, how far apart are those holes? They don't use slots do they?
 
This is the adjuster to raise or lower the axle to change cross. The back side, which is not seen in the photo, is a standard three bolt bearing hangar pattern. The floating hubs that Derek is speaking of allow slight movement to accommodate chassis flex etc.
 

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This is the adjuster to raise or lower the axle to change cross. The back side, which is not seen in the photo, is a standard three bolt bearing hangar pattern. The floating hubs that Derek is speaking of allow slight movement to accommodate chassis flex etc.
Real interesting. From the picture I can't quite see how it works, but I get an indication. How rugged is it?
Thanks for the heads up.
 
It's made of two pieces that slide up and down against each other. The four bolts facing are loosened, the top bolt is either tightened or loosened to change the ride height, then the four bolts and jam nit are tightened to lock in place. Common in the dirt world and used to a lesser extent in sprint racing.
 
Seat and engine location have very negligible influence on cross weight. Sounds like the rf needs bent down about 4mm and you’d be much happier with your numbers. 50% cross and left and 44% front would be a great starting point.
 
Seat and engine location have very negligible influence on cross weight. Sounds like the rf needs bent down about 4mm and you’d be much happier with your numbers. 50% cross and left and 44% front would be a great starting point.
Thanks Gary, that's kind of the direction I'm going myself.
So for someone who has never done that, is it just kind of a case of trial and error i.e. bend it a little, put it back on the scales and just keep repeating until it's correct? Is there any damage that can occur by going too far and then having to bring it back the other way?

My 15yr old son also runs the same chassis and class as I do. He's 5'9" and 125lbs though, so he has to add around 50lbs of lead to get up the 365 minimum. We were able to scale his kart out perfectly by adding most of the weight around the left side of the seat, mostly towards the rear though, and then adding 10lbs to the front on the steering brace to get the front % up over 44%.

Back to my original question, do the purpose built 4 cycle chassis help in this aspect some how, and if so, how?

Just going off pictures, they don't look much different from the Praga chassis.

Thanks
 
You may want to check the ride heights front and rear. Kart should be basically level front to rear or a tad nose down,

My son is 6'4" and right at about 190 lbs. We can set the seat and engine in the kart and scale out lest the 5lbs across from and rear.

We have our seat moved back and close to the rear vertical plane of the axle. All our lead gets bolted on in neutral locations.
You really should check the ride heights. And then tweek the chassis a if needed.
 
Be sure to set your air pressure and MEASURE your rollout on both sides before scaling. I had a set of new Hoosiers that were off by 5/8" rollout. Would surely throw the cross numbers off by that small amount.
 
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