Curved scuba weight orientation: In or out?

I’m using 3# and 5# scuba weights for ballast. They’re slightly curved.

Should contact between the weight and the seat (or frame) be minimized to reduce any possible stiffening? Or it doesn’t matter?

Placed curve-in, there’s a slight gap between the center of the weight and the seat, while the edges are braced against the seat.

In contrast, if placed curve-out, the center of the weight is in contact, while the edges are not.

And a side-by-side comparison:

If curve-out is better, should contact be further reduced with a washer between the weight and the seat?

Note: I have to add 50+#, so the number of potential contact points is high.

I’m curious to see if you get stress fractures in the seat eventually. It seems like the flexing of the seat would be an issue but maybe it’s so minimal that it wouldn’t deform the bolt hole.

Maybe some sort of foam rubber shaped between the seat and the curvature?

Curve out is how we mount ours. We do put fender washer under though just in case. No issues for a few years now


Isn’t mounting weight to the seat normal? That’s what I’ve seen on others, including a tent team.

Or did I misunderstand the potential cause?

I got the impression you were saying that the weight isn’t sitting flat against the back, that the weight edge is making contact as opposed to the broader surface. But from the pics it doesn’t seem like there’s an excessive curvature that hinders it making contact. Also, looks like the scuba weight is coated in some sort of rubber surface so even then the edge wouldn’t scrape and wear the seat.


Is there a reason you have them so high? Usually we try to put them as low as possible to keep the c of g low.

As for the curvature I doubt it makes any difference to the stiffness.

Depends on your situation. Many times you want the weight high to generate transfer and lift, especially with a smaller driver.

You would want weight low if you were very tall and already were transferring too much weight.


Hmm… so for a light driver (that’s 5’8”), the priority is vertical placement over fore/aft balance?

I placed most of my weight under the front of the seat to get the fore/aft balance correct. But that means only 18 of my 53 pounds is on the seat back.

CG is all about averages, so you can move some weight to the top of the seat back and some weight closer to the nose. The average can stay the same while raising the rear CG and lowering the front CG. The main downside to this vs having all the lead close to the CG is that spreading it out increases the “polar moment of inertia” (think of it like rotational momentum). This can make the kart a little harder to start and stop rotating.


Good explanation, like “rotational mass” in a bike wheel.

I mount it curved-in, but use rubber washers + fender washers on both sides, so it’s almost flush along the entire length.

That said, I’ve been using weight clamps when possible: Aluminum Frame and Weight Clamp :: Frame & Bodywork Mounts, Clamps :: Chassis & Parts :: Comet.... This lets you clamp 8lbs to the frame opposite the engine (i.e. better L/R weight balance), and saves a hole in your seat.

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Theoretically , single point of contact should affect seat stiffness less than a full surface or 2 line contact with the weight curved in. Whether in practice it does, I don’t know but we do spend a lot of money getting the seat stiffness right. The downside with single point mounting is that the weights aren’t as secure, which causes the mounting hole in the seat to wear and most importantly, it is uglier. Some racing organizations require two bolts for weights greater than 7 pounds.