Measurable benefits to reducing rotating mass?

Curious what measures you may or may not have taken to reduce rotating mass in Karts and if there were measurable benefits to those changes.

I’ve stated this here before…I’m 40 and fairly heavy…down to 212 right now from 234 earlier this year but have a ways to go and then have to figure out how to maintain it through the race season in order to make min weight…so anything I can do to make sure I’m at min weight is going to be great but I’d like to know if there are other measurable results to be had by looking to reduce rotational mass at the same time.

Coming from tracking cars it floors me that nobody publish’s weight of the various wheels on the market. I’d think that would be a selling point but it seems stiffness and heat transfer is much more important in karts.

I see various titanium hardware options for the rotating mass, lighter rotors like Frixa and or the carbon Ven05 brakes from CRG. Kinda interesting that there aren’t more lightweight rotor and or carbon rotor options as it seems to be a big lump of rotating mass.

Has anyone measured weights on chains and compared them?..seems like another large chunk of rotating mass.

Are all the components just soo small and light already that there’s really not much to be gained there?

Thanks for any feedback.

Cost vs. benefits doesn’t make much sense, pretty simple. Ti bolts are freaking expensive. You’ve only got roughly 80 small nuts/bolts on the entire kart, so going straight Ti/Aluminum isn’t going to gain much net savings.

Another fairly large weight reduction is running without a sticker kit/graphics.

If you’re racing in a Masters class you should be right there to make weight already.

Plenty of guys at 200 in masters.

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OP I think was specifically mentioning rotating mass. Don’t think He wanted to know if Titanium floor pan bolts would help. I do see his point to the weight of wheels, brake and sprocket hubs. Otk MXC’s are magnisium and pretty freaking light though. So not sure there is much to gain there.

I guess the context was is spending money on x vs y more beneficial if I’m looking for those last couple pounds.

I also need to get the new Tony Kart on scales because at 217 in my last race, on my old Birel, I was still 25 lbs over.

A couple real-world experiences on lighter weight stuff…

Several years ago there were some lightweight composite/plastic sprockets that were fairly common in some of the air-cooled classes. I saw more than one race lost because a driver ran over a curb and the sprocket exploded, rather than just bending like a metal sprocket.

With chains, it isn’t directly down to weight, but comparing a heavier o-ring chain against a normal DID chain, I’ve heard builders say it’s worth .5 hp. Not sure how much lap time that translates into, but I’ve won races on o-ring chains so I’d rather have the peace of mind that it isn’t going to break as easily.

The brake rotor seems like it could be a fair chunk of weight saved on lighter rotors. I’d be curious to see how much rotor you really need on a kart, given that we have one rotor to do all the stopping. Would it be more beneficial to have more surface area to keep the rotor from overheating or more beneficial to reduce weight on a smaller rotor?

My personal theory is that all of it is such a small savings in weight that it doesn’t really matter much. There may be some things to be gained by reducing weight, but like with wheels, there could be much more to be gained by using a certain alloy of metal etc.

As they used to say at Bontrager when I worked there: “Light, strong, cheap. Pick two.”

I can’t speak to rotating mass but the rule of thumb I was told is that 10lbs is 1-2 thenths (depending on class & track). Here are some weight saving tricks we’ve used or looked at and the very approximate weight savings:

Front bumper mount 1lb
Sidepod mount 2.5lb
Rear bumper mount .75lb
Stearing column .5lb
Seat bolts .5lb

Switching to a light weight lithium battery 3lb
Removing start/battery box 1lb
Remove third bearing 1.5lb
Softer seat (less glass and resin) 1lb
Carbon fiber floor panel 1lb
Smaller gas tank (so you can run less gas and still cover fuel line)
Stickers 1.5lbs

Removing things is free :+1: and the rest you can price out to get $/weight ratio. Seat, bearing and floor pan will change how the kart handles.


On rotors - I can see no need for a heavy ventilated rotor. Karts are light, it’s not like you are heating it up at all and need that cooling. It’s just for looks.

And yes, eliminating rotating mass is very important, more than static mass. You have to accelerate it, and then slow it down. The less mass, the more power from the engine is transferred to the pavement instead of being put into a flywheel.

Narrow tires. 6.00-5 are lighter and less drag than 7.10-5. Enduros can go to 11-5x6 and still handle -maybe better

All of the rotating mass on karts has a small diameter… So it isn’t anywhere near as critical as you might at first expect.

I did the math on brake rotors… I don’t have the figures any more unfortunately… But from memory even with the weight calculated as all being on the outer edge of the rotor, it still ended up being very tiny gains vs all of the weight being static instead.

I think tire roundsness is an important factor to go along with reducing. Drag or brake calipers drag when both piston are not applying the same squeeze to the disc

" Light, strong, cheap. Pick two."

KB words of wisdom…great slogan for t-shirt.

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Can anything be said for the higher speed of the rotating masses (in a kart vs car) as well as their weight relative to the vehicle.

ie did it still seem insignificant when that was accounted for?

Inertia=mass * radius² so any mass you can drop of a rotating object or unsprung mass will lessen the force to accelerate or decelerate it.
steel verses carbon fiber would take 80% less force to accelerate and decelerate. Speed up faster and slow down faster. Aluminum takes 65% less energy to accelerate and decelerate but shortening the radius is more important if length in half it will take only 25% of the force to move it and stop it.

Here is a drive shaft difference for strength steel verses carbon fiber composite for strength and it would be

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When we changed the rear rotor on a CRG from steel to the aluminum ceramic it was worth about 3/4 of a horsepower on our Dynojet chassis dyno. I think it was about 3 lbs. less weight.

Something else must have changed surely? Essentially you’re lightening a flywheel which helps with inertia/acceleration, but not in HP.

How did area under the curves compare?


James, lightening the flywheel does increase the horsepower to the rear wheels. I don’t remember exactly what the curve looked like, it was a few years ago that we did the test. It really just increased the horsepower at all RPM’s, same shape just 3/4 more horsepower almost everywhere. It does make the curve a little more jagged so the power delivery isn’t quite as smooth.