Gears in a non gearbox/shifter kart?

I was trying to explain to my son how the engine revs down under hard braking but I was having trouble explaining it in that I don’t really understand how a kart works in terms of gearing.

If I am not mistaken your typical non-shifter kart does in fact have gears? The difference is that there is no transmission per se. The engine is directly connected to the chain which is directly connected to the axle.

So, as you slow down the revs drop because they are being pushed down by the slowing of the axle? When you hear that big whirr from the engine under braking, it’s the kart downshifting?

The centrifugal clutch is what controls the shifting? If there are gears on an x30, how many are there, 2?

Non shifters (aka gearbox karts) have a single (but adjustable while between sessions) gear ratio determined by the number of teeth on the sprocket on the motor and axle.

The vast majority of these karts have a centrifugal clutch on the motor which allows the kart to stop while the motor continues to run. The basic principle is the same as a moped or chainsaw.

How a centrifugal clutch works:

The speed of the motor will stay in harmony until the assembly is slowed enough for the clutch to disengage. Up until that point, they are locked together.

Adjustment of the gear ratio comes in the form of different sized sprockets that you can change on the motor and axle.
There are kart motors that run without a clutch, but they are not very common. Those karts have to be push started and seizing a motor is an interesting experience.

I think you’re getting a little off track with that. In a lot of engines nowadays, there are gears in a gear case. However, they don’t affect final driver ratio–at least you can’t adjust their size in most cases. Most 50-100cc engines that are air cooled do not have gears, aside from some TaG engines such as the KA which I suppose you could argue the starter ring gear is a gear…

‘Non-shifting’ gears in say a Rotax or an X30 are either to balance the hi-revving crankshaft, power the water pump, I guess in some 4 strokes an oil pump although I have yet to see one at most kart tracks.

The whirring you hear as you slow into a corner is the engine braking while the clutch is still engaged, just like you would in any car or any engine, and then as the clutch disengages the noise changes slightly… This should happen extremely fast, however, so I suppose it is mostly just the noise of the engine returning to near idle as you release the throttle.

Because karts have a centrifugal clutch, the engine Is connected directly to the driveline for most of the ‘on power’ portion of your lap, and theoretically disconnected for a decent portion of the corner as you are off the gas, depending on the RPM of the engine.

To be clear: a centrifugal clutch doesn’t ‘shift.’ It simply engages at a set RPM, although in many clutches over time you find this engagement can never be exact, although with heavy-duty clutches it is more consistent.

Ok so it’s just one gear. The shifting noise is actually just the clutch disengaging. Thanks!

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A “shifting” noise where you have a wave of sorts is probably from cadence braking near the limit. The clutch is usually still engaged at that point unless you drop the motor rpm below the stall point for long enough for the shoes to retract.

Yep the noise I am referring to is when you brake really hard.

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Dom, I don’t get to participate much on this forum but I thought this was an interesting topic. To be clear there are two types of karts 1) karts with a single speed/no gearbox and 2) karts with multiple speeds/gearbox. A kart with a single speed can either have a clutch or not.

In the early days, most karts were direct drive, meaning the engine was directly connected to the axle by a chain. In order for the engine to run the kart had to be moving and vice versa. Later clutches were used so that a kart could be started while standing still. There are dry clutches and wet clutches. Wet clutches can engage at higher RPMs but tend to slip more, as dry clutches can lock fully. Clutches were very useful on US tracks which tend to have narrower pavement and tighter hairpin type corners than European tracks, which tend to be much wider and have higher speed corners. For many years there was a general rule that US karts used clutches and European karts stayed direct drive. There was an argument that direct drives were more efficient and could put more power into the axle, the rotational weight of the clutch would slow down the engine which did not allow as many RPMs. This is pretty much fact as the highest revving clutched engines reach about 14,000 RPM and the highest revving direct drive engines reach up to 20,000 or more RPM.

When you brake a clutched engine if you brake too hard, you can actually turn the engine off. And you want to keep the clutch close to initial engagement. So the fastest way to drive tends to be braking and throttling at the same time. Honestly I don’t remember hearing much of anything peculiar when slowing down.

On a direct drive kart its a totally different story. Because you can actually stop and restart the engine during braking. During very hard braking it is not uncommon to intermittently turn the engine off. You will hear a very distinct “chirping” sounds as the engine stops and starts rapidly during braking. The advantage is that you can brake “very hard” and that you can control the direction of the kart under braking much better, it is virtually possible to turn 90 degrees without losing much speed at all. We used to recon this to watching UFO’s. There was a move made famous in the 80’s and 90’s called the “Swedish flick” in which you could actually turn the kart the opposite direction during braking (i.e. turn right, before a left hand turn), and then set the kart in a single motion to pivot around the direction you needed to turn, and the kart would instantly be set off in that direction. This makes a lot of very strange sounds for sure and it is very entertaining to watch! However this is not possible in a kart with clutch.

Later they added starter motors to karts and virtually all karts got clutches, direct drive pretty much disappeared. This was in an effort to reduce the cost of engine tuning and was packaged with a lot of other things (water cooling, larger bores, lower RPM, pop off exhaust valves etc…). In theory engines running at lower RPM need to be rebuilt less often, can have bigger tolerances and are not as sensitive to casting deficiencies.

Very recently, I think just last year, direct drive engines were reintroduced to karting. In general, the more complex starter engines are just a lot heavier, and not very fun to drive compared to the old days.

Then there are gearbox karts. They definitely shift gears up and down! Just like a motorcycle. So you will hear them downshifting into a corner and upshifting on the way out. The gears are generally close and shifting happens pretty fast. They have a clutch but it is operated by the driver, not centrifugal force. So when the clutch is locked it is also for all intents a direct drive system. The clutch is not used between gears because the gearboxes are synchronous and sequential just like a motorcycle and the shift directly from one gear to the next. It is not possible to jump gears from 3rd to 5th for example or short shift. You have to pass through all the gears so during a given lap, you shift a lot of times! You will hear this very distinctly and it will be obvious!

I hope this helps.


THAnks. Very interesting. This also adds some color to the whole rotating the kart while braking driving style that has gone out of fashion.



YEP! Thats Garda and its exactly how we used to drive em! Real kart driving :slight_smile: (in fact we used to actually call it “riding” the kart because it was so intense!) Thats Gandolfi going into the 1st turn at Garda at about 45 degrees at the European Championship 1994. Trulli, Manetti, Orsini, Pantano and Gianniberti were all racing that year. It was beautiful!


Can we do that again nodaway ? In a direct drive cathegory like the OK. I have read that first time in the book how to master the art of kart driving but I didn’t test it yet.