Axle diameter

Does the axle diameter change how fast your tires turn?
I’m looking at a used margay kart that’s set up for 206 but it has a big diameter axle. I know the the thicker they are the stronger they are. But I feel that the tires will turn slower. Is this true?

No. The axle has no bearing on wheel speed. The tire and the sprocket size remain the same diameter on a 40mm, 50mm, 30mm etc. axle.

Axle size will not effect wheel speed the ratio of the sprocket size and the tire size stay equal if the same combination is used on any size axle.

Now, why we have different size axles is far more interesting and the upside/downside to them all is equally interesting. Maybe, I will cover this one day but I still have not released the first of my content so we will see.

Sprocket size OD (# of teeth) remaining the same would result in the same axle RPM regardless of axle diameter. The axle being anchored to the sprocket hub means that those two rotate at the same rate no matter what. Changing sprockets (front or rear) would change wheel speed, as would changing tire circumference.

I understand that but I’m going to check it out for myself.
It’s just hard for me to believe because I run the same gears in my car, I can change different size axle and change wheel spin. It’s just something I need to see for myself that’s all I was saying in my last comment. It’s just something I need to do that’s all. Because if I can machine something to get it to turn faster I am. I just like doing things like that. You learn by doing it yourself right. So that’s what I’m going to do.

What’s catching you out is how you use the word axle. In the car you’re really talking about the axle gear ratio, which may or may not result in you having to change out the axle assembly (specifically the final gear.)

In a sense, for gearing, changing the axle in the kart is like changing the size of the driveshaft in the car. It doesn’t impact the gear ratio. It’s just a different size shaft that has the same gears.

What you’ll change are the sprockets on the kart, which in a sense is like being able to change the gears inside the axle of the car to modify the final ratio.

Hope this helps.

1 Like

In a car you got splines on the end of your axles to match the gears that you already have in your car in the rear end right.

In a kart you got a sprocket with a bolt pattern on it to accommodate the same sprocket.

This is the same type of method as a spline in a axle in the rear end.

The gear has the same outside diameter as so does the sprocket. So there for the axle should turn faster.
Because the diameter is different on the axle.

Maybe I just need to do it my self it see for myself. Because I’m just not getting it. This is why I like to see for myself.

If you want to go to the work to try it yourself, go ahead, but multiple people here with literal decades of karting experience are giving you the correct answer.

The engine has no idea what size axle you’re running, as long as the sprocket and tire circumferences are the same. The sprocket determines how fast the engine spins the axle. If the sprocket remains the same circumference, it will rotate the same amount for every engine revolution, regardless of how big the axle is it’s mounted on.

A 75 tooth sprocket on a 40mm axle is the same circumference as a 75 tooth sprocket on a 50mm axle.

Okay, we don’t have splines at the end of our axles, so I think that’s where the confusion is coming from. It’s a one piece axle the connects the wheel to the other wheel.

The gear sits on a hub that slides on the axle. No matter what diameter the axle is, the gear will have the same diameter and number of teeth of it no matter which axle you use.

There is only one sprocket on the axle, so the diameter of the axle doesn’t change any gear ratios from the engine to the wheels.

The wheels, no matter what diameter axle you use, are all the same diameter, so the different axles have no way of changing the wheel speeds of the kart at a given RPM, without changing the direct gear ratio from the driving sprocket on the crankshaft to the driven sprocket on the axle.

In this article is my cousin he has many championship in kart racking. More then most people. He told me that the axle does without a doubt change tire speed even if you keep the same gear. He told me that some people just don’t understand simple mechanical physics of the wheel and axle.

I think we’ve all spent too much time on this already, but let’s go ahead and assume that what you’re saying is true. Why on earth would you want you want to go to the trouble of changing axles when, by your theory, you can accomplish the same effect just by changing one of the sprockets?

Also, nice article about your cousin, but he is also incorrect regarding this topic.

I think you probably misunderstood him, or the two of you have some sort of different terminology for these things. It’s essentially like saying changing the drive shaft in your car changes the gear ratio. Or changing the size of a gear shaft (vs cog) inside a gearbox affects it’s ratio.

I can say for sure, he’s right. Plenty of fast drivers out there that have no idea why they are so fast or understand mechanics.

Axle diameter is a handling change.

What he might be referring to is the much debated topic of using a smaller engine sprocket to attain the same gear ratio. Some believe that running the smaller front sprocket provides an advantage… I’ve yet to see evidence of that… only hypothesis so far.

Again, think of the karts axle as being the driveshaft. It has no bearing on gear ratio. The gears set the ratio, in the case of the kart the gear is bolted to a hub which resides on the axle (think ring gear in the promod) on which you have adjustments available from as low as 60 and as high 90 depending on chain pitch. Drive\engine side (pinion in your pro mod) is anything from around 14 - 20 something teeth.

He explained to me the size of the axle allows you to add more torque or power. The smaller the weaker. He also said that is like a 3rd gear. Because the bigger the faster. He told me you put a dot on the gear, and put a dot on the shaft the two are turning but the smaller is not traveling as far as the one on the gear, but at the same speed. Then he told me that if you put a bigger axle in the dot will travel less them the one before it. He said there for the tire will turn slower but go faster in the top end. If I’m understanding him right. That’s the axle will turn at the same speed as the gear. No matter the size of the axle. The difference you see is at the tire not the gear or the axle. He said if this was not true then you would not see so many people complaining about changing axles with the same gear set up. I’m just trying to get an understanding. I’m 7sed to promod dragracing and people telling me all kinds 9f thing that’s not true so I won’t beat them. This is why I got to do everything my self. Because I’m tired of getting lied to. To only way for me to understand it is to do it my self. Or it’s me and I’m not understanding. I just know when I change axles in the promod car not drive shaft. I change tire speed. I just thought it would do the same in a kart.

Everything you mention above that he stated points to gearing…

One scenario I can think of that might throw people off… When people change the axle it’s a handling decision… Without a differential a kart will scrub/bind in turns unless you unload the inside rear wheel. Axle choice (Along with other factors) influences how and when the inside rear wheel unloads and reloads while cornering.

When changing the axle helps the kart “free up” it may feel like the gearing has changed, but it has not. The kart just has less rolling resistance while navigating the turn.

I will say though, generally an axle change will maintain the same axle diameter, but with different wall thickness, metal type or both which is what plays a part in the handling change.

It’s the naming of the parts that’s throwing you off. The kart’s axle IS the driveshaft equivalent of the car. When you change the car axle, the gear ratio is changed. On the kart, this gear ratio is controlled by the sprockets on the engine and axle.

Once you get the kart I think it’ll make more sense. Also consider that to change the actual axle size you will need different axle bearings, sprocket carrier, brake disk carrier and wheel hubs at the least.

Now I completely understand that. I can relate to what you are saying, and I’m sorry for all the doubt. But being competitive like I am with every thousands of a second counts. You can see how I have not got to much trust in what people say. I’m a type of person that if I’m going to do it. I want to make sure I can understand every aspect of what it is I’m getting into. I’m getting into karting for the fun of it. Not like drag racing where I’m in it for the money because I got a big fortune tied up in it. Unlike kart racing you got a few thousand tied up in it. What I’m running I got 227k tied up in my car. That’s just getting it built, and running. Not counting the expenses keeping it running. With one engine in the car, and one in the trailer, and one at shop. 60k each. You can see why I ask question. I build everything myself. Machine, and fab some parts if I need them. For the chassis. Everything is done in house in my speed shop. I’m trying to get out of drag racing. I’m trying to find something fun I just had a little girl so I want to do something that family friendly.

Drag racing seems a thrill but with a baby, you are probably making a good move. Karting can be what you choose it to be, pretty much. If you want to run in super competitive teams that try to find the edge, they are out there. If you want the total opposite, same thing. If you want to fly solo, that works too.

Finding your spot would be important. There’s low-power, low-cost stuff like 4-stroke, all the way up to gearbox 175cc.Oh and then there’s vintage and also superkarts. So many different ways to race or mess around. Money wise, there’s guys who manage to do race weekends for a couple hundred bucks and guys who spend 8-10K a race weekend at the pointier end of the spear, internationally.

The USA is mainly club karting and then regional and national/intl stuff. It is SUPER family friendly at the club level. There is no cooler thing than karting with your son or daughter or wrenching for them, imho.

Anyhow, good luck with your search. If we can be of help, let us know.

1 Like

I think you’ll enjoy karting, especially if it becomes a family activity for you. The atmosphere is generally laid back, especially at the club level. In my years of karting (14+) I have found that the vast majority of people (even the fastest guys on the grid) are eager to help newcomers, and are very open and honest when answering questions. Everyone shares a love for the sport, and wants to see the sport grow.

I understand being competitive and having questions, but karting generally doesn’t have much mind games or people throwing out advice to send you in the wrong direction. At the club level, everyone will probably be willing to help you out if they can. And this forum is for everyone to talk about karting and share advice for others to try.

If you look at just this thread, you’ll have:

  • TJ who’s been racing since before I was born I’m pretty sure.
  • Evan just said he’s been racing for 14 years, which is plenty of experience for this question.
  • James founded the site and has raced for a while as well, I would go with the idea that he’s not here to send you down the wrong path.
  • Ronald I’m not super sure on with his experience but he’s provided good advice before.
  • Dom is relatively new but has learned a lot about karting in his time here with his son.
  • I’ve only been racing for 5 years, so I probably have some of the least experience in the forum, but I try to listen to the people that have been established in karting for multiple times more than me.

In my experience on this forum, these guys will all try to steer you in the right direction and help out as much as they can.

There’s not a lot of “voodoo”, “black magic”, or “tricks” in karting, so people are pretty open and honest about setup and things.

Hell, I’ll give out my full race setup at the end of a race day. There’s nothing to hide really, as it mostly comes down to driving and understanding how a kart works. Maybe I won’t walk over and hand out what gear I’m on or explain my handling issues to a competitor in the heat of the day, but at the end of the day I don’t mind sharing setup things.

Plus there’s basically no money on the line, so people don’t care as much.

I didn’t respond to this before as for some reason I couldn’t see this post specifically, but now it is showing up.

The larger axle allows a higher torque or horsepower engine to be used on the kart. I wouldn’t run a 30mm axle with a 175cc Shifter engine, I wouldn’t even run a 40mm axle with a KA, although it is possible. It does not mean that changing the axle adds torque or horsepower directly.

This sentence is correct, the edge of the axle is not covering the same distance as the edge of the gear, even though they are moving at the same rotational velocity. Due to the difference in radius, the circumference of the sprocket is larger, and will then have a longer traveling distance than the axle. However, the sprocket is bolted directly to the hub, and therefore any rotation of the sprocket will be equally matched in rotation by the axle, so they will always have the same rotational velocity, or rotations/second.

This part confuses me. If you put a larger axle in than the previous axle, then the dot would be moving a longer distance for an equivalent rotation compared to the smaller axle.

You’re getting lost in the connection from the kart axle to the wheel. In the same way that the sprocket is connected directly to the axle, and the axle will only rotate the same amount as the sprocket does, the wheel is directly bolted to the axle, and will only rotate the same amount as the axle rotates.

Let’s follow this through one rotation from the sprocket to the wheel. The kart is running, and the sprocket moves one full rotation. You agree that the axle will rotate at the same rate as the sprocket, so the axle will also move one full rotation. The wheel is directly connected to the axle, in that the wheel can not rotate without the axle rotating the same amount, just like how the axle cannot rotate without the sprocket rotating. So, when the axle moves one full rotation, the wheel will move one full rotation as well. This means that when the sprocket moves one full rotation, the wheel will always move one full rotation to keep up.

If you change the size of the axle, all that you’re doing is changing the size of the hub that connects the sprocket directly to the axle, and the hubs that connect the wheels directly to the axle. Since changing these hubs still connect the sprocket and wheels directly to the axle, it does not affect the rate at which either of these rotate relative to the axle. It is still a 1-1-1 rotation ratio, going sprocket rotation-axle rotation-wheel rotation.

James is correct, the naming of the parts is likely throwing you off. An axle in a kart is similar to a driveshaft in a car, in that you can change the radius of the part and still have the same number of rotations/second. He is also correct that if you see a difference in, say, top end speed at the end of the straight after changing an axle in a kart, that is going to be due to the kart handling better and exiting the corner faster than it was with the old axle. If you exit a corner faster, you’ll carry that added speed all the way up until either 1) you brake for the next turn or 2) you hit a rev-limiter, depending on your engine.