1kw electric kart gear ratio, gearbox, driveshaft and weight saving?


I’m new to this forum, so please forgive me if I use it the wrong way or post something on the wrong forum.

Together with 5 other young guys, I’m participating in the eracup (www.eracup.net). A competition where people aged between 16 and 24 build their own electric go-kart like vehicle. A chassis and requirements to get it running are supplied. From there on, teams need to design and create a bodywork and upgrade their kart to their liking to be the fastest.

We’re working with a 1kw electric engine (which we unfortunately can’t change). The standard gear ratio is 11:44 (11 teeth on engine and 44 on rear axle) and a T8F chain is used. We’re looking into changing this gear ratio, but aren’t sure what ratio would be ideal. The rpm of the engine is in between 2700 and 3100 (we haven’t been able to test that very well yet). Our kart is quite heavy, since the chassis is compeletely made out of steal and has the form of a small racecar, so it goes over the legs etc. too. We have considered designing a simple gearbox, to improve both acceleration ánd top speed, our current top-speed is namely only about 27-28kmh, but we aren’t sure whether this would even be beneficial.

Would you have any tips on gear ratio? What is your opinion regarding a 2-speed gearbox? Or does someone know something else that could help us with this?

Thanks in advance!

Kind regards,
Lars - Black Lions Racing Team

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Welcome Lars. Are you allowed make changes to the motor and/or controller?

We have installed a new motor controller (KBL - General Brushless Motor Controller with Regen (12V-96V) (100A-400A) - Kelly Controls) which should allow us to optimize efficiency. Our motor, however, has to remain unchanged internally.

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Damn. My mind immediately goes to overpowering the motor while keeping temps in check…
On gearing it’s hard to say, where is the race being held?

It might be worth having a simple two speed gearbox, but I don’t have the knowledge on how you could calculate it, vs just testing. Do you have a torque curve for the motor?

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You might have done this before. Here is a link for the gear ratio calculator them you should be able to get a rough idea of top speed. https://apx6u.app.goo.gl/GyCV.

I have seen people made a simple 2 speed gearbox, here is the a link just for ideas. He also made a simple transfer box if I recall.

Based on my knowledge of electrical motor, the best way to improve power output is to over voltage it with good temperature control.

A lot of good suggestions here.

With an electric motor in general, more voltage will get you more RPM while more current will get you more power. Depending on how long the races are you may be able to over drive the motor significantly.

What I can see without registering:

Motor: 800W 36V Brushed DC motor
Chain driven rear axle
Battery: 20 Ah, 36V battery pack

Kelly controller - not sure what voltage or amperage specified

Do the rules place an absolute limit on the voltage? 36v isn’t very much, you can usually get away with up to a 50% increase on the motor.

While it does state 20 Ah, can you replace the battery every run? If it’s something like the Formula Student or Formula ASE then it’s really short like an Autocross course and batteries can be replaced.

If replaceable or rechargeable between runs, I’d minimize the cell weight by using the highest C rated cells you can. Hobby cells work good here, but the quality is questionable (don’t ever charge indoors and store cells in a fireproof bag). 18650 cells from major manufactures are reliable, but the C ratings are lower because of that.

If you can answer a couple of these questions I’ll plug it into my spreadsheet to see what it recommends.

I would guess that you’re geared too tall, 2700 RPM isn’t much. For the other Americans on this thread, 1 kW is 1.3 horsepower, barely more than a toaster. Perhaps skip the battery pack and carry an extension cord?

Recommendation 1: shorter gearing
Recommendation 2: much, much lighter weight! If your machine is over 50kg and your driver over 50kg then you’ll be beaten like a drum.

Better yet… if remote control is allowed… I’d go that direction.

At that speed aero doesn’t matter at all so just go as light as possible. I’m not too familiar with electric engines but I know if you get a speed controller and some other things you could overpower the engine without harming it

We’re unfortunately also not allowed to overpower our engine. Personally, we would have opted for a motor more in the range of 5-15kw. It doesn’t have to be too powerful (however, the fastest the better ofcourse) and since it’s a competition for young students it’s understandable that the organisation doesn’t want the cars to go too fast. 1kw is however not a lot… But, as I said, we have to do it with the motor and accu we get sent by them. We have been thinking about creating a gearbox just like suggested here.

But thinking about it again, our car doesn’t even get to the maximum speed of this gear. With the weels of the ground it should be able to go (without load) 40kmh. So if it now only reaches about 27, the only thing we can do is install a lighter gear ratio i guess? Because I assume that it means that 27 is its top speed and that that can’t be changed by gear ratios etc. ?

Coming to the point of weight, that is a difficult topic. The chassis as it is now is designed by the organisation and is really heavy for the engine we have. We haven’t been able to measure it, but it’s all from steel (I’ll put some pictures below) and as you can see it’s not particularly ‘light weight’. The idea has come up to drill a lot of holes in the chassis to save all the weight possible, but that wouldn’t be very good for the strength of it ofcourse. Any ideas?

Bodywork is mandatory, so we’re thinking of making it from carbon or glass fibre. And as it is mandatory, we figured we could just as well make it as aerodynamic as possible

Does electric work different than gas? With our normal karts, top speed is gear limited. There’s a limiter and we try to gear to hit the limiter towards the end of the main straight. So, depending upon track, top speed might be 60ish or 70ish, depending upon gearing.

Could you make it faster by gearing better or does the electric nature make gearing irrelevant?

Well, our max. top speed now is 27kmh. According to the gearing combined with max. rpm it should be 40kmh. This means that our engine isn’t powerfull enough to get it to this speed, so we shouldn’t be able to increase our top speed by gearing at all (if my way of thinking is correct)

There’s ways to calculate how much you could safely drill the frame, but I don’t know exactly how to that. The floor of course would be safe to drill or even remove sections if it.

With a power to weight ratio being so poor, you’re going to need a gearbox is my thought. Two gears.

What can you tell us about the race itself?

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Datasheet motor:

Do you think two gears would be benificial? Even with the added weight? That’s the doubt we’re having at this moment, also since it would only be from 0-27kmh. However, it would be a good chance to do something different from the others

There would be two races this year. They would be something like 15 minutes long (we have not received a lot of details about them) and they would be held on homemade go-kart like tracks. One would be at a school, one in the paddock of circuit zolder. It is unknown whether it will be wheel to wheel racing or based on laptimes

Ok since there’s going to be turns… Live axle (rear differential) is what you need. I doubt that chassis has a lot of caster to unload the inside rear wheel. This will kill momentum on the turns.

I guess it’s hard to say for sure if a gearbox will help because we don’t know the full nature of the track or race. If it’s a longer race with a high average speed, taller gearing might be the way to go. If it’s a circuit that will require lots of acceleration points… then you’ll have to be setup for that.

So… can you make changes to the kart on the day to suit the track? If not then having something that gives you options without having modify the kart in some way would be advantageous.

This is a fun topic :laughing:

I think we’re allowed to make changes, but they can’t be too big ofcourse, since it would be undoable. So making it some kind of modular would be preferable indeed

But, just a wild thought, what would happen if we would put the engine on only one rear wheel? It would save us a large amount of weight, since there’s no rear-axle plus a differential wouldn’t be necessary. Or would it be pulling the kart to one side perhaps and make the handling very bad?

It truly is a very interesting project with a lot of challenges, but a lot of fun too!

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It shouldn’t add too much weight to drive both wheels and the advantage could be pretty huge.

Driving one rear wheel can work as long as it’s not hindering traction coming out of the turns. There are some fun\fairground gokarts setup this way… They are left wheel drive… the downside is that if the inside wheel gets unloaded enough under hard cornering to lose traction you have to wait for it to get back down again before you’ll see acceleration.

So again, maybe the best way is to have a means to adapt the drive. Maybe a center primary driveshaft and a collar system that you can use to connect the left, or right wheels as needed for the track.

Ah, I see, that’s fair. Do differentials suited for kart-like projects exist? Or would we need to design one ourselves. Seeing the space we have, building a differential and gearbox could be quite a challenge, but it might be worth the shot since we have got to try something to be different from the competition

We have thought about shortening the wheelbase, since there aren’t a lot of rules about the chassis itself. You can choose whether you make your own or buy the kit from the organisation. That implies that we can change this chassis to our liking. However, I’m not sure whether it’s even realistically possible to shorten the wheelbase with this chassis