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

I believe there are some live axle setups out there for gokarts. For sure building a gearbox and differential would be challenging.

OK this is key… It still has to meet certain specifications correct? Do you have those specs?

We have a rulebook, I’ll link it in a minute. However, before changing this chassis, I’d have to check with the organisation. It’s there first season and as far as I know everyone drives with the kit. I know there intention was letting us design our own frame eventually, but whether we actually are allowed to change it this year is a little uncertain. But according to what they said up till now, it should be allowed

ERA+Cup+Rules+2021.pdf (406.1 KB)

That’s a lot of rules for a pretty slow vehicle, but the journey is the reward. No min weight specified, that’s pretty significant. I saw that the roll bar materials and size were determined… but didn’t see anything regarding the frame?

Overall, the driver’s body build and ability is going to matter more than the vehicle design, and maybe that’s the point. You need someone light and fast behind the wheel of this thing.

No machining or drilling of the casing is allowed. T1.3. Motor cooling is only permissible using passive or forced air, without any prior energy input or power from batteries other than the main vehicle batteries.

What about adhesives, straps or clamps :smiley:
You could strap a series of copper fins to the body of the motor… although I’m not sure if cooling will be a concern…

A 6 mm diameter hole should be drilled through any solid floors adjacent to the
batteries to allow height measurement. T4.2.
The driver’s seat including any padding must be at or below 100 mm from ground level. A six mm hole should be drilled through the base of the seat to allow height measurement.

Consider having ability to offset batteries and driver seat to one side or the other in the event that the track is an oval, or has a heavy dominance to one direction at critical corners.

T7.9. There must be a solid, rigidly mounted, bulkhead forward of the driver’s feet, with 200 mm depth of foam with a compressive strength of 300 – 700 Kn/m2* affixed to the front side of this bulkhead, to protect the driver from frontal impact. Any material forward of the bulkhead
must be easily deformable. This must be available for inspection at scrutineering

OK there’s your nosecone specs, for aero this is probably the most important part… But overall aero might not big a big factor, I’d obsess over making it as light as possible before worrying too much

I guess cable power loss might be something else to consider. Minimum cable length runs, appropriate gauges.

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I agree, find the smallest and lightest driver you can with that weak of a motor and battery.

I’d personally recommend structural fiberglass for the body work, unless you have a budget for cf. Good video comparing the plus and minus attributes of each.

For bodywork, go to the fabric store and go get your hands on some nice thin mesh or polyester/Spandex blend - it’ll be FAR lighter than carbon or fiberglass with a bucket of heavy resin dumped into it!

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In the SCCA class Formula 500 they use 500 cc motor cycle engines, but to save weight and space they have a simple counter weight and spring CVT transmission. This allows them to achieve the variable gearing for low speed acceleration and better overall top speed.

There are smaller versions for kart and snowmobiles:
https://www.ebay.com/itm/221558481260?_trkparms=ispr%3D1&amdata=enc%3AAQAFAAAAkATmepnMl8rxeGObTzS1AleCBs3GTpfGvF8iMXZnDrgQX%2BeT6lzIUM679MFAwY8CSH43rfT7orAiGLmijnI7zI82yDT%2FJdkRQHJX%2FHIG1rS56laBwLTa4CtOndEiHTMHVnIZPYNT%2FDSGnjUE7F3knCQGC2YUF%2Fvl2AgLkS%2BgM15NiX49ScATZbHX8rpDaBv6Iw%3D%3D&chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=221558481260&targetid=1192081308624&device=c&mktype=pla&googleloc=9026919&poi=&campaignid=12873833227&mkgroupid=120757694159&rlsatarget=pla-1192081308624&abcId=9300536&merchantid=101687977&gclid=Cj0KCQjw4v2EBhCtARIsACan3nzhwvV6TLZcxFy2lTnSLCOWEeHm0QwtXvGw345CuhTmyHtc0GL4_vAaAijBEALw_wcB

You may have to play around with the springs and weights to get you what you need out of it for your 1kW motor, but I think it will do what you need it to.

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Something that is as light as possible. Don’t need it to be particularly strong like carbon fiber as it isn’t going to expedite large forces. Fiberglass can be light if done well but a lot of the time people build up too much material and it ends up being heavy. Does it have front brakes?

There comes a point where an engine may run out of power to continue accelerating even if it hasn’t hit its limiter. If you have 2hp, and the friction and forces acting against you equates to 2 hp, you won’t accelerate.

@LvBrussel i think beyond everything else you want to make it as light as possible. That wheelbase looks pretty long to me, seems like it could bind easily In turns. I think shortening may be beneficial but ask some of the real gurus here. Is there anything against putting pedals and a chain so you can power it with your legs…?
CVT transmission is certainly a possibility, don’t know much about them but they seem good.
Get the lightest driver possible, that’ll be huge.
Does the steering work like a normal kart where when you turn the outside wheel goes up? If not you could look into that.
Again, as light as possible, carbon is heavy compared to some things that you could use as aero. I think a transmission could give you a huge advantage, or maybe nothing, but it’s for sure worth a shot and is probably your best chance at finding a big advantage.

It would be so cool to just go full bonkers on a kart. Too expensive unfortunately. The idea of the live axle is intriguing, how do you think they would make it, since it is RWD

I actually might be getting terms mixed up. I thought a live axle in karting terms meant that it had a differential… but I don’t think that’s the case.

To answer the question, one way a differential has been implemented in karts is via special sprag hubs. Of course if you have two engines, it’s a lot easier as you can essentially just cut the axle in half, provided it gets supported appropriately.

Basically a clutch in the hub itself, correct? I think at that point it may not be worth it, as you may lose efficiency to all the middle men between the motor and ground, and it hardly has enough power at 100% efficiency. Some sort of Dino or torque test at the wheels with a transmission vs without a transmission would be interesting

It’s a sprag clutch, there’s no loss perse. Not as much as they would be from wheel scrub I suspect.

Fair point. There’s no moving parts unless it’s engaged, in which case it doesn’t matter

Looks like the frame is “as supplied” or “be build with a kit-car kit
bought through ERA or build to the exact same specification.” I would take this to mean the same dimensions and materials.

There is no spec on the speed controller other than it cannot boost the max voltage of the battery output. I would focus on the most efficient controller you budget would allow and not run any electronic off of the main battery. Sort of like an RC Car setup, where the main battery powers only the motor and the controller and servos run off an auxiliary battery. Batteries get heavy quick, so might want to limit your use of them.

Lightest driver possible is a must. Many of the safety regs sound heavy enough that I would try to save weight everywhere you can.

Looking at the spec sheet you posted on the motor, your best usable power window is between 1100 and 2800 rpm. You should be able to ball park a gear range for optimal acceleration and top speed.

I still think a torque converter can widen this window for you for max torque and top end speed as they go into overdrive. It will have to be adjusted to match the output of your motor/controller combo (meaning engage at a very low rpm/peak torque and max out ratio change before your usable power drops off). Increasing the size of the pulleys could offer a wider ratio of change as well. Changing belt to a wider belt and softer drive pulley springs to net a very low belt engagement rpm along combined with changing the spring rates and pre-load tension on the driven pulley will adjust the rpm window from lowest to highest ratio. Lastly you can change out the axle sprocket ratio to match the track layout. It gives you the gearing variability and tune ability in a light-weight compact package.

I’m sorry, it have been a busy few days filled with exams. But wow, so many great ideas from you, thanks!

@Charles_Kaneb unfortunately we have to make use of “rigid” bodywork, so fabric won’t be allowed. We have considered using thin aluminium plates, but as far as I know, that would still be heavier than glass or carbon fibre.

@GregF One of my first ideas was a Cvt actually! But we didn’t know enough about them to be able to confidently tell whether it would be advantageous when used in combination with an electric motor. On top of that, we couldn’t really find one suited for 1kw/1hp, but there is definetely potential

@E13 The steering doesn’t work like a normal kart as far as I know. The frame is far too rigid for that I think. We only have one rear brake, just fixed to the axle.

@KartingIsLife Cooling indeed is one of our concerns. At least, the organisation has told us that it gets hot quite fast, so that would be a good idea!

Then use cardboard! It floats, carbon fiber composites sink.