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Old 11-25-2019, 05:26 PM
sjgv sjgv is offline
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Default Rear Axle Fabrication

Hey guys, I'm new to the forum and go karting!

The Setup:

So I want to build my first go-kart as much from scratch as possible. Every tutorial or video I've seen seems like people buy a rear axle kit that includes flangettes, bearings, a keystock and a keyed axle.

The Question:
Do I need to have a keyed axle and a keystock? Can't I just make my own rear axle with some steel tubing and some 5/8" bolts welded to it?
and finally, could I just simply weld the sprocket to the axle?

I'm not really a mechanical engineering guy so I don't know why the axles are keyed and why the kit comes with a stock (I've read online that the concept is that the axle has a groove that is half the width of the keystock and then whatever part you want to attach has another groove with the other half and the keystock mates into both...probably just an extra layer to protect the axle???)

Cheers!
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Old 11-25-2019, 05:51 PM
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J.S.@SMS J.S.@SMS is offline
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Yeah, keys are pretty helpful. I lost one on my drifter once and barely made it home on one tire. They keep the axle from spinning inside the wheels. I don't think steel tubing will be strong enough, axles are usually 1 in. solid steel. And I don't get the 5/8" bolts. Also, welding a sprocket, or anything for that matter, to the axle is ill-advised, and leads to headaches when something breaks or wears out. And a keyway is half the depth of the keystock, that way there us an equal amount of keyway in the axle as say the wheel hub, which allows them to transfer torque without shearing as easily.

You know what, I just realized what you meant by the bolts. I think you're thinking of a dead axle setup, where both wheels freewheel on bearings on the bolts, and one wheel has the sprocket and (usually) band brake drum attached to the wheel. That is the simplest setup that I know of for these karts. With that you don't even need an axle.

I'm just not sure that building a kart from scratch is the best thing to do when it's you're first kart. I'd look around for a cheap frame on craigslist, FB marketplace, letgo, offerup, newspapers, etc. to build off of. However it's your kart not mine, so you can do what you want. Although I am assuming you know how to weld, or know someone who does.
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Old 11-25-2019, 06:25 PM
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I made a live axle out of a piece of 1" OD by 1/4" wall DOM seamless tube. Hand cut the keyways (not full length, just the ends and for the sprocket and brake) with a cape chisel and a die grinder. Bored and tapped 5/8" threads into the ends and use a bolt instead of a nut to retain the wheels.

It works fine. It was incredibly labor intensive and time consuming. BMI sells steel axles for dirt cheap. Probably cheaper than that piece of tube would have cost. I just had it leftover from another project.

I really wouldn't recommend making one.

I do however plan to do a bit of experimenting with a keyless live axle, using loctite 680 retaining compound to bond the sprocket hub to the axle shaft purely out of curiosity to see how strong that stuff really is.
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Old 11-25-2019, 06:46 PM
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Thanks guys!
This was super helpful. I guess I'll be purchasing a live axle kit from BMI for this first build (@J.S.@SMS I've welded some MIG before but I'm undertaking this project partly to learn some stick welding)
(@anickode I don't have much more than just very basic tools so it sounds like I would have a very big headache trying to fabricate that).

I'm looking at a 1" steel live axle (https://www.bmikarts.com/1-Steel-Live-Axle_p_517.html) my next question is, do you guys think this would withstand the power output from a small motorcycle engine (under 250cc) or would a 1-1/4" aluminum do better do you think?


cheers!
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Old 11-25-2019, 07:01 PM
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It'll be fine. Depending how wide your frame is (and thus how far your bearing carriers are), you might consider adding a third bearing carrier somewhere in the middle of the frame next to the sprocket location to help alleviate the overhung load (force pulling laterally on the shaft) resulting from the tension of the chain under acceleration.

However, a motorcycle engine probably wouldn't be my first (or even third or fourth) choice for a first kart build. Even a small one has the potential to push a kart to very high speeds, which can be particularly dangerous without good brakes, proper tires, well-designed steering, and on a frame that you taught yourself to weld on.

Not saying you can't, just saying it may not be the best choice.
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Old 11-25-2019, 07:33 PM
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Hahaha, I appreciate the advice but life is too short for best choices!

I really like the idea of a third bearing carrier, thanks!
Shifter kart or bust, man.
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Old 11-25-2019, 08:46 PM
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Just be smart about what you build. Good tires, brakes, and steering are absolute must-haves for a fast kart.
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Old 11-25-2019, 09:44 PM
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I'll keep y'all updated.
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Old 11-26-2019, 12:47 AM
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I would still not advise welding ANYTHING to your axle. Use keys. Not only can you never remove and swap sprockets when they wear out, but welding on the axle does weaken it.

You mention that you just have the bare minimum and basic tools available. I am not too sure if from that comment you might also mean that you haven't done much fabrication work before, but if that is the case, then a motorcycle engine is probably not the best choice for you. You are going to have to do a lot of fab work to get the engine mounted, get the shifter working etc. etc. etc.
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