Engineering Student Building 250cc Road-Kart! Need Advice

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Angus

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Intro

hi there, my name's Angus, and I'm new to this forum. I'm a second-year engineering student in Canberra, Australia. I joined because I'm planning on building myself a road kart in the upcoming uni holidays, and I need some advice.

The kart is not intended to be a competition racing kart, just something that handles well and goes fast. I'm building it to get some practical experience with making things rather than just designing.

The Kart

I'm copying the frame design of a production 125cc shifter-kart frame, namely the Tecno SS30. I'll be making the frame myself out of mild steel.

(Available here: http://www.tecnokart.com/tecno3/chassis_ss30_ss32_ing.html )

I'm going to buy a 250cc twin from my local bike wreckers. The power output of these engines is roughly 35hp when new, but the power I get might be lower than that because the engine will be older.

For the wheels, tyres, bearings, axle etc I'm probably going to end up buying proper racing kart bits.

My Questions

I'm planning on making the frame out of 30mm outside diameter steel tubing. How thick do the walls need to be?

Where the production frame is bent, could I cut those sections and weld them at an angle instead? Would this affect the handling, and if so, how?

Axles are bloody expensive. Can I just buy a 50mm tube from a local metal shop and use that instead? (What wall thickness would I need?)

Do motorbikes and karts use chains with the same distance between the teeth? Is there anything to stop me using a rear sprocket designed for karts with a motorbike chain and front sprocket?

What are typical camber and caster angles for 125cc shifter karts? I don't want to make mine adjustable, because it's too complicated. So I need to get it right first go.

Do I really need front brakes? They look like they'll complicate things enormously and be fairly expensive. Will going without front brakes make my kart just slower stopping, or will it actually be incredibly dangerous? (Bear in mind that I could be going up to 150km/h or 90m/h.) I'm happy to have the braking performance of a regular large sedan rather than a super-fast open-wheel racing car.

Do motorbike sprockets and brake discs use the same mounting geometry as kart sprockets and discs? If I buy motorbike sprockets and discs, will the bolt-holes in them line up with the bolt-holes in a 50mm-axle shifter-kart sprocket carrier?

Should I be putting seat-belts on this thing? From what I can tell, no racing karts have belts, at all. I find this very, very strange. Is it actually safer without belts somehow?



I'm sure half of what I've asked is stupid noob-questions, so please bear with me. Thanks in advance for any help or advice.

Angus
 
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jace

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i wouldnt use tube for an axle it would porbally bend, and also the seatbelt thing. racing karts do not use a seat belt unless they have a roll cage. without a roll cage you should be thrown out of the cart this is becuase if you were strappd into a cart without a roll cage you would flip and tumble witht he cart and probally break your neck and a bunch of other bones DO NOT use a seat belt unless you have a roll cage
 
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modelengineer

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Hi there,

I'm also a second year engineering student, but I'm at UNSW.

I'm going to buy a 250cc twin from my local bike wreckers
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A better idea would be an entire written off bike. It ends up being MUCH cheaper after you've sold off some parts (which is easy) and you get the sprockets, chain, brake calipers and discs, tacho, all electrics. It's a much better deal than buying an engine from a wreckers.
To give you an idea, I went to a wreckers and asked for a GS-500f engine, they offered me a 1995 model with over 60,000k's for $1500 complete. I went to the auctions and bought a 2007 GS-500f with 9,500k's for $1,000. So far I've sold $650 of parts and have still got heaps left to sell. It's a much better deal, trust me.

Axles are bloody expensive. Can I just buy a 50mm tube from a local metal shop and use that instead? (What wall thickness would I need?)
Click to expand...
A better idea would be 30-40mm solid steel, preferably alloy 4140 or 4130. That way you can get a keyway cut in it and it will be pretty much identical to the axles you buy for heaps. It also comes ground to size so you don't need a lathe to machine it down for bearings.

Do motorbikes and karts use chains with the same distance between the teeth? Is there anything to stop me using a rear sprocket designed for karts with a motorbike chain and front sprocket?
Click to expand...
They use different chains unfortunately. I'd just stick with all motorbike stuff for the chain and sprockets. Much easier to get a hub for a bike sprocket than to get a kart sprocket with the right splines for the engine. Also, you can use the chain you get from your donor bike which saves $$$.

Do I really need front brakes? They look like they'll complicate things enormously and be fairly expensive. Will going without front brakes make my kart just slower stopping, or will it actually be incredibly dangerous? (Bear in mind that I could be going up to 150km/h or 90m/h.) I'm happy to have the braking performance of a regular large sedan rather than a super-fast open-wheel racing car.
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I'm sad to say it, but at those speeds and with a 250cc engine I'd be going with front brakes, others might disagree. While you may be happy with the braking of a large sedan I feel it would be very, very dangerous. You need decent brakes. Remember we're talking about a vehicle which will go from 0-100kph in < 4 seconds.

Do motorbike sprockets and brake discs use the same mounting geometry as kart sprockets and discs? If I buy motorbike sprockets and discs, will the bolt-holes in them line up with the bolt-holes in a 50mm-axle shifter-kart sprocket carrier?
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I reeeally doubt it, but you might be able to find a bike which has the same bolt pattern as your hubs. Motorbikes often use different bolt patterns for the sprockets so there's no "motorbike bolt pattern" as such.

Should I be putting seat-belts on this thing? From what I can tell, no racing karts have belts, at all. I find this very, very strange. Is it actually safer without belts somehow?
Click to expand...

The rule is:
Roll cage AND harness
No roll cage and NO harness

Any other combination is dangerous. The reason is in a kart with no cage you don't want to be smashed into the ground by the kart in a roll, you want to slide away from it. Imagine if motorbikes had seatbelts and they had a crash. Not good.

Hope this helps,
Andrew
 
devino246

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JHen

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If you use 4140 or 4130 you can get away with say a 1 inch axle and 3/8 to quarter inch wall thickness I think.
 
Kaptain Krunch

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JHen said:
If you use 4140 or 4130 you can get away with say a 1 inch axle and 3/8 to quarter inch wall thickness I think.
Click to expand...

Not with a motorcycle engine. Unless you used like 5 bearings. I would go for 1 1/4" solid as a minimum.
 
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Angus

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hi everyone, thanks for all the replies.

Modelengineer, you got an '07 GS500 for $1000?! Wow! Could you tell me more about these auctions? Who runs them, where they are etc? (Do you know what sort of prices 250's were going for?) Can you check if the engines run before bidding?

My local bike wreckers sell what they call 'kart kits', where they take a wrecked bike and remove everything you won't need for the kart, and give you the engine with all ancillaries still in the frame. However from the sounds of it I could get a much better deal in Sydney, even with $100 in petrol costs.



I've still got a few questions:

Is 3mm an appropriate wall thickness for the frame?

With up to 40hp, can I get away with a 30mm solid axle? I've noticed that 125cc racing karts all use 50mm hollow axles. Is this because smaller axles can't handle the power, or because the larger hollow shafts behave better in some other way, perhaps during cornering?

I don't really care about having race-winning cornering, but I do care about my axle not breaking. On the other hand, if I can get away with a 30mm axle that would be great, because all the bearings and hubs will be cheaper.

Are racing wheels made for specific hub sizes? Can I mount the same wheel on a 50mm hub and a 30mm hub?

Thanks in advance,
Angus
 
Fast Eddie

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Glad to see another karter joining the fray! Engineering it is half the fun.

However Angus, I have to warn you of two things. One-you are taking on a BIG project. If this is like a school project, I can understand. But just designing the frame alone takes a bit of chassis fabrication knowledge. Not including designing (from scratch) steering, braking, and drive systems. Two-you are putting on a crazy-big engine for a street kart. I know 35 HP doesn't sound like much, but mate that to a 5 speed gearbox, and you have the recipe for a suicide machine. Especially if the design of the kart itself is not property sorted-out. A heavy offroad kart with suspension geared properly would be a better fit for an engine like that.

But hey, this is DIY Go Karts, right? That's what we do! Good luck and I'm sure you'll get lots of good advice here.
 
bajagokart

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pretty sure if you and a 100 lb kart or however much it weighed were strapped together and it flipped and rolled a crap load of times at 100 km/h, you would probably be dead
 
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modelengineer

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Modelengineer, you got an '07 GS500 for $1000?! Wow! Could you tell me more about these auctions? Who runs them, where they are etc? (Do you know what sort of prices 250's were going for?) Can you check if the engines run before bidding?
Click to expand...

There are 2 main companies which auction off all the insurance write-off bikes in Australia. What you're looking for is a "statutory write-off" which means it can never be registered again in Australia. Because of this it should go for *much* less than the sum of all it's parts since it's only wreckers bidding on them. Hence, if you buy it you can almost always make your money back.

http://www.pickles.com.au/
http://www.manheimfowles.com.au/

Here's the links for bike auction's in Canberra:

http://www.pickles.com.au/damaged/l...=description&ed_search_type=AUCTION&x=38&y=13

http://www.manheimfowles.com.au/fin...odel=&rm=view_results&state=Aust+Capital+Terr

As you can see there are not many bikes available in Canberra at the moment, but that could be because they just had a bike auction (I don't know). Repairable write-off's are not usually worth looking at because you're bidding against people who want to repair and register them. They will almost always bid higher than the wreckers.

I bought mine from Pickles and it was relatively painless. $900 under the hammer + some percentage + admin fees came out to about $1030. I sold the fuel tank for $250, front master cylinder for $100, and I actually can't remember what else I've sold but all up it's $650 back and I still have the front brakes ($200+) and the wheels and race tyres ($200+), so that's all my money back :)



That's the bike I bought. The engine was in pristine condition, just a few small scratches on the engine side cover and it runs like a champ.



In a way the GS-500f is an ideal donor bike because they are SO common, and have been made for over 20 years in one form or another so it's not worth all that much. Coupled with the fact that they are pretty cheap new and they look pretty good.

... & the engine has 47hp, 6 spd, 11,000rpm, twin mikuni carbs, and a GREAT sound. The only downside is it is one HEAVY beast of an engine.
 
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Flexibel_kampfe

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Not to hijack the thread, but does anyone know any similar places in RI/MA/CT? I realize its outside of the AU... Sorry again, I dont mean to hijack the thread, I just figure while its a current topic/post....

PS: what has to happen for something to become a statutory write off? Is it one that has been involved in a fatal?
 
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