my soapbox build, in need of some advice

[audipiglet]

Hey everyone, I've been racking my brains but I can't figure out a good solution to my problem, I've made a wooden soapbox which has 20" bmx wheels which I used in the local soapbox derby, We had two runs down the course which had a slalom then a ramp, Last year's ramp was 4" tall which was fine, this year it was 18" tall! So it had a slalom to go around it with a 5 second time penalty, I opted out of the jump on my first run and got a time of 23 seconds which was the fastest run but with the time penalty I sat in 3rd place, so on my second run I had to go over the jump if I wanted first place, I hit the ramp at 20mph, flew 6 foot which missed the down ramp the other side and hit the road so hard I snapped the left rear axle clean through and bent the from left axle 45°, scraping down the road I lent out of the kart as far as I could to counter balance it and rolled the last 15m over the finish line with 2 1/2 wheels and with a time of 23 seconds and 1st place! After I stopped the front wheel pinged off in a hilarious cartoon fashion and the crowd were going nuts, it was the best, most terrifying 23 seconds I've had for a while.

So my problem is I need to beef up the wheels and axles somehow, they are 3/8" axles which I know is very small and I want to go to about 20mm but unsure how and the bmx wheels are not designed to take a side load so would bigger spokes help with this? Or is there a easier way I should be thinking? I guess folks here have had wheel problems when building electric recumbent bikes and trikes that need to take a lot of strain, this whole project is made of odd bits of wood and metal I've had lying around my workshop so the budget isn't very big, the whole thing has cost £40 so far including a tin of varnish so looking for a low cost solution, thank you for any help!






If anymore pictures would help I can take some more

 

[Chalo]

20mm through axle hubs are generally available, but it's unusual to lace them to 20 inch rims. You'll have to do it yourself, or have someone do it for you. Such hubs are always intended for disc brakes, so they have narrower flange width than most BMX front hubs. 20mm bolts and pins are not especially easy to find in the USA. [Edit: I see you denominated your expense in £, so this last issue probably won't apply to you]

There are also cart hubs, which use common wheelbarrow bearings and cost very little-- roughly $10 each, new. The included bearings are typically 5/8" (about 16mm) inside diameter, but can be swapped out for 3/4" bearings (about 19mm). It's easy to get a high strength bolt in either size to use as an axle.

Much easier would be to get some BMX wheels with 14mm nutted axles, which might or might not be sturdy enough for your application. Those you can simply buy prebuilt in 36 or 48 spoke versions.

Thick spokes won't help. The wider the flange spacing of your hub, the stiffer and stronger the wheel will be when side loaded. Also, a double walled rim is stiffer and stronger than a single walled rim of similar size and weight.

I wouldn't worry too much about the details of the spokes and rims, because the axle is by far the weakest link in your system.

 

[SlowCo]

First off: What a beautiful "soap box"!

As you're in the UK you could source 16"or 17" spoked moped wheels preferably front wheels. They have bigger and stronger bearings and can be mounted to custom axles. Just a few quick examples:
http://www.ebay.co.uk/itm/Garelli-S...014822?hash=item5d5ee4d9e6:g:90AAAOSwo8hTqyDZ

http://www.ebay.co.uk/itm/74-Piaggi...300328?hash=item488c963828:g:2nMAAOxyhodRrRMC

 

[audipiglet]

Thank you for the replys, I like those cart hubs, they look like they could be easily fabricated so I can make them wider to help make them more stable, lacing wheels isn't a problem, and in the mean time I will keep an eye out for some motorbike wheels, there is a motorbike breakers near me so I'll see what he has, I really need 20" wheels because it's 2.5" off the floor which is the minimum it says it can be in the soap box rules, (should of been 4" but I mis-measured the hight of the front axle )

 

[Voltron]

Lovely detailing on the wood work behind the cockpit....
And second what they said.. work on the axle until a wheel breaking is the problem, and keep the rotating weight as low as possible.

Or built a cambering front end for next year

 

[Drunkskunk]

Really nice cart.

Those cart hubs would work. so would wheelchair wheel hubs, or even tricycle hubs. And if thew BMX hubs you have use a cartridge style bearing, you might even be able to throw out their axle and use a grade 8 bolt for the axle.

A 16" motorcycle rim is the same size as a 20" bicycle rim. That might be overkill, and overly heavy, but it's an option.

Since your wheels broke because of the shock they took on landing, you might be able to prevent that by absorbing the impact with some form of shock absorbing device..... Hmmm, if only modern science had invented a device for the absorption of shock.....
( http://www.monsterscooterparts.com/cheap-assed-rear-shock-absorber one of these might do.maybe one for the front, too )

Another option would be to use larger air volume tires. 3" or 4" wide tires will absorb a lot more impact than 2"

 

[Chalo]

Thing about soapbox derby is that you have only a small gravity assist for propulsion. If you go squandering your little pile of energy on suspension damping, fat tires, etc., your car might work great but it won't win. The answer to such issues around here is always, "more motor more battery", but in soapbox racing, everybody gets the same motor and battery. So you gotta conserve.

A stiff, undamped spring on the seat or the front axle might be able to store and release some bump energy, reducing the highest structural loads and maybe even allowing a small net speed increase. Short of that, the best approach is to use supple thin-walled tires at high but not rock hard pressure.

 

[audipiglet]

Thanks for the kind words everyone, there was a lot of work go into making it look good, I am a cabinetmaker so I love working with wood, as gravity is the only power camber would cause more friction so I went for none, bmx wheels are a good, cheap lightweight option, the kart doesn't have to be that light because there is a starting ramp and we had a push start as well, so more weight meant more stored energy, I like those cheap ass shocks drunkskunk and the price is right

I think new hubs so I can get bigger axles is the way to go and since I was going to upgrade them I thought I'd skip 3/8" and go up to 3/4" but can't find the exact thing I'm looking for without spending loads, so I was thinking a bit of brainstorming would really help,


I have found this picture of a homemade hub, I've got a friend with metal lathe so can get the outside discs made so they hold a big sealed bearing, then I can get a big high strength bolt for a new axle, if I made some wider hubs to help with the side load how much wider would you go?

 

[Chalo]

For 20" wheels, I wouldn't want hubs wider than about 90mm, lest the spokes kink at the nipples. Frankly, I don't think the hub width will be a limiting factor for you unless you use very flimsy rims.

The cheap cart hubs I linked to are convertible to from 5/8" to 3/4" by prying out the bearings and replacing with the larger size.