Board logo

Soapbox go kart build
nick205 - 23/12/15 at 10:10 AM

Hi All,

After 3 years of 2nd place in my local downhill soapbox go kart race I've thrown out the old go-kart and decided to build another.

I'm after ideas for wheels if anyone's got any thoughts???

Must be cheap and easily available to meet budget and time constraints.

I can weld and have a MIG welder so fabrication isn't an issue.

nick205 - 23/12/15 at 10:14 AM

This was the last one...



Wheelchair wheels up front and MTB front wheels at the back.

[Edited on 23/12/15 by nick205]

blakep82 - 23/12/15 at 10:15 AM

Doesn't look heavy enough more weight, better acceleration?

Kids bmx wheels?
This kind of thing
Sunbeam Challenger 14" Inches Children's BMX Bike blue with Training wheels

[Edited on 23/12/15 by blakep82]

r1_pete - 23/12/15 at 10:32 AM

the bigger wheels would have less rolling resistance, and skinny tyres is the way to go, look at the sprint cyclists setups.

What do the winning carts run?

ETA

How about going with a single front wheel to eliminate any scuffing from toe in/out??

[Edited on 23/12/15 by r1_pete]

nick205 - 23/12/15 at 10:35 AM

The many time winner has large skinny wheels like me, but a lower CoG and 3 pushers for a faster start.

The race is down a steep cow field so quite bumpy, big wheels are definitely the way forwards.


ETA...rules specify 4 wheels, no power. Already thought about a 3 wheeler, but it would be stopped at the start line unfortunately.

[Edited on 23/12/15 by nick205]

nick205 - 23/12/15 at 10:37 AM

quote:
Originally posted by blakep82
Doesn't look heavy enough more weight, better acceleration?

Kids bmx wheels?
This kind of thing
Sunbeam Challenger 14" Inches Children's BMX Bike blue with Training wheels

[Edited on 23/12/15 by blakep82]


Weight is an issue, if you flipped it it would break your neck - a roll bar/cage may be an option for the next cart.

ReMan - 23/12/15 at 10:39 AM

That looks good to me it must be the driver, bad workman blames his tools and all that !

Seriously could you drop the seating position a bit to lower cog
How about reverse tricking, lose a rear wheel for less drag
What are the "rules" ?

garyo - 23/12/15 at 10:45 AM

Is suspension allowed - you must lose a lot of forward energy doing down a bumpy field.

blakep82 - 23/12/15 at 10:47 AM

quote:
Originally posted by nick205
quote:
Originally posted by blakep82
Doesn't look heavy enough more weight, better acceleration?

Kids bmx wheels?
This kind of thing
Sunbeam Challenger 14" Inches Children's BMX Bike blue with Training wheels

[Edited on 23/12/15 by blakep82]


Weight is an issue, if you flipped it it would break your neck - a roll bar/cage may be an option for the next cart.


Roll bar adds safety and weight!
Any more info on rules and track?
Straight run down a field will have different needs to a road with a few corners surely? Big skinny wheels would bend on a corner, small fat tyres would be too bouncy in a field?
Any other rules on size, weight, equipment etc?

What about some basic suspension?!
Even if its just like the old tamiya rc f1 and group c type cars had? The front upright could slide up and down on its king pin, and had a small spring at the top

[Edited on 23/12/15 by blakep82]

nick205 - 23/12/15 at 10:49 AM

quote:
Originally posted by ReMan
That looks good to me it must be the driver, bad workman blames his tools and all that !

Seriously could you drop the seating position a bit to lower cog
How about reverse tricking, lose a rear wheel for less drag
What are the "rules" ?


We've now tried 3 different drivers, me and two BILs - still 2nd place (by fractions of a second).

Did consider lowering the seated position, but it's gone now so a new one to build.

ETA...new one can be designed and built closer to the ground from the outset which may help.

[Edited on 23/12/15 by nick205]

nick205 - 23/12/15 at 10:52 AM

quote:
Originally posted by garyo
Is suspension allowed - you must lose a lot of forward energy doing down a bumpy field.


The rules don't say no suspension, but my guess is the complexity and movement would make it difficult to do well.

It's also part of the day that you have to get the cart from the bottom of the hill to the start at the top so weight can be a penalty in that respect.

That said, we use a lawn tractor to pull it up the hill so it's not all bad.

Sam_68 - 23/12/15 at 11:09 AM

quote:
Originally posted by r1_pete
the bigger wheels would have less rolling resistance, and skinny tyres is the way to go, look at the sprint cyclists setups.

What do the winning carts run?


^^^ This. Rolling resistance is critical, so skinny tyres with minimal tread and pumped up to very high pressure.

There's a compromise between the lower rolling resistance of a large wheel and the lower rotational inertia and aerodynamic drag of a smaller one. Do a Google image search for 'gravity racing' for ideas... but the 'big boys' often run quite small diameter tyres, with disc wheels rather than spokes to limit drag.

I've got a mate who is seriously into this sport, and the technology gets quite trick, to say the least.

ETA: with regard to suspension, the quicker cars do run it, again often quite trick. Some use composite wishbones that are designed to flex and so act as springs, with pushrod-actuated dampers and inboard ARB's!

It can get quite serious...

[Edited on 23/12/15 by Sam_68]

nick205 - 23/12/15 at 11:20 AM

To give some perspective, it's free to enter and there's no prize money at stake.

The art I believe is to build cheap, push hard and hold on tight.

Experience with those running small wheels is bad, they don't make it all the way or break.

As a keen cyclist on and off road, big thin wheels seem to offer the best mix of speed and toughness.

I opted for wheelchair wheels on the front as they're designed for single side mounting. I fabricated the steering with castor and camber in favour of swing arm steering as the latter seems to foil others to good effect.

nick205 - 23/12/15 at 11:21 AM

Race date is e/o June 2016

loggyboy - 23/12/15 at 11:35 AM

quote:
Originally posted by nick205
The many time winner has large skinny wheels like me, but a lower CoG and 3 pushers for a faster start.

Looking at the website only 1 push starter is allowed?
http://www.thewackyraces.co.uk/winchesters-wacky-races---last-sunday-in-june

nick205 - 23/12/15 at 11:41 AM

Loggy

There in may be an issue as the start marshalls don't stop you using more than one pusher.

One of my BILs is an ex army body builder so we have a good start, just need to build/maintain speed all the way.

Mr Whippy - 23/12/15 at 12:23 PM

I wish there was something like this near Aberdeen, looks good fun

nick205 - 23/12/15 at 12:38 PM

quote:
Originally posted by Mr Whippy
I wish there was something like this near Aberdeen, looks good fun


It is excellent fun!

I'd be surprised if there wasn't something local to you, there are lots of them around.

Ours is organised by my next door neighbour who runs a 2nd hand Merc garage - he hasn't won yet and is getting fed up with being beaten

kenton - 23/12/15 at 01:40 PM

A few of the SKCC guys entered one this year at glynde.

https://reachvalleygravitygrandprix.wordpress.com/

What great fun it was, a couple of pics of my winning entry





Kenton

nick205 - 23/12/15 at 02:00 PM

OK - I don't have a tube bender, but I do have space for one if I have to

blakep82 - 23/12/15 at 02:12 PM

quote:
Originally posted by Mr Whippy
I wish there was something like this near Aberdeen, looks good fun


There's one in Brechin around Halloween time every year I think?

steve m - 23/12/15 at 02:43 PM

I was at the same run as Kenton, watching unfortunately, due a broken arm

Both Kentons who won, and Richards were of similar design, and very quick
both had the seating only a couple of inches off the ground, and that is the first thing I would do looking at the very high stance
of the OP's version

Personally I would use 16" wheels for more control

steve

garyo - 23/12/15 at 02:58 PM

quote:

The rules don't say no suspension, but my guess is the complexity and movement would make it difficult to do well.


I've done one myself and used an old ski right under the front acting as both composite lower wishbones. Worked really well as long as you deal with the bump steer, which isn't really all that difficult given that you already have most of the ball joints.

You probably don't want to go too deep though - when you cross the line between having fun and lying in bed thinking about it, you have to double check that you're happy why you're doing it all :-)

smart51 - 23/12/15 at 04:55 PM

quote:
Originally posted by blakep82
Doesn't look heavy enough more weight, better acceleration?


Gravity is an acceleration of 9.81 m/s/s whether your cart weighs 20kg or 200. Bearing friction and wind resistance are comparatively smaller if your momentum is higher, Heavier carts will roll further but not accelerate faster.

nick205 - 23/12/15 at 04:59 PM

quote:
Originally posted by smart51
quote:
Originally posted by blakep82
Doesn't look heavy enough more weight, better acceleration?


Gravity is an acceleration of 9.81 m/s/s whether your cart weighs 20kg or 200. Bearing friction and wind resistance are comparatively smaller if your momentum is higher, Heavier carts will roll further but not accelerate faster.



True and heavier carts are harder work to push start as well!

nick205 - 23/12/15 at 05:00 PM

This is no. 1 boy's go-kart, smaller wheels and a bit slower, but very stable and safe.

MikeR - 23/12/15 at 06:56 PM

Guy Martin did one, if you watch that program he explains the principles.

ken555 - 23/12/15 at 08:53 PM

quote:
Originally posted by Mr Whippy
I wish there was something like this near Aberdeen, looks good fun


Deeside Speedfest

I plan to be going the other way for the hillclimb if you need a tow to the top.

[Edited on 23-12-15 by ken555]

dmac - 23/12/15 at 09:15 PM

quote:
Originally posted by nick205

I opted for wheelchair wheels on the front as they're designed for single side mounting. I fabricated the steering with castor and camber in favour of swing arm steering as the latter seems to foil others to good effect.


Trike rear wheels are designed for single sided mounting, or alternatively re-spoke the wheelchair wheels for a larger/narrower rim.

Dmac

nick205 - 24/12/15 at 08:55 AM

quote:
Originally posted by dmac
quote:
Originally posted by nick205

I opted for wheelchair wheels on the front as they're designed for single side mounting. I fabricated the steering with castor and camber in favour of swing arm steering as the latter seems to foil others to good effect.


Trike rear wheels are designed for single sided mounting, or alternatively re-spoke the wheelchair wheels for a larger/narrower rim.

Dmac


The wheelchair wheels weren't cheap and a fellow LCBer helped immensely by collecting them and delivering them for me!

They are strong and have good bearings too - I fabricated the steering uprights for them. Hard work, but they worked well and made for good steering too.

nick205 - 24/12/15 at 11:00 AM

No. 1 boy's go-kart was derived from this, although a pretend V8 motor would have been nice they were quite pricey at the time...

ETA...I even copied the rear hinged suicide doors as well and my neighbour donated a GRP cart seat to finish it, which really helped!



[Edited on 24/12/15 by nick205]

[Edited on 24/12/15 by nick205]

StevieB - 24/12/15 at 10:32 PM

Could you use a bit of suitably sized steel flat bar across the chassis to act as suspension, in a similar vein to cooper 500 single seaters? It'd be very crude, but coupled with a simple damper might be all you'd need (like a boot lid damper or something).

[Edited on 25/12/15 by StevieB]

ettore bugatti - 25/12/15 at 05:37 PM

Maybe streamlining is worth pursuing?


Wide front track, narrow rear track put into a streamlined monocoque shell?