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Author: Subject: Soap Box Racer Camber and pin lead
v8kid

posted on 16/8/20 at 12:55 PM Reply With Quote
Soap Box Racer Camber and pin lead

Having decided it would be a good bonding exercise with the grandkids I've acquired some large diameter pram wheels but the track is too narrow so I have to make new axles anyhow.

So if doing this should I add negative camber at the back? The logic is twofold, most of the weight is at the back so most of the cornering force is at the back, and as we all know (?) negative camber adds cornering power, particularly where the CofG is high relative to the track. Also, I have to say pram wheels do not look too sturdy to me so the negative camber will reroute some of the cornering force perpendicular to the axle taking some strain off the dodgy spokes. If so how much? Too far and the spokes will fail due to the weight rather than the cornering force.

By the same logic should the front wheels have positive camber? There is less weight on them and as we all know (?) positive camber makes steering easier, see old non-power-assisted tractors. Again if so how much? Or should it be negative?

I was not intending to add any toe as its a downhill racer after all and scrub is the enemy but what about pin lead? I have to return the grandchildren in one piece to their parents so it would be advantageous if the Soap Box Racer was steerable by 5-year-olds and especially as the whole front axle will steer and it might be a bit unstable. So if the steering point was in front of the axle that would add stability but at the expense of making the steering heavier- so how much lead if at all?

Finally, bearing in mind the returning the little darlings in one piece condition, I will be chickening out and adding linked brakes to both rear wheels. But do I need front brakes? After all the original Austin 7 managed fine without them.

Cheers





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russbost

posted on 17/8/20 at 07:16 AM Reply With Quote
I always feel 5 - 10 dgerees of camber for something like this is a good guide/max, as you say if you go too steep you simply are applying a sideways force to the spokes the whole time whilst reducing the cornering force.

Pin lead, I have absolutely no idea, but why not make some sort of mounting plate at the centre of the axle with several alternative pivot points so you can try different settings easily - from what I remember of my soap box carts from my youth steering was very twitchy with a "standard" centrally mounted pivot

Brakes adds complexity, unless you're going to be going down something truly massive/steep I'd have thought just rears would be fine, just get them to wear crash helmets & padding for the inevitable falls!





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Mr Whippy

posted on 17/8/20 at 09:36 AM Reply With Quote
I'd not put any negative camber in tbh as over bumps that will just lead to twisting on the spokes anyway so your not gaining anything. I think your right about pin lead most karts don't have this but it makes sense.

As for brakes two linked at the back will be great, 4 wheels what? boring! how is he meant to do handbrakes turns?? I've seen quite a lot of crashed Austin 7's.... Just make sure his feet can't fall of an hit the ground or he'll end up with broken ankles. I'd also suggest some sort of rudimentary seat belt and some padding on the steering wheel would be a good idea remember the 2.5mm rule on sharp edges

We had one of these as kids pulled by our bikes like a drunk water skier, so many spills so many cuts

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coyoteboy

posted on 17/8/20 at 11:02 AM Reply With Quote
quote:
Originally posted by Mr Whippy
I'd not put any negative camber in tbh as over bumps that will just lead to twisting on the spokes anyway so your not gaining anything. I think your right about pin lead most karts don't have this but it makes sense.

As for brakes two linked at the back will be great, 4 wheels what? boring! how is he meant to do handbrakes turns?? I've seen quite a lot of crashed Austin 7's.... Just make sure his feet can't fall of an hit the ground or he'll end up with broken ankles. I'd also suggest some sort of rudimentary seat belt and some padding on the steering wheel would be a good idea remember the 2.5mm rule on sharp edges

We had one of these as kids pulled by our bikes like a drunk water skier, so many spills so many cuts


Spent most of my youth using foot-braking on my soapbox, on the hills around my house. Don't think I ever broke an ankle!

I did roll the tyres off the rims several times, and in times when I crashed, I did not want to be tied to that flying piece of wood - the ability to jump off and evade a bush, fence, passing car was vital!





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Mr Whippy

posted on 17/8/20 at 11:14 AM Reply With Quote
yeah our tyres kept coming off too, their not even glued on!
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coyoteboy

posted on 17/8/20 at 11:18 AM Reply With Quote
Certainly makes it switch ends quickly





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v8kid

posted on 25/8/20 at 01:43 PM Reply With Quote
Ok s0 that's it settled then 5% degrees negative camber
Description
Description

and 48mm pin lead
Description
Description

As it is centerpoint steering I don't think the negative camber will make the steering too heavy, quite the contrary I recall center point steering as far too light - but that was a few years ago

I will make up large diameter polypropylene washers for the steering pin to go between the chassis and the axle as I definitely do recall some seriously heavy wear taking place in this area. In fact the pin (wore) fell out at Melville Ave Hill and I ran all the way home with bleeding knees - couldn't have been that bad if I ran Huh! And I was only 38.

So the wooden chassis I see on the internet look quite squalid with simple Lap joints and lots of blocking to make a level floor. As the little darlings will have to pull it up the hill again weight has to be minimised I wonder if lightweight canoe building techniques could be used?





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Mr Whippy

posted on 25/8/20 at 02:16 PM Reply With Quote
Looks fine for a kids weight, I think an adult would collapse the wheels

You could buy a big block of blue foam and carve a body shell with a hot wire cutter (very easy to make) and knife

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v8kid

posted on 25/8/20 at 03:16 PM Reply With Quote
quote:
Originally posted by Mr Whippy
Looks fine for a kids weight, I think an adult would collapse the wheels

You could buy a big block of blue foam and carve a body shell with a hot wire cutter (very easy to make) and knife


Yup its for the grandchildren as a grandad/kids bonding exercise.

Interestingly the wheels have a 1/2 inch dia axle which is the same size that wheelchairs use so I would be mildly surprised if there was an issue there. As for the wheels themselves, they are from that great British institution a Silver Cross pram. Yes you've guessed it they say " Made in Holland" on them. Still, they are almost built to imperial sizes having a 1/2" axle a diameter of 20" and a width of 28mm. I think they will be strong enough, if not its not my knees this time.

Like the idea of a big block of foam there's loads of protection there just not sure about durability. I'll put it to the collective as the kids will have an input to the design as well.





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