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Author: Subject: Gravity racers
Benzine

posted on 17/11/19 at 07:03 PM Reply With Quote
Gravity racers

Anyone of this parish (classic forum saying) been involved in gravity racing/soap box races? I've moved to a new village and they have one every year. There's a single jump involved (less than a foot high), otherwise it's smooth road.

Tips on a making a fast racer? It's a short course, winning times are usually sub 20 seconds so aero might not be a massive factor.

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JAG

posted on 17/11/19 at 07:52 PM Reply With Quote
Never done it - but I've often thought how I'd do it

Aerodynamics is a very difficult thing to get right as an amateur so I agree with you about forgetting it except for one thing; keep the frontal area as small as possible and the wheels narrow.

After that I'd focus on parasitic losses or rolling resistance. Big wheels and tiny wheel bearings will help. As will brakes that don't rub or drag.

After that - add lightness (as someone famous once said ).





Justin


Who is this super hero? Sarge? ...No.
Rosemary, the telephone operator? ...No.
Penry, the mild-mannered janitor? ...Could be!

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SteveWalker

posted on 17/11/19 at 09:37 PM Reply With Quote
Judging by some of the videos, keep the centre of gravity as low as possible and maybe add a bit of width and length for stability - width to keep the CoG within the wheels and so stay upright when turning and length so as to make it less likely that you get into a fast steering wobble (and pilot induced oscillation!)
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fishywick

posted on 17/11/19 at 09:46 PM Reply With Quote
Currently watching Guy Martin and the Worlds Fastest Tractor. I think he did a Worlds Fastest Gravity Racer at 85mph ish, so that's your target!
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adithorp

posted on 17/11/19 at 10:06 PM Reply With Quote
quote:
Originally posted by JAG

After that - add lightness (as someone famous once said ).


Not in this case. You want max weight and min frontal area.





"A witty saying proves nothing" Voltaire

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AdamR20

posted on 18/11/19 at 09:19 AM Reply With Quote
Yeah, about 200kg all in is a good target. Must be strong enough to withstand the jump!

100% Ackerman quite important to minimise scrub, as is steering that isn't too sensitive. Low CoG as has been said.


I spent quite a bit of time researching these recently (there's good info on the web) but then decided to build a pedal car instead

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HowardB

posted on 18/11/19 at 11:28 AM Reply With Quote
if you want to learn from other peoples bruises then watching the youtube of RedBull soapbox racing is worth it,. there is everything in that for all types of racer.
Personally one of the interesting features of the winning (and crashing) carts was the level of sophistication in suspension.
Also I have experience of steering system failure where the steering arms are in compression and have buckled on landing!





Howard

Fisher Fury was 2000 Zetec - now a 1600 (it Lives again and goes zoom)

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Mr Whippy

posted on 18/11/19 at 12:29 PM Reply With Quote
One thing to remember when using bike wheels is to use the BMX wheels with their 14mm axles and reinforced rims, not the normal bike ones which are far too weak. Another point I see many not take into account is the camber of the wheels you must build in so that the king pin center line points at the tyre contact point, it seems 15 degs caster is a good target too.

I'd recommend you go to the site Atomic Zombie and purchase their plan for the StreetFighter QuadCycle, costs £14 to download and use that as your basis or at least a general guide if say you wanted to add suspension etc. Just leave out the drive train etc and you have a perfect base for a scary fast machine



linky


I'm almost finished their StreetFox Tadpole Trike and if the plans were £50 it would still have been a bargain as it showed me loads of things I would not have taken into account if I just tried to design it all myself.

Btw, one thing I learnt is to stick to only using BMX forks and heads (the steering part of the bike frame) and avoid any front suspension mountain bike forks which are not usable for the atomic zombie designs.

I'd use at least 2mm wall thickness for the frame of a gravity racer instead of the specified 1.5mm for the pedal version, maybe even more considering the jump...

Oh and one other thing - steering damper... fit one unless you want a tank slapper, you can get them off ebay






[Edited on 18/11/19 by Mr Whippy]

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Benzine

posted on 18/11/19 at 03:11 PM Reply With Quote
Thanks for all the replies! I think you're allowed a push at the start for a short section. Would light weight be best as it's such a short course and it would help the pushers get the initial speed up quicker?
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coyoteboy

posted on 18/11/19 at 03:45 PM Reply With Quote
quote:
Originally posted by Mr Whippy
[Edited on 18/11/19 by Mr Whippy]


Pedals look a bit far from the saddle there!

Recumbent always look incredibly hard to get uphill - you see them struggling up to lights where on a normal bike you'd be swishing along at 15mph.





Report your local potholes, it actually works!

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Fred W B

posted on 19/11/19 at 06:54 AM Reply With Quote
have a look at this thread, after page 1

https://www.gt40s.com/threads/gravity-racer-take-iii.52915/

https://www.gt40s.com/threads/gravity-racer-take-iii.52915/

[Edited on 19/11/19 by Fred W B]





You can do it quickly. You can do it cheap. You can do it right. – Pick any two.

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cliftyhanger

posted on 19/11/19 at 08:15 AM Reply With Quote
Having spent/wasted soem time watching the redbull stuff, one thing that became obvious is that even well-designed carts often had very poor bump-steer. That and as already said, steering that is way too sensitive leading to terminal tank-slapping. Castor angle will be important...
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nick205

posted on 19/11/19 at 10:27 AM Reply With Quote
I've built and raced a couple. I should highlight the race was local to me and held in a field, aerodynamics didn't come into it.

Cart 1
I built from 25mm RHS box welded together and clad in 6mm ply wood.
Wheel chair front wheels with Ackerman steering and used a set of bicycle handlebars.
MTB rear wheels with MTB V brakes on the rear wheels only, brakes operated by bicycle brake levers on the handlebars.
Centre of gravity was too high and given the chance to redesign I'd find a way of lowering it.
No suspension, but then there were no jumps to negotiate.

Cart 2
Simple wooden structure built for my sons to use.
Simple swing bar steering operated with a rope.
6mm ply body, built around scrap bits of 25x18mm pine.
Cheap inflatable wheels (wheelbarrow) from Screwfix.

Cart 1 was very fast and got 2nd place 3 years running.
Cart 2 wasn't so fast, but was very stable to avoid kids rolling it.

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nick205

posted on 19/11/19 at 12:10 PM Reply With Quote
Here's cart 1







[Edited on 19/11/19 by nick205]

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