Benzine
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| posted on 17/11/19 at 07:03 PM |
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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
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| posted on 17/11/19 at 07:52 PM |
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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
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| posted on 17/11/19 at 09:37 PM |
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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
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| posted on 17/11/19 at 09:46 PM |
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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
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| posted on 17/11/19 at 10:06 PM |
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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
http://jpsc.org.uk/forum/
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AdamR20
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| posted on 18/11/19 at 09:19 AM |
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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
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| posted on 18/11/19 at 11:28 AM |
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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
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| posted on 18/11/19 at 12:29 PM |
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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
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| posted on 18/11/19 at 03:11 PM |
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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
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| posted on 18/11/19 at 03:45 PM |
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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.
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Fred W B
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| posted on 19/11/19 at 06:54 AM |
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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
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| posted on 19/11/19 at 08:15 AM |
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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
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| posted on 19/11/19 at 10:27 AM |
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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
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| posted on 19/11/19 at 12:10 PM |
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Here's cart 1
[Edited on 19/11/19 by nick205]
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nick205
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| posted on 19/11/19 at 02:51 PM |
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Last comment to add, in cart 1 I got top speed of the day at 21.5mph (local police brought a speed gun to verify). That doesn't sound fast and
I've been considerably quicker on bicycles, but when it's you in the cart moving at that speed, steering and having to slow it down after
the run it seems quite a speed. My next design phase would probably include a roll over bar/cage of some kind.
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starterman
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| posted on 19/11/19 at 08:18 PM |
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First thing I'd do is geared steering. Direct steering as per a go kart is the biggest downfall of most gravity racers. 2 cogs and a chain and
you're away.
[Edited on 19/11/19 by starterman]
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nick205
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| posted on 20/11/19 at 09:41 AM |
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quote:
Originally posted by starterman
First thing I'd do is geared steering. Direct steering as per a go kart is the biggest downfall of most gravity racers. 2 cogs and a chain and
you're away.
[Edited on 19/11/19 by starterman]
Probably a very valid point - steering is often the biggest downfall off gravity racers!
I took the time with my cart 1 to make as close to proper steering as I could, but not with a geared steering rack. If you look at my 2nd photo the
series of holes in strips at the end of the steering column allowed for very minimal geaing adjustment. If I had the skills, tools and time I'd
have either gone for a car type steering rack or a cogs and chain arrangement. I built the whole cart in a few evenings and finished it with 3
minutes to spare before the event started 
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Mr Whippy
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| posted on 20/11/19 at 08:11 PM |
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My trike has very direct steering... Like a bike you have a handle bar lol however when the steering is very light a small damper does wonders and
prevents wobble or isolations developing. If you've heard of a tank slapper you'll know how scary it can get. Ebay sells ones for bikes
their not expensive. Mind too that having a long wheel base also increases stability at speed.
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