Gravity Racer Suspension - Is 3 The Magic Number?

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jimcroisdale
jimcroisdale
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Gravity Racer Suspension - Is 3 The Magic Number?

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Hi all,

I'm designing my gravity racing car for the 2021 UK season. This car will be the culmination of all that I've learned with my previous builds, and I hope to be able to compete with the very best cars out there.

The car will feature a suspension system with a small amount of travel. The roads we race on can be rough in places and suspension will help, but it has to be well controlled. Sloppy suspension seems to slow gravity cars down.

The first design stipulation is that the wheels (front and rear pair) must always face in the same direction. Gravity cars roll best with no toe, no camber, and a few degrees of caster. This originally got me thinking about beam type suspension. Initially a traditional beam with panhard rod and control arms - but this system seems so complicated for what may end up being an inch of travel at the wheels. So I looked at simpler torsion beam designs, before alighting on my idea.It will have caster gain on bump, but that's okay.

Imagine a triangular piece of card on a table. You can lift any of the three corners, and the other two will stay touching the table. The magic of three :-) The suspension works in a similar way - it is essentially a triangle. A corner each for the wheels, and one corner for the chassis mount via a rose joint. A single panhard rod locates it laterally, and shocks or viscoelastic polymer pucks are mounted off the "triangle" to give the suspension travel required.

I've spent a few months looking at it now, and to my eyes it's the simple and effective solution I was hoping to find. However, I welcome your comments and suggestions on this. :-)

Image

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Greg Locock
Greg Locock
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Joined: Fri Jun 29, 2012 11:48 pm

Re: Gravity Racer Suspension - Is 3 The Magic Number?

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Looks OK to me. I'd terminate or raise the structure at the axle allowing a longer panhard. Bump castor really isn't an issue unless you have a driveline.

AngusF1
AngusF1
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Re: Gravity Racer Suspension - Is 3 The Magic Number?

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How did you go with your design? What do the rules say about maximum weight?

I looked into gravity racing some years ago and realised that linear acceleration was determined by the ratio of vehicle mass to aero drag. Experimentation determined that my car was much quicker than my bicycle and motorbike.

Can you pursue a high weight, large tyre strategy?

jimcroisdale
jimcroisdale
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Re: Gravity Racer Suspension - Is 3 The Magic Number?

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AngusF1 wrote:
Thu Jun 25, 2020 2:52 am
How did you go with your design? What do the rules say about maximum weight?

I looked into gravity racing some years ago and realised that linear acceleration was determined by the ratio of vehicle mass to aero drag. Experimentation determined that my car was much quicker than my bicycle and motorbike.

Can you pursue a high weight, large tyre strategy?
Just getting round to this project again, and this solution is looking like the likely candidate.

Most gravity races have a weight limit of 100kg for the car, so building super heavy isn't an option. Then again, some tracks are quite twisty so a heavy car would need a lot of grip (fat tyres) but then you would get beaten by lighter cars with skinnier tyres that roll much quicker off the line.

My new car will have an aerodynamic body based off the NACA 0024 profile, as close to 50/50 weight split as I can, and 12.5in tyres on 8" rims.

tok-tokkie
tok-tokkie
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Joined: Mon Jun 08, 2009 3:21 pm
Location: Cape Town

Re: Gravity Racer Suspension - Is 3 The Magic Number?

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Elegant solution.
How about rubber in tension rather than compression. Rubber rings. Irrigation & other PVC piping systems have nice rubber rings. Possibly too long for your needs.

e36jon
e36jon
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Location: California, USA

Re: Gravity Racer Suspension - Is 3 The Magic Number?

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If you google "bungee cord suspension" (Bungee cord" = elastic cord) you'll see a number of light aircraft solutions. No damping though.

If you google "torsion spring trailer axle" you'll see some solutions that might be ideal for you. The challenge will be a low enough spring rate. You could make longer arms for this style to get the spring rate down where you want it: https://www.truckspring.com/products/Fl ... 5-2-S.aspx The Flexiride brand is showing that they have better inherent damping than an undamped leaf spring: https://www.theuniversalgroup.com/mwdow ... ink/id/63/ Hard to say what's 'good enough'. You see these lightweight trailers on the road and they're not all jumping around, so something is working...

thisisatest
thisisatest
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Re: Gravity Racer Suspension - Is 3 The Magic Number?

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I'm nervous about the panhard bar on a solid axle with only one mounting point. It will steer a bit with travel. If you are using so little travel (which would limit the negative effects of the panhard, i realize), couldn't you use whatever shock or bumper assembly to manage the lateral location? Or twin leading links made of a flat plate to allow some twist...

tok-tokkie
tok-tokkie
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Location: Cape Town

Re: Gravity Racer Suspension - Is 3 The Magic Number?

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Instead of panhard you can use a Watts linkage. Alfa used it on the Alfetta. Here is a video showing it. Beware: the 2 chassis mounts are at different heights.

Tommy Cookers
Tommy Cookers
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Re: Gravity Racer Suspension - Is 3 The Magic Number?

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e36jon wrote:
Sat Aug 22, 2020 10:34 pm
If you google "bungee cord suspension" (Bungee cord" = elastic cord) you'll see a number of light aircraft solutions ....
No damping though .....
yes damping - if you have many strands (twisted - so on deflection giving friction amongst themselves)
as a proper tailwheel light aircraft will have (eg Pitts or WW1)

though the value of 3 point landings is to keep the pilot alive if the designer 'solution' was to have negligible damping
as bouncing on touchdown if already at the stalling AoA won't increase as the Cl won't be increased by the bounce attitude

e36jon
e36jon
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Re: Gravity Racer Suspension - Is 3 The Magic Number?

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[/quote]
yes damping - if you have many strands (twisted - so on deflection giving friction amongst themselves)
as a proper tailwheel light aircraft will have (eg Pitts or WW1)
[/quote]

Instead of 'no damping' I should have said 'not easily adjusted damping'.

Even with multi-leaf leaf springs there is the ability to wrap them to increase the friction between the leaves, increasing spring stiffness and damping. That system still wants a separate shock absorber...

I had mentioned earlier that microcellular urethane 'bump-stops' as used with most modern shock absorbers could be a potential solution. They still aren't tunable but can be swapped easily and are available in a huge range of properties. Check the selection here: https://www.speedwaymotors.com/shop/sho ... 6-566-7564. Girvin Pro-Flex mountain bikes used this system initially (Google images has info-o-plenty.).

Hoffman900
Hoffman900
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Re: Gravity Racer Suspension - Is 3 The Magic Number?

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e36jon wrote:
Sun Aug 23, 2020 9:23 pm
yes damping - if you have many strands (twisted - so on deflection giving friction amongst themselves)
as a proper tailwheel light aircraft will have (eg Pitts or WW1)
[/quote]

Instead of 'no damping' I should have said 'not easily adjusted damping'.

Even with multi-leaf leaf springs there is the ability to wrap them to increase the friction between the leaves, increasing spring stiffness and damping. That system still wants a separate shock absorber...

I had mentioned earlier that microcellular urethane 'bump-stops' as used with most modern shock absorbers could be a potential solution. They still aren't tunable but can be swapped easily and are available in a huge range of properties. Check the selection here: https://www.speedwaymotors.com/shop/sho ... 6-566-7564. Girvin Pro-Flex mountain bikes used this system initially (Google images has info-o-plenty.).
[/quote]

I was thinking along these lines as well reading this thread. Those bump stops have different rates to as they ramp up an infinite spring rate (at least feel that way).

Jolle
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Re: Gravity Racer Suspension - Is 3 The Magic Number?

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I’m very novice about gravity racing, but the weight of the thing looks like it’s small compared to the driver. Can’t you get away with sprung seat? Or, if you build a chassis with the three wheels and steering, and spring the body and seat on that?

AngusF1
AngusF1
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Re: Gravity Racer Suspension - Is 3 The Magic Number?

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It seems obvious to me that with a car weight limit of 100kg, the fatter the driver the better. The car with the fattest driver will accelerate much quicker, and the weight limit is high enough to allow tyres large enough to corner the fatter man nearly as quick as a lighter man. A large-boned individual with dense muscle would do the trick in terms of maximising weight for the smallest drag.

I wonder whether there is a limit on driver "clothing". One could carry ballast in the back pockets of one's trousers - "which was the style at the time"....

Greg Locock
Greg Locock
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Re: Gravity Racer Suspension - Is 3 The Magic Number?

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We used to worry that Panhard was more assymetrical than Watts. As you will see in June (ish) those fears were not insurmountable.

Rodak
Rodak
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Re: Gravity Racer Suspension - Is 3 The Magic Number?

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AngusF1 wrote:
Tue Jan 26, 2021 5:48 am
It seems obvious to me that with a car weight limit of 100kg, the fatter the driver the better. The car with the fattest driver will accelerate much quicker, and the weight limit is high enough to allow tyres large enough to corner the fatter man nearly as quick as a lighter man. A large-boned individual with dense muscle would do the trick in terms of maximising weight for the smallest drag.

I wonder whether there is a limit on driver "clothing". One could carry ballast in the back pockets of one's trousers - "which was the style at the time"....
I wonder why you think a large stone will accelerate more quickly than a small stone, ignoring air resistance.