GH80

Some of us in my work are building a soapbox (gravity) racer over the next two weeks, does anyone here have any experience or insider information?

I have designed a basic chassis using some 0.9mm stainless steel folded up, with some light weight 20mm tubing for reinforcement. I was aiming for it to be as light as possible - although some in the team say it should be heavy as possible. Anyone want to back up or counter my argument? It is working out at estimated 40kg currently which I would like to improve on, but there is a maximum weight restriction of 120kg.

I am thinking increasing weight will slow acceleration, as there is more mass to move - acceleration is constant so a heavier object will not fall any quicker, but accelerate slower in the case of rolling down a hill. Also friction will rise, as F = uR hence more rolling friction. Only benefit I can see making it heavier would be to overcome drag at higher speeds?

What about aerodynamics though? Do they really make any difference at 30mph? I had thought about leaving the car bare without any aero, as the projected frontal area is less than what it would be if you gave it some nice aero panelling. Or would a nice slippery shape pay off?

Some pics so far below. Suggestions criticism and general mocking all welcome.

markjmd

iTrader: (11)

You should definitely go for a carbon-fiber monocoque tub, many hours of wind-tunnel testing, and ...


... some brakes

*matthewturb2000*

iTrader: (11)

Surely a smoothed pointy nose cone would make a slight difference?

dpb

Probably down to how smooth your bearings are for all I know

PaulC72

why not make it in Aluminium? you will be able to make a more robust shell using thicker material but it'll be lighter than stainless.

PaulC72

also maybe round off the end where the opening is otherwise the air will go up the inside and slow you down with the drag on the body etc.

It seems logical anyway

vindaloo

What are the rules pertaining to the karts? Can you have say go-kart wheels and tyres and some form of proper suspension? Or does it have to be a bone shaker steered by string?

dpb

There's a million an one by the sound of it

Don't be tempted to fit any electromagnets!

GH80

Rules are - max 300mm ground clearance, max wheelbase 2.5m, max width 1.5m, must have mechanical steering (no string) and max weight 120kg.

We have some aluminium but not much in the larger size. we have hundreds of sheets of stainless, MD has said we can use whatever is out in the factory (no ordering in special parts or material)

We can do whatever we want with the wheels, max diameter is 550mm. looking to get some bike wheels.

We are going to round off the nose cone similar to the main body, haven't drawn it in though. I am still unsure if it makes a significant difference to be honest.

yabbadoo4

iTrader: (8)

needs a big spoiler on the back.

yabbadoo4

iTrader: (8)

on a more serious note, having the front covered must make a difference surely with aerodynamics? and if weight makes no difference why is there a limit? saving weight in a powered vehicle makes obvious sense but going down a hill having more mass gives more acceleration does it not? im no expert mind you lol.

dpb

Dt swiss make some very smooth hubs , I have them on my tt bike

MOTORS S GT

iTrader: (2)

The clue is in the rules, MAX weight, not minimum, when we built one as apprentices years ago it was to get as close to max weight as possible, as its mass you require.

ditchmyster

iTrader: (7)

And some stickers.

Oh and let the fat bloke drive.

Used to make loads of these as a kid, Big wheels at the back small ones at the front.

dpb

Or is there a max weight, including driver ?

MOTORS S GT

iTrader: (2)

Should be including driver.

Any idea the length & nature of the course your running down, angle of initail starting ramp etc ?

Carnut

Firstly if you want to know how important aerodynamics are at 30mph there is a simple test, put your hand out of the car window at 30 and you can feel the affects. With regards to the weight, it has a lot to do with the track, weight may be great in a straight line but in the twisty bits light is your friend.