Spring Powered Truck

You need to consider the maximum energy your spring can hold. It's not a matter of what's the maximum force your spring do, but the maximum potential energy it has when compressed. It's given by the expression

E = 1/2 k x^2 where

k is the spring constant;

x = spring deformation.

Check out the formula. As it's dependent with the square of x, you can store more energy with softer springs and more deformation than with harder springs with less deformation for the same force.

Of course, you'll have to calculate, with the spring options you have, the maximum power (energy/time) you will be available to transmit to wheels.

I'd use rear axle transmission. But, if you guarantee theer's no slipage, makes no difference. If it's a competition, it's better choose the best transmission efficiency at last. Is it possible to connect the spring directly to the wheel? Zero losses.

Well done!

Reverse engineer one of those wind up radios and work from there. They have particularly strong springs in them to get lots of time on air.

You can probably use the parts direct but remove some gearing so your speed is higher.

your range and weight are pretty small you coukld build it into an old plastic toy dump truck pretty easily I should think. May be even include headlights!

The wind up radio idea is a good one. 60 turns of the crank lasts an hour. When the drive belt to the dynamo comes off it takes about two to three seconds to completly unwind.You might need a slightly larger spring for your purposes. Anther wind up motor to investigate would be one of the Mc Donalds pull back cars from ten or so years ago. Pull it back six inches and it would go for fifteen or twenty feet . I always intended to see if I could scale that one up, but never did,sounds like a heck of a way to beat the oil companies.

I had another think about this last night.

I noticed your 'power' limitation is only in in Newtons, not newton metres per second (watts).

Your spring is only limited in the instantaneous pressure, it pulls 7N. The time it pulls (unwind speed) and the torque (Nm) is unlimited.

Selecting your spring is the key. you need a very long spring with a fast unwind rate. the fast unwind speed multiplies the applied force and the 'long' spring keeps the torque coming (the longer the spring the longer is stays wound up).

Given 7N is B-All force (gravity is 9.8) I'd also look at 1 week clock mechanisms and my favourite at teh moment is roller blind springs. These have along runout time with little force.

You can get belts and gears and stuff from junked video recorders, they have heaps in them.

Springing the drive wheels and using soft rubber tyres (not soft enough to deform) will also help by keeping the wheels in contact with the road over surface imperfections.

after looked at the track i realized that the friction factor is very small.

my question is can i counter this by wider the wheel size ?

Yes, you can, because enlarging the wheel will cause the transmission ratio to be longer, thus, reducing slipage. The problem is that if you have some extra load, or a ramp, your ability to overcome the obstacle will be reduced.

Select a good tire to the job. Pick up some scrapped toy cars, and do some experience with them to check the friction coeficient. Select the best combination and install in your truck.

Don't use too soft tires, they will increase the drag too much and you'll loose energy.

With regard to the earlier comments about friction, don't forget that when accelerating, the front wheels will be slightly unloaded & the rear wheels more loaded. Thats why you never see a front wheel drive dragster. What sort of spring are you using? It could be a torsion spring but the best bet would be a constant force spring.

When power is applied to the front wheels, loss of traction would be greater, more power means more loss of traction due to the weight/torque (applied power)calculations. When the power is applied to the rear wheels, the weight of the vehicle shifts to the rear, as well. Ergo, any tractive effort should be applied to the rear-most wheels. Of course, all this depends entirely on the type of tire you utilize. Softer tires mean greater adhesion, harder tires mean less adhesive characteristics. This does not take into account the level of abrasivity of the surface upon which you conduct your test runs. Good luck