Strange question about an AWD hand car

[justinpba]

Hi all! I'm new here. I'm a mechanical engineering student at ODU and I'm designing a hand car. Yes, like the ones that ride on train tracks. This is being designed for the Santa Rosa California Hand Car Regatta and will be pedal powered. The HCR is part art show/part drag race and I'm building it to race. My question is: is there any mathematical advantage to having the car be AWD vs RWD or FWD. There will be 2 riders and I'm thinking of setting one up to power each axle. All other calculations are being made regarding material, thicknesses, stresses and what not to minimize weight and maximize speed, but I don't know how to calculate what the best drive configuration would be. Consider that either setup will require the same parts and weight. Thanks for any help!
-Justin

 

[justinpba]

Anybody? Mathematical formula for which will transfer more power over short distance AWD or RWD is all I'm really asking for.

 

[HowlerMonkey]

Only if you have issues with traction.

 

[mender]

Otherwise the axles can be considered mechanically coupled by the rail.

 

[justinpba]

Okay. Thanks. Now as a follow up, because it will be pedal driven via a chain, for weight distribution purposes would it be best to minimize chain length and position a driver at each end and operate via AWD? My concern that if weight is evenly distributed via RWD that would require placing both operators at the center of the car, but that would increase power loss due to a longer chain. Both operators are of similar weight. Again, any help is greatly appreciated.

 

[HauntedMines]

I think the more important issue is the efficiency of delivering power from the source (the two people driving the cart). At too slow and too fast a pumping/pedaling speed your people will be inefficient at delivering their maximum power. So the goal would be to find the optimum gear ratio that let's them work comfortably and efficiently to propel the cart down the track. Unfortunately, the cart will be going at different speeds (starts at 0, ramps up, etc...): That's where different gears ratios come in, just like for a car engine, to keep it working in it's optimal range.

It seems that for something like this, a continuously variable transmission may work out, since it doesn't have to handle too much force. Maybe you could pickup a small one from a junkyard and hack it to work on your cart?

Another idea is that instead of pedaling, look into rowing motions; it seems to me that rowing motions are more natural for humans and you might get more power out from the drivers. Maybe hook up with a physiology major for some ideas?

To your original question, I don't see much advantage to using 4wd over 2wd, unless you think the track is "slippery". If the latter, add a thin rubber sleeve around the drive wheels (like from a strip of bicycle tire tube) to increase friction. Since it's human-powered, you'll want to minimize energy loss; you'll loose more energy in a 4WD system than a 2WD system

fwiw.