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Post Info TOPIC: Design Questions


Newbie

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Design Questions
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Some back story first. I was on a team in high school from 2015 to 2016 so i have some basic understanding but there are still a few concepts that I'm unsure about. These concepts include:

Shocks or no Shocks

Car Bottom close to road or held up a few inches 

As light as possible or is that not as important 

Fiberglass or metal frame with a skin 

Large diameter tires or small diameter tires 

Fat tires or skinny tires 

Electric motor or electric tire hub motor  

2 batteries or 3 batteries

 

Any advice that you may have on any of these matters would be greatly appreciated. Thanks for all of you help and advice.  



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I do not know whether we will be racing this year but I will give a shot at answering some of these easier questionssmile

Two batteries vs three. Electrical losses are greater with higher currents. If you run a higher voltage, then you can use a lower current to get the same power (amps*volts = Watts). Top lithium teams are running in the 65-85 volt range. The theoretical advantage is there for lead acid teams but the trick would be to find good batteries. Most teams run two 12 volt Optima batteries because they are a proven quality battery. We ran four Deka batteries as a 48 volt system very successfully but we never had the capacity (Watt-hours) that two Optimas would supply. A number of teams have tried using small UPS batteries to make higher voltage lead acid systems. The problem is that these batteries are rated for 20 hour discharge, when we are discharging them in 1 hour. Do not believe what it says on the label. Buy one and test the capacity at the 1 hour rate. The only way to take advantage of a three battery system is if you can build a pack that can deliver the same Watt hours as a two battery pack.

Hub motor vs Chain drive. A chain drive should be cheaper, more efficient and more flexible to gear for the track but requires care and very good design. We have lost races repeatly by throwing chains. A hub motor is super reliable. The most important quality is high efficiency in the rpm/amp draw range that you intent to operate the motor in. I also highly recommend a motor/controller combo that has regenerative braking. There is a great online simulator here that allows you to play with voltage, weight, motor and more. Keep your eye on efficiency, not just speed. https://www.ebikes.ca/tools/simulator.html

Fat or skinny tires. Running high pressure gives low rolling resistance. I have read some theoretical arguments that wider tires should offer lower rolling resistance than skinny tires but I do not know of any go kart tires that will hold shape at 100 psi. On the other hand, on the tight many corner parking lot tracks we often run in the Tampa Bay series, giving up some rolling resistance to be able to carry extra speed through the corner might be an advantage. At the moment, there are no competive cars running wide tires. We are trying wider BMX tires but no answers yet.

Large diameter vs small diameter wheels. The bigger the wheels, the more aero drag. We found more side flex to bigger wheels which hurt handling.  In theory, there might be an advantage to bigger diameter wheels for rolling resistance. 

Fiberglass vs a metal frame with skin. There is a saying that perfect is the enemy of good. A well designed carbon fiber chassis could be stronger, lighter and stiffer than an equally well designed metal chassis. This is probably true for a well designed and built fiberglass chassis with vacuum bagging and proper directional laid glass and epoxy. Every race team for go karts to F1 have limits to their work time and cash. What use of time will give the best results? A huge investment in building the perfect chassis or a modifiable chassis that you can improve as you spend time racing?

As light as possible or not that important. Depends on the track. On a tight track, where the car slows for the corner and then accelerates again, weight will make a difference. On an fast track, like an oval, once you are up to speed, weight is not that important.

Car bottom close to the road or held up a few inches. Rules say a minimum of 1.5 inches off the ground. For handling, which is important on tight tracks, the lower the center of gravity the better. So it is not just the bottom of the car but any heavy parts such as battery and driver, should be as low as possible. For aero drag, there is an optimum height above the ground for the least amount of drag (according to solar race team studies). So if you plan to race on fast tracks and spend some time getting the bottom of car smooth and the bottom edges rounded, there might be something to gain in aero efficiency by raising the car. It is a complicated calculation involving shape, wetted area, surface texture, velocity. A rough calculations for the ProEV Coupe puts it around 1.5 inches, so close enough.

Shocks or no shocks. Again perfect vs good. There is no doubt the vehicle will corner better with a flexible suspension especially with 100 psi tires. However, building and tuning the basic double A-arm suspension is a big time suck. There are a lot of fiddly things to get wrong, like camber curves, bump steer, shock bottoming. On a big smooth oval, a suspension might just be extra weight. We race often on tight bumpy parking lot courses where a suspension should help. The compromise that a number of teams have adopted is a suspension on the single rear wheel. We use a shock spring combo off of a BMX bicycle. It improves the grip overall, though more on the rear wheel. There is also less harsh vibration on driver and equipment. 

Are you running High School or Open? If Open, look closely at Advanced Batteries. ProEV's expericne has been that they are cheaper, faster and the pack should be competitive longer.



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Cliff

www.ProEV.com



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ProEv covered it pretty well. I would just add that a lighter car will be more efficient than a heavier car everything else being equal. Also, I have only ever run packs of 3 of the group U1 batteries and have been very happy with the results. I find three of them to be cheaper than two Optimas too. Just be sure to pick quality brands and test them to verify capacity.

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