Using differential gears for forward wheel drive? (Trike configuration.)
Zahrack said
Jun 11, 2016
To clarify... this is just an idea/debate I have with myself. I'm in the middle of doing a paper design that I don't plan on building till I have funds around a year from now. The hopes are to compete with the top racers in a new fresh 3 wheel design with a trike configuration. (2 wheels front & 1 in the back.)
For future reference the front wheels are located with their axis(s) under the driver's kneecaps. The rear wheel is located slightly under the driver's helmet and just 3 to 5 inches behind the shoulders. The batteries are located from the driver's ankles to just below their knees.
So the original idea was to put my drive motor behind the driver's head and shoulders with the ESC as near the motor as possible. With my current design I'm not entirely sure this would work best due to two reasons. First and primarily, the location of the motor in the back means that the power wires need to travel from the batteries around the drivers torso then to the motor. There is guaranteed to be some power loss there. Maybe not a lot but still more than the least it can be. Second, with the motor and ESC crammed behind the driver's head and shoulders it may be difficult to put ballast back there when keeping the weight balanced across the rear wheel axle. Yes I could move the ballast forward between the driver's legs but I'm not sure the handling will do well with an additional ~22 lbs of the weight concentrated near the front wheels.
The solution I came up with solves these problems and introduces some of it's own. Adding a torsion differential directly driven by the motor with the ESC located above it. The whole assembly would fit between the drivers legs with some room to spare for the steering mechanism. It would sit just inches away from the nearest battery and clear up space behind the driver's head and shoulders for ballast and possibly shocks for head comfort. The drive gear on the differential can be designed to be changed out for different motor gearing. No sprocket and chain required. The differential may actually increase the strength of the wheel assembly. (See below)
The disadvantages of this design may outweigh the benefits. Obviously it would cost more to develop. I can do all of the design and prototyping myself but producing the actual parts to be used would cost a small fortune in machining or 3D printing metal costs unless I find out a type of 3D printed plastic is strong enough. The differential assembly would likely weigh around 5 to 10 pounds. With the motor and ESC this increases to around 15 pounds. (Not much less than the ballast... though the ballast would still be further back.) Differentials are more fragile than solid wheel axles but this difference is negligible being as the wheels would otherwise be free spinning off of small half inch pegs/axles. (The differential may actually increase the strength of the wheel assembly.) The differential has more moving parts so it will require more attention and maintenance. It may be difficult to design the steering mechanism around the differential but certainly not impossible.
If ya'll need some pictures or diagrams I would be willing to post a couple of sketches. Right now I'm just too lazy. Thanks!
Zach
Nitoragro said
Jun 11, 2016
I think it would be REALLY cool to have a two wheel drive car and it could help with handling if done correctly. I would love to see some rough ideas of how you want to do it.
So you want to have a 2 wheel drive car, both in the front. I would assume you would still use front wheel steering which is better but I think would be more difficult. It sounds like you would need two U-Joints so you can still turn the wheels and have power. One thing I have learned from Electrathon is that ANYTHING uses energy. When my car hits a bump and the paneling flexes and makes noise I waste energy, while it is VERY little it is still something. What I am trying to say is that everything you do in anyway will cause a waste of power, some more than others. I know another page in this forums talks about chain efficiency and it can be very good, I want to say 5%-ish. But I know on my car we have had some chain problems, oblong sprocket, slightly crooked chain, chain slipping off, etc, so eliminating a chain could very well help you.
I bet if you spend a lot of time getting the differential perfect every gear inside of it waste energy. In one of my engineering classes we found that chain drive has around 95% efficiency while gears have around 85%. While that might not be to much you also have to look at the U-Joint efficiency when transferring the power to the front wheels. A rear wheel steer would from what I think be more efficient since you would not need two U-Joints it does bring up more problems.
There is a team in my league I was talking to about battery/motor/controller placement. What they believe in is keeping it all VERY close together. On one of their car's they claimed it had 2 feet of 2 gauge wire. This is because the motor, batteries, and controller were all behind/below the driver. On my car this is very different. I tried to put the weight closer to the front axle by putting the batteries up front under my leg's. I use about 8-10 feet of 8 gauge wire since it goes from the controller in the back, to the batteries in the front, to the motor in the back. The weight distribution on my car is not the best. All of my batteries are in front of the axle and I sit about 8 inches behind the axle. So i have a LOT of weight on the front, about 70-75% up front. So I would not worry about moving that 22ish pounds up front.
I love experimenting when building, right now I am building my 3rd car this summer and I have tried some unique things on my first two, from new batteries to use, to steering set-up, to using an aircraft canopy. One big thing I learned is that sometimes things turn out better than you would think. I got some really cheap batteries that no one has used that I know of, they are for solar panels. But after using them for a year they work GREAT, I thought I would suffer through the season because my team tried to save money, but it was a huge success, these batteries brought us 1st twice, and 2nd 4 times, over 8 races. However I also tried to use duel levers on my 2nd car and I learned exactly how NOT to do it. I spend many many hours trying to design my own and it kinda didn't work. Sometimes what you think will happen won't, or it could surprise you beyond imagine.
Archer321 said
Jun 12, 2016
I suspect the mechanical losses in a differential would exceed the electrical losses of a longer cable run.
ProEV said
Jun 12, 2016
Zach,
Every design choice is a compromise. Choosing the right balances of compromises is what makes a successful racing vehicle.
You are deciding between the electrical losses of an extra length of cable against moving the motor forward and adding a differential to drive the front wheels.
Can you quantify the losses of these two choices?
Let's say the rear mounted motor will require around an additional 8 feet of #2. Pack capacity is 800 Whrs and it is a 24 volt system so the vehicle will average 33 amps (800/24=33.3). Wikipedia tells us that 2 gauge wire has a resistance of .1563 Milliohms per feet ( https://en.m.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizes ).
So the wire resistance is .1563 milliOhms and we are running a current of 33 amps through it. What is the voltage drop from one end of the wire to the other? V= I*R so 33*.1563= 5.1579 milli volts.
Next, how much power is actually lost? 33 amps * .0051579 volts = .17 Watts. So, over the course of an hour race, each extra foot is wasting .17 Whrs.
Conclusion: That extra 8 foot of cable wastes around 1.36 Whrs during the race and adds less than a pound of weight.
On the other hand, a differential is somewhere between 85% to 95% efficient, so wastes between 15% and 5% of our 800 Whr pack which comes out to 120 Whrs to 40 Whrs. It will also add more than a pound of weight.
I would conclude that compromising on the length of the battery cables is better than moving the motor and using a differential to drive the front wheels.
Zahrack said
Jun 15, 2016
Thanks guys. And thanks Cliff for the clear math!
I was actually planning on a 36 volt system. I get cheaper batteries, more space around the batteries, easier/cheaper replacement and greater efficiency possibility with a good motor selection.
I'd say the electrical losses will be quite worth it. The alternative with a differential offers ~15 pounds more weight, more engineering/prototyping, extra parts' cost, a greater chance of part failure and increased maintenance cost. After I got my head screwed on straight the choice became quite clear. I'll take the pound of #X gauge wire and no more than .2 watts of power loss.
The greatest problem I actually have after that decision is finding out how to make more room for ~22 pounds of ballast.
If anyone runs into an actual "need" for a differential in their design I'll be willing to share idea's and offer any help I can.
To clarify... this is just an idea/debate I have with myself. I'm in the middle of doing a paper design that I don't plan on building till I have funds around a year from now. The hopes are to compete with the top racers in a new fresh 3 wheel design with a trike configuration. (2 wheels front & 1 in the back.)
For future reference the front wheels are located with their axis(s) under the driver's kneecaps. The rear wheel is located slightly under the driver's helmet and just 3 to 5 inches behind the shoulders. The batteries are located from the driver's ankles to just below their knees.
So the original idea was to put my drive motor behind the driver's head and shoulders with the ESC as near the motor as possible. With my current design I'm not entirely sure this would work best due to two reasons. First and primarily, the location of the motor in the back means that the power wires need to travel from the batteries around the drivers torso then to the motor. There is guaranteed to be some power loss there. Maybe not a lot but still more than the least it can be. Second, with the motor and ESC crammed behind the driver's head and shoulders it may be difficult to put ballast back there when keeping the weight balanced across the rear wheel axle. Yes I could move the ballast forward between the driver's legs but I'm not sure the handling will do well with an additional ~22 lbs of the weight concentrated near the front wheels.
The solution I came up with solves these problems and introduces some of it's own. Adding a torsion differential directly driven by the motor with the ESC located above it. The whole assembly would fit between the drivers legs with some room to spare for the steering mechanism. It would sit just inches away from the nearest battery and clear up space behind the driver's head and shoulders for ballast and possibly shocks for head comfort. The drive gear on the differential can be designed to be changed out for different motor gearing. No sprocket and chain required. The differential may actually increase the strength of the wheel assembly. (See below)
The disadvantages of this design may outweigh the benefits. Obviously it would cost more to develop. I can do all of the design and prototyping myself but producing the actual parts to be used would cost a small fortune in machining or 3D printing metal costs unless I find out a type of 3D printed plastic is strong enough. The differential assembly would likely weigh around 5 to 10 pounds. With the motor and ESC this increases to around 15 pounds. (Not much less than the ballast... though the ballast would still be further back.) Differentials are more fragile than solid wheel axles but this difference is negligible being as the wheels would otherwise be free spinning off of small half inch pegs/axles. (The differential may actually increase the strength of the wheel assembly.) The differential has more moving parts so it will require more attention and maintenance. It may be difficult to design the steering mechanism around the differential but certainly not impossible.
If ya'll need some pictures or diagrams I would be willing to post a couple of sketches. Right now I'm just too lazy. Thanks!
Zach
So you want to have a 2 wheel drive car, both in the front. I would assume you would still use front wheel steering which is better but I think would be more difficult. It sounds like you would need two U-Joints so you can still turn the wheels and have power. One thing I have learned from Electrathon is that ANYTHING uses energy. When my car hits a bump and the paneling flexes and makes noise I waste energy, while it is VERY little it is still something. What I am trying to say is that everything you do in anyway will cause a waste of power, some more than others. I know another page in this forums talks about chain efficiency and it can be very good, I want to say 5%-ish. But I know on my car we have had some chain problems, oblong sprocket, slightly crooked chain, chain slipping off, etc, so eliminating a chain could very well help you.
I bet if you spend a lot of time getting the differential perfect every gear inside of it waste energy. In one of my engineering classes we found that chain drive has around 95% efficiency while gears have around 85%. While that might not be to much you also have to look at the U-Joint efficiency when transferring the power to the front wheels. A rear wheel steer would from what I think be more efficient since you would not need two U-Joints it does bring up more problems.
There is a team in my league I was talking to about battery/motor/controller placement. What they believe in is keeping it all VERY close together. On one of their car's they claimed it had 2 feet of 2 gauge wire. This is because the motor, batteries, and controller were all behind/below the driver. On my car this is very different. I tried to put the weight closer to the front axle by putting the batteries up front under my leg's. I use about 8-10 feet of 8 gauge wire since it goes from the controller in the back, to the batteries in the front, to the motor in the back. The weight distribution on my car is not the best. All of my batteries are in front of the axle and I sit about 8 inches behind the axle. So i have a LOT of weight on the front, about 70-75% up front. So I would not worry about moving that 22ish pounds up front.
I love experimenting when building, right now I am building my 3rd car this summer and I have tried some unique things on my first two, from new batteries to use, to steering set-up, to using an aircraft canopy. One big thing I learned is that sometimes things turn out better than you would think. I got some really cheap batteries that no one has used that I know of, they are for solar panels. But after using them for a year they work GREAT, I thought I would suffer through the season because my team tried to save money, but it was a huge success, these batteries brought us 1st twice, and 2nd 4 times, over 8 races. However I also tried to use duel levers on my 2nd car and I learned exactly how NOT to do it. I spend many many hours trying to design my own and it kinda didn't work. Sometimes what you think will happen won't, or it could surprise you beyond imagine.
Zach,
Every design choice is a compromise. Choosing the right balances of compromises is what makes a successful racing vehicle.
You are deciding between the electrical losses of an extra length of cable against moving the motor forward and adding a differential to drive the front wheels.
Can you quantify the losses of these two choices?
Let's say the rear mounted motor will require around an additional 8 feet of #2. Pack capacity is 800 Whrs and it is a 24 volt system so the vehicle will average 33 amps (800/24=33.3). Wikipedia tells us that 2 gauge wire has a resistance of .1563 Milliohms per feet ( https://en.m.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizes ).
So the wire resistance is .1563 milliOhms and we are running a current of 33 amps through it. What is the voltage drop from one end of the wire to the other? V= I*R so 33*.1563= 5.1579 milli volts.
Next, how much power is actually lost? 33 amps * .0051579 volts = .17 Watts. So, over the course of an hour race, each extra foot is wasting .17 Whrs.
Conclusion: That extra 8 foot of cable wastes around 1.36 Whrs during the race and adds less than a pound of weight.
On the other hand, a differential is somewhere between 85% to 95% efficient, so wastes between 15% and 5% of our 800 Whr pack which comes out to 120 Whrs to 40 Whrs. It will also add more than a pound of weight.
I would conclude that compromising on the length of the battery cables is better than moving the motor and using a differential to drive the front wheels.
Thanks guys. And thanks Cliff for the clear math!
I was actually planning on a 36 volt system. I get cheaper batteries, more space around the batteries, easier/cheaper replacement and greater efficiency possibility with a good motor selection.
I'd say the electrical losses will be quite worth it. The alternative with a differential offers ~15 pounds more weight, more engineering/prototyping, extra parts' cost, a greater chance of part failure and increased maintenance cost. After I got my head screwed on straight the choice became quite clear. I'll take the pound of #X gauge wire and no more than .2 watts of power loss.
The greatest problem I actually have after that decision is finding out how to make more room for ~22 pounds of ballast.
If anyone runs into an actual "need" for a differential in their design I'll be willing to share idea's and offer any help I can.
Thanks again!