2 motors- chain & sprocket

[ido]

hey all,

This year will be our first time to participate in the fsae electric event.

As wrote in the title, we gonna use 2 motors, one for each rear wheel, with chain and sprocket transmission. all car made from carbon monocoque.

Any idea to chain tensioner? and... it is necessary?

Any help/answer/reply will be VERY appreciate!

Thanks,

Ido
BGR-e recing team.

[apalrd]

If you are not constrained by the restrictions of an IC engine (which generally has a sprocket rather than an integrated differential/final drive), is a chain necessary? Would a gear be suitable? or a direct-drive even?

Depending on the length of the chain, tensioning is preferred or required. We (IC team) use an eccentric disc with the diff mounted to it, and rotate the assembly to change the chain center-center distance. A very short chain will likely not require this.

I suggest you rethink your choice of 2 motors, as well as the choice of carbon monocoque, as they are more complicated (both mechanically, electrically, and controls/software wise) than a simpler single motor space frame car. Since this is your first year, complexity is something you should seriously consider minimizing, even at the expense of ultimate performance. Once you have a working system which you have learned from, you can design the best car possible from your mistakes.

[Z]

Ido,

At the recent FSAE-Australasia 2014 competition (~1 month ago) the #E17 Swinburne Team brought a car similar to what you seem to be designing. I suggested potential improvements to that car at the end of this "Why More R%" post. Note that I had to split the comments up into two parts, so the Swinnie "Case Study" is right at the bottom of the second part.

I believe that such a low-CG, high-R% concept is the easiest way to build a world-beating E-car. Aim for 60-70%R if you have NO torque-vectoring, or 70-80%R WITH torque-vectoring. I would use a single-stage gear-reduction, ~4:1, to take advantage of the (small-ish, but still useful) gyroscopic anti-Roll effect. This gear-reduction would also allow low-mounting of the motors, and a compact, reliable, and overall low-mass driveline.

Most importantly, remove as much unnecessary junk from the car as you can find. There is a lot of it! And look at the Delta-Wing underbody aero. Very efficient ...

Z

[sekl]

Hello,

Most importantly, remove as much unnecessary junk from the car as you can find.

Just to throw it out there... Looking at the numbers with just two questions in mind:
1: What top speed is desired and what does that mean for maximum gear ratio?
2: How much torque is desired per wheel and what does that mean for minimum gear ratio?

https://docs.google.com/a/ba-racing-...#gid=565451337


As mentioned in the spreadsheet: Is a single Emrax 207 maybe already enough and a second is unnecessary (junk)? Single 228 has been seen, seems to work well as far as I can tell...

At least at FSG, it was a single Emrax 228 which put down the quickest time in acceleration when comparing all 11 entries using Emrax motor(s). 4 did run a single 228, might well be that the extra torque is worth the extra weight, but decide yourself based on the numbers you got.


Some more arguments that make a single Emrax motor seem interesting to me when the alternative is two of them:

Eliminate:
Motor (~10kg)
Shield, Mount, Chain, Sprocket, ...
Controller (~10kg)
Plumbing, Wiring, Splitbox, ...,
Frame / Monocoque material, ...

Reduce:
Effort for controls development and test time needed for debugging, tuning, ...
System complexity and amount of new things

Add:
Differential, Mount, ... Things you know already from a combustion car.



On the Efficiency note from Z (in the other thread).

Judge yourself what impact gear ratio realistically got on efficiency score.

Energy in % means, 0% is lowest energy needed by any team (here, 3.8kWh) and 100% is max (here, 7.5kWh). Similar for Time, Tmin is 0%, Tmax is 100%.
The plot is as per 2014 FSE rules.


Gruß
Sebastian

[JulianH]

I think Sebastian could be right. For a first year electric team, I'm quite sure, that the single motor solution is quite good.

If you look at FSG2014 (electric), the fastest car, which was not one of the impressive 4WD monsters, was the first-year electric car from Trondheim with a single motor. They were nearly as fast in AutoX as 2nd overall from Stuttgart. If they didn't DNF in Endurance, they would probably clinched the 4th overall position.
The 2013 Amberg-Weiden car with only one Emrax was also very fast.

It takes a lot of time to get the electric "toys" (TV mostly) working properly and in the first year you probably have enough problems with other stuff so that the "GFR-solution" to run a single motor as long as you have electric gremlins is probably a good one.

[ido]

Thank you all for your replays!

Our concept is already close. Too late to change it. We gonna use 2x emrax 207 motors and our calculation takes us to the reduction ratio between 3.6-4.2. We want to keep the option to change the ratio by replace the rear sprocket, so i think we need some mechanism to keep the chain in the correct tension.

We trying to keep it simple and really our humble goal for this year---> to take part in the competition with our car and hopefully to finish the endurance.

Alumnus- i saw picture of your 2010 car, the concept look exactly like our concept (so we are just 5 years beyond you... )
how was it? is it worked properly? did you use something to ensure chain tension?

Again thank you all, your answer was very helpful!

cheers,

Ido
BGR-e racing team.

[mech5496]

We trying to keep it simple and really our humble goal for this year---> to take part in the competition with our car and hopefully to finish the endurance.

Not trying to discourage you, but if this is your goal, I cannot see how your concept fits it...

[Z]

Ido,

I have to clarify myself by agreeing with Sebastian, Julian, and Harry here.

If your aim is to compete with a "keep it simple" car in a 2015 European competition (!!!), then you best grab an E-motor/driveline out of the first golf-cart or old-person's mobility-scooter that you can find, and bolt it in the back of your car NOW. The ~six months to comp is almost tomorrow! My suggestions above were more for a Team doing "concept" work for maybe 2016...

One Emrax 207 motor with ~4:1 chain reduction to a spool differential is enough to get you started. For chain tensioning you just need to provide means to move the motor forward, OR the diff-mounts rearward. LARGE FORCES are involved, so provide a solid "load path" between the motor and diff-mounts. I doubt you will have time to do much swapping of sprocket sizes, but changing the front sprocket by +/-1 tooth is probably easier than rear sprocket changes because less change in overall package size, chain-guards, etc.
~~~o0o~~~

Thinking "Big Picture" (ie. for Teams aiming for 2016 Northern comps), I see the aces up the E-car's sleeve as being;
1. Ease of achieving preferred mass distribution. Specifically, low-CG, low-Yaw-Inertia, and almost any desired F:R% are all easy to get by rearranging the many small and relatively independent parts, namely batteries, controllers, and motors.
2. Ease of delivering constant thrust to driving wheels, especially at low speeds, because no clutch + gear-swapping required as in C-cars.

It then seems sensible to me to try to capitalise on these strengths.

The 4-Motor-4WD approach might seem "the optimum" here, but I see increased Yaw-Inertia from the four widely separated, and quite heavy, motor/gearboxes, and at least four longish high-amp cables (or two widely separated power controllers?).

The better solution, IMO, is to head in the direction of ZTR lawn-mowers, or as a more extreme example, those "Segway" scooters.

For those who don't know, ZTR mowers have a ~20 hp IC-engine that drives the mower blades, and also two "swash-plate" (ie. variable displacement) hydraulic pumps. These then each drive a hydraulic motor in each BIG-ish rear-wheel. Result is driver-controlled IVT to each rear-wheel, which means both forward thrust and "steering" are done by the rear-wheels. The front-wheels are uncontrolled "castors". When one rear-wheel drives forward and the other backward, the whole vehicle spins on the spot for "Zero-Turn-Radius".

(BTW, when I was a teenager I proposed almost exactly this sort of thing to some knowledgeable hydraulic Engineers who worked in a factory next to Dad's workplace. They were obviously TOO experienced because their response was "No way! It'll never work ... way too expensive for a start...". There are squillions of these ZTR mowers being sold nowadays...)

Anyway, an FS E-car with all the batteries, controllers, 2 x motors, etc., compactly packaged between the two rear-wheels, with the driver sitting well back and with minimum size front-wheels and structure, all giving a high R% car, would make a relatively simple but potentially very potent package.
~~~o0o~~~

Sebastian,

In your "Efficiency" graph, I take it that the coloured bands represent the Times of different cars between Tmin (top band, winner) and Tmax (bottom band, lowest score)?

But does this graph reflect the fact that a more efficient car should (?) be able to carry a lighter battery pack, and therefore should (again->?) be faster than a less efficient car, and so score more points from its speed? (Note that "slip-angle-drag" is probably the main first-order-factor that makes the lighter car better here.)

Also, my concerns with Swinburne's choice of running their motors in an inefficient part of the map has more to do with the amount of IN-efficiency they had. Namely, a motor running at 12% INefficiency (ie. eta = 88%) must reject THREE TIMES as much heat as one running at 4% INefficiency (ie. eta = 96%).

Does anyone know why Swinburne had to stop on almost every lap of Enduro while a track marshal pushed a button on the back of the car? Maybe an overheat reset button?

Anyway, I don't see much sense in coal-fired steam-engine cars generating even more steam out there on track...

Z

[sekl]

In your "Efficiency" graph, I take it that the coloured bands represent the Times of different cars between Tmin (top band, winner) and Tmax (bottom band, lowest score)?

Correct. Tmax is Tmin*1.33

But does this graph reflect the fact that a more efficient car should (?) be able to carry a lighter battery pack, and therefore should (again->?) be faster than a less efficient car, and so score more points from its speed? (Note that "slip-angle-drag" is probably the main first-order-factor that makes the lighter car better here.)

I personally do not think the effect you describe is very relevant. FSAE cars are too small and 5% energy is maybe 2.5 kg less on cells (if it was that easy to scale, which it is not).
I am not the person to talk vehicle dynamics and have no idea what "slip angle drag" is though..


The plot is just the FSE scoring formulas for Endurance and Efficiency. It shows any possible score (within the given limits) and therefore reflects all facts.

Below the same thing. It shows how the rules changed for FSE13 (colormap) compared to FSE12, emphasizing to run faster in endurance.



Whatever...
https://docs.google.com/a/ba-racing-...50UGdJWW8/edit

[mech5496]

...I see the aces up the E-car's sleeve as being;
1. Ease of achieving preferred mass distribution. Specifically, low-CG, low-Yaw-Inertia, and almost any desired F:R% are all easy to get by rearranging the many small and relatively independent parts, namely batteries, controllers, and motors....

Sounds easier than it is...It might be a lot easier than C-Cars, but at some point you have to make compromises between MoI and CoG height and F:R distribution, even before starting considering overall dimensions and ergonomics, access for maintenance, electrical connections...oh and Rules of course. Trust me, it is way harder than it sounds! I have to agree on everything else though...