Motor & Controller for Efficiency Racer!

[jimcroisdale]

As you say, seems like timing belts are the ticket. Obvs this graph is useless for us as we have no idea what belt size and torque it represents, but certainly shows what type of belt is best, and perhaps what efficiency could be achieved.

As you say, our usage scenario ought to be ideal. I dont know anything about belt drive, but that makes sense to me at this stage!

 

[jimcroisdale]

I should add....

While our driven wheel will be around 600mm diameter, it'll be enclosed in body work, with only approx 50mm of tyre protruding through the bottom of the shell.

This will mean that a safe pulley diameter would be around 350mm (being on the safe side).

Centre distance could be whatever was deemed most efficient, within reason.

 

[PaPaSteve]

Learn something new every day ! ! !
Greenpower racing looks like fun, I had no idea it existed.
Have you heard of Electrathon racing ?
re : gearing . . . most of the human powered speed bikes will multiply the RPM with a secondary shaft.


In your case the RPM needs to be divided but the same approach can be used.
Typically the secondary shaft is created from a repurposed bicycle freehub body as shown in photo.
Example shown in speed bike photo is from a BMX source but a road bike derailleur style like shown below might add some gear range.

The following picture shows an example of a bike with a secondary shaft and dual derailleurs.
The bike is a 35 speed sporting a extra low gear for climbing and extra high gear for speed.

Is there any efficiency loss with this scheme ?
With only 2 more bearings in the system it will be low.
Also, on the speed racer bike, the bearings are replaced with high quality ceramic bearings with dust shields ( rubber contact seals are removed )

 

[scianiac]

I believe the primary loss in a belt drive (and in fact a chain drive) is the friction of bending around the pulley and so this can be quite dependent on the belt itself. So a thicker belt, generally one with a higher pitch like an 8m will be pretty stiff so if run at a high speed and low torque around a small pulley it will not be very efficient but the opposite is true. So larger pulleys will be better so you bend the belt around a larger radius. And running the belt at a good amount of torque for it's size, I assume at some point you go too far and start causing other issues. So that means sizing the belt appropriately, a smaller pitch belt like a 5 or 3 will be much more flexible and then of course the belt width has the same effect, narrower belt, less rubber to bend.

Chains are similar, less bend radius the better (within reason, you just don't want a tiny motor pulley if you can) and since you won't need more torque than a bike chain can handle you can just go with the lowest friction option with the best lube. So just whatever zero friction cycling recommends.

Either option will be far easier to change the gearing than gears, although large fine pitch belt pulleys you may need to make yourself but that's not that difficult. If you machined them from a low friction plastic you may even gain some very slight efficiency. Same goes for large chain sprockets if you want to go 1 stage reduction but I agree with Steve 2 stages may be fine as long as designed well. And even an additional 1% loss in drive train from two reductions may be worth it if you can run the motor more efficiently which could gain you much more.

A lot of this is going to come down to testing which depends on what hardware you have for that and then you're going to want to be able to change things easily to test things.

 

[PaPaSteve]

Bicycle transmission studies have shown that modern quality bicycle chains and sprockets are allegedly in the 98~99% efficiency range.
11 tooth seems to be a break point in efficiency with smaller sizes jumping in losses.
A small chain pitch obviously means small sprockets which might save some weight.
Although, weight verse ultimate strength a bicycle chain is a clear winner.
Acceleration can chew up power, having a shift-able transmission can be helpful for reducing that type energy loss.
Somewhere in the shop there are extra freehub bodies left over from my bicycle building days and CAD drawings to make custom sprockets.
BTW . . . 1996 World Solar Rally Class Champion Akita Japan

 

[AnthonyC]

No, pack voltage can't be reduced unfortunately. And yes, 100% throttle is most efficient as you don't have any PWM switching losses in the controller. So basically, we're designing to be most efficient at full throttle - around 30-35mph @650w. Motor will be air cooled. The particular event we're looking at is a 24 event, so it's most laps in 24 hours.

I've no racing experience but it sounds as though rider endurance (for fewer stops) and higher power (for speed) are the main factors in a win (assuming reliability). Why so much emphasis on efficiency when your voltages suggest you're only using half your battery capacity?

 

[jimcroisdale]

I've no racing experience but it sounds as though rider endurance (for fewer stops) and higher power (for speed) are the main factors in a win (assuming reliability). Why so much emphasis on efficiency when your voltages suggest you're only using half your battery capacity?

How do our voltages suggest we'll only be using half of our battery capacity? We'll be using ALL of the battery, AND recharging it three times.

 

[jimcroisdale]

Learn something new every day ! ! !
Greenpower racing looks like fun, I had no idea it existed.
Have you heard of Electrathon racing ?
re : gearing . . . most of the human powered speed bikes will multiply the RPM with a secondary shaft.
View attachment 349607
View attachment 349608
In your case the RPM needs to be divided but the same approach can be used.
Typically the secondary shaft is created from a repurposed bicycle freehub body as shown in photo.
Example shown in speed bike photo is from a BMX source but a road bike derailleur style like shown below might add some gear range.
View attachment 349611
The following picture shows an example of a bike with a secondary shaft and dual derailleurs.
The bike is a 35 speed sporting a extra low gear for climbing and extra high gear for speed.
View attachment 349610
Is there any efficiency loss with this scheme ?
With only 2 more bearings in the system it will be low.
Also, on the speed racer bike, the bearings are replaced with high quality ceramic bearings with dust shields ( rubber contact seals are removed )

Looks terribly lossy to me. And complex. And unreliable.

Basically, any time the chain goes round a pair of sprockets you lose 2% approx. So that little lot would account for what, over 6% losses? Thats the difference between a motor costing £350 and a motor costing upwards of twenty grand. Anyway, with a maximum ratio required of 10:1, we'll only need a single stage, regardless of what it is.

My teammate that competed in Greenpower says that the default position for chains is "off" There does have to be some reliability built into all of this. Three years of development and many thousands in cost could all come to naught if something breaks that we can't fix (or can't fix QUICKLY).

As they say, to finish first, first you must finish.

 

[PaPaSteve]

Looks terribly lossy to me. And complex. And unreliable.

Basically, any time the chain goes round a pair of sprockets you lose 2% approx. So that little lot would account for what, over 6% losses? Thats the difference between a motor costing £350 and a motor costing upwards of twenty grand. Anyway, with a maximum ratio required of 10:1, we'll only need a single stage, regardless of what it is.

My teammate that competed in Greenpower says that the default position for chains is "off" There does have to be some reliability built into all of this. Three years of development and many thousands in cost could all come to naught if something breaks that we can't fix (or can't fix QUICKLY).

As they say, to finish first, first you must finish.

Looks can be deceiving.
Using a two stage system like this on a human powered streamlined vehicle :
200 meter sprint record speed is 89.59 mph (144.14 kph) *list*
One hour record is 56.89 miles (91.556 kilometers) *list*
24 hour 676.47 miles (1088.67 kilometers)

Ratio change on my own speed bike is also in the 10/1 range.
A 100 tooth sprocket does not fit inside the bike so a two stage system is required.
My personal "finish first" stats are : 50 wins from 82 events entered.

Best regards

 

[AnthonyC]

Battery pack will vary from 94v at its highest (right off the charger), quickly dropping to 86v at first use, and then gradually tapering off to a minimum of around 83v.

83 / 94 * 4.2 = 3.7

What we CAN'T do is drop the pack voltage, for reasons I won't bore you with.

My bad, I assumed you'd be using Li-ion when actually I know nothing.

 

[jimcroisdale]

Looks can be deceiving.
Using a two stage system like this on a human powered streamlined vehicle :
200 meter sprint record speed is 89.59 mph (144.14 kph) *list*
One hour record is 56.89 miles (91.556 kilometers) *list*
24 hour 676.47 miles (1088.67 kilometers)

Ratio change on my own speed bike is also in the 10/1 range.
A 100 tooth sprocket does not fit inside the bike so a two stage system is required.
My personal "finish first" stats are : 50 wins from 82 events entered.

Best regards

Well, with the rpm of human legs, and the wheel rpm required to do those speeds, I don't think you have a choice - there's no other way to do it. But that doesn't mean that it's efficient per se, it just means you can't get around it - it's the most efficient way to do what you need.

 

[jimcroisdale]

83 / 94 * 4.2 = 3.7

My bad, I assumed you'd be using Li-ion when actually I know nothing.

LiFePO4

 

[jimcroisdale]

Well, with the rpm of human legs, and the wheel rpm required to do those speeds, I don't think you have a choice - there's no other way to do it. But that doesn't mean that it's efficient per se, it just means you can't get around it - it's the most efficient way to do what you need.

And the most efficient way to do what we need are direct drive motors with 97% efficiency and no transmission loss AT ALL that cost £20,000+, which we don't have (and are unlikely to find )

 

[PaPaSteve]

System efficiency . . .
For an example, lets look at an internal combustion engine.
Comparing a 2 valve, single cam design to a 4 valve, twin cam design.
The 4 valve, twin cam actually has higher friction losses yet it achieves better overall engine efficiency.
Unfortunately, there isn't a 100% energy transfer system that has ever been discovered.

Examining the Greenpower F24 racing rule book :
TECHNICAL
T1 - MOTOR
T1.1. The vehicle will only be powered by one 24 volt DC electric
motor, supplied by Greenpower. Only one motor may be
installed in the car. Hybrid systems are not permitted.

Am I looking at the correct set of rules ?

 

[PaPaSteve]

Whoops . . . my bad
Clearly not the Greenpower series as the voltages don't match up to the first post in this thread.

 

[jimcroisdale]

Yes, it's not Greenpower - we're running in a much more open formula.

As regards system efficiency - here's something to ponder. We have a simulator, and currently, with the car as light as we're likely to get it, with optimal drag coefficient and minimal transmission/motor losses, we'll be averaging about 30mph using standard motorcycle tyres, at a given wattage.

If we use Bridgestone Ecopia eco tyres (designed for solar cars and very difficult to get hold of), that speed increases to.....

52mph.

Just from the reduction in rolling resistance.

Crazy, isnt it?

Fortunately for us, there's a middle ground, but still.... 22mph gain just for different tyres.

 

[PaPaSteve]

Several studies in the bicycle world that point towards good choices for low rolling resistance tires.
Conversely though low rolling resistance tires are more prone to punctures.
Coast down testing is a good tool to point the design towards good decisions on a number of fronts including tires, tubes, bearings, body shapes etc.

 

[PaPaSteve]

BTW : Several of the speedbike racers were attracted to the Michelin Solar Car tires for the published low rolling resistance numbers but didn't find the results they were looking for.

For the bulk of my wins I ran IRC Panaracer tires.
They weren't the lowest rolling resistance, they were reliable and had predictable traction in the corners.
For awhile IRC moved production out of Japan and the quality suffered so a switch to Schwalbe was required.
. . . Production for IRC is back in Japan again . . .
Nylon casings are often the lowest rolling resistance for any brand of tire.
Look for casings with high thread counts.
For the race season I would purchase a dozen or more, inspect them and keep the best for the races.
Store flat in a cool dry space.
Never start a race on a used tire or tube.

We certainly could be of more help if you would point us toward the race and rules etc.

 

[jimcroisdale]

Yes, Greenpower use bicycle tyres and achieve crr figures very similar to the solar car tyres, and at a fraction of the cost. The problem with the race we're entering is that bicycle components aren't allowed - whatever we use has to be load/speed rated, which leaves us with motorcycle tyres. So really, as the manufacturers don't know/quote the crr figures, it will be a case of choosing a tyre that seems right (relatively skinny, high pressure, minimal/no tread, high threadcount etc etc) and testing them as best we can.

 

[DaLanMan]

What about scooter or moped tires? Dealt with a Honda Elite 250 while on vacation a few years back, was amazed at how light the tires on it were. granted they were also smallish.. like 14"

Mopeds I know have ratings in Cali, because here it is a motor vehicle managed by DMV. So has to have a Highway patrol cert on anything safety related.

 

[PaPaSteve]

Average race speed of 30 mph on speed and weight rated motorcycle tires ?

Motorcycle tires . . .
The lowest speed rating is "B" which equals 50 km/h (31 mph)
Lowest weight index is 20 equaling 80 kg (176 lbs)
That will be a unicorn tire.

Besides winning with bicycles of my own designs I also have experience prepping racing vehicles that have won or set records in a wide variety of motorsports. Dirt, road and Bonneville. Gas and electric.
Care to bet I could point you to some high performance, lightweight options ?

Share the rules ?

Motorcycle Tire Guide | FortNine Canada

FortNine is your source for Motorcycle, Dirt Bike, Snowmobile and ATV apparel, accessories, and parts. Everyday savings and free shipping in Canada.

fortnine.ca

 

[jimcroisdale]

Average race speed of 30 mph on speed and weight rated motorcycle tires ?

Motorcycle tires . . .
The lowest speed rating is "B" which equals 50 km/h (31 mph)
Lowest weight index is 20 equaling 80 kg (176 lbs)
That will be a unicorn tire.

Besides winning with bicycles of my own designs I also have experience prepping racing vehicles that have won or set records in a wide variety of motorsports. Dirt, road and Bonneville. Gas and electric.
Care to bet I could point you to some high performance, lightweight options ?

Share the rules ?

Motorcycle Tire Guide | FortNine Canada

FortNine is your source for Motorcycle, Dirt Bike, Snowmobile and ATV apparel, accessories, and parts. Everyday savings and free shipping in Canada.

fortnine.ca

PM'd you Steve!