Paging bicycle and engineering gurus

Someone has asked me about a school electric racing car project. I gave
a bit of general advice on the need for correct gearing, and ideally
'gears' in the plural. So he is thinking of trying to use a BMX wheel.
Here are some details:

"Motor details are 24volt, 240 watts at 2000rpm, torque 1.15Nm. The
graphs all show 2000rpm is the magic figure of efficiency. Each team is
allowed 4 x 12v YUASA batteries, 2 on the car and 2 off. We change them
about every 30 minutes initially...we cannot recharge during the race
which lasts 4 hours! The course is a proper raceway like Rockingham or
Darley Moor! The best cars can do 40+mph and seem to go on doinf that
for 4 hrs! They can cover 140 miles in thec 4 hours...we have a best of
about 80 miles! Hence the need for more speed. We made a start
tonight...looks difficult to attach cycle wheels!"

The latest query:

"I need to buy and mount a 3/8 57T fenner type sprocket...standard
Reynolds chain.. to my 20 BMX wheel...ideally to simply screw it on
where the deraillier sprockets are mounted...any ideas???? Where can I
get the sprocket? Can I get it ready to screw on?"


This is way outside my area of knowledge. I would have thought a
freewheel thingy using bolt on sprockets might be the way to go.
Like this (if the thread is right):
http://www.electricscooterparts.com/SPR-SB4H.htm
If so anyone know of a UK supplier?

I have pointed him to Wyko and also a thread on an electric car forum
talking about using bike wheels, but have any of you bicycle engineering
boffins out there got any thoughts?


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Using the patented Mavis Beacon "Hunt&Peck" Technique, Pete Fisher

Hmm...

Last I checked, wisdom with leccy motors was that gears were
unnecessary, since leccy motors produce max torque from a standstill?

 

Good point. Whatever the overall final drive ratio?

"Unlike other electric cars, the Tesla has two gears: one a high-torque
option to take the car from 0-60 at maximum speed, the second offering a
gentler rate of acceleration, taking the car up to its top speed of
130mph."

TBF, that's probably why he is going for a single sprocket then innit?
So any ideas on how to mount a sprocket carrier on a bicycle wheel to
allow experimentation with drive ratios?

The forum I found included this.
"Well, we bore our sprocket to be a tight press over the outer edge of
the bike chain sprocket (it needs pressing on in a vice). Then we place
several nuts/bolts/washers through the various holes and do them up
tight. Never had a problem except you probably need a TCT tool to bore
it."

Is a TCT tool the sort of kit a school DT dept might have available?

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| Pete Fisher at Home: |
| Voxan Roadster Gilera Nordwest * 2 Yamaha WR250Z |
| Gilera GFR * 2 Moto Morini 2C/375 Morini 350 "Forgotten Error" |
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Using the patented Mavis Beacon "Hunt&Peck" Technique, Pete Fisher

Well, there's obviously a point where starting results in a fire.

I think I have to play the "Whatever, I read it on the Internet" card at
this point.

Well, *that* was helpful, wasn't it?

 

I think TCT tool is a lathe tool - tungsten carbide tipped.

At a guess, they bore out the sprocket to suit the hub and then bodge it
to fit with whatever they can.

If you get a taper-lock type fitting, you can change sprockets within a
range reasonably easily. Depends on the wheel hub dimensions

http://www.lenze.co.uk/lenze.uk_en_...che_Spannelemente.uk.jsp?cid=0b0164e0800ec623
and
http://www.industrialmotion.co.uk/products/power/taperlockbushes.htm

Electric motors produce high torque right from start but you can
overload them if you gear too high. You can calculate or get specs on
the torque output from a motor

 

Have you looked here
http://www.powerchair-review.fsnet.co.uk/

An acquaintance has a powered one and it *is* fast....

 

Useful links, thanks. He seems to have already calculated an optimum
final drive ratio, though different sprockets for different tracks might
be a good idea.

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| Pete Fisher at Home: |
| Voxan Roadster Gilera Nordwest * 2 Yamaha WR250Z |
| Gilera GFR * 2 Moto Morini 2C/375 Morini 350 "Forgotten Error" |
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The motor the project uses is an electric wheelchair item. All
participants use the same type.

Some very useful stuff in the section on the four wheel drive chair
project - thanks.
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| Pete Fisher at Home: |
| Voxan Roadster Gilera Nordwest * 2 Yamaha WR250Z |
| Gilera GFR * 2 Moto Morini 2C/375 Morini 350 "Forgotten Error" |
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Because you cannot work with brute force to overcome any gradients or
head winds, being able to change gearing would be a good idea.

So-called plate sprockets are relatively cheap and can be welded or
bolted together like a push-bike set-up. We used a similar set-up to
optimize line speeds when working on new machines.

Now the price of electronic speed controllers has dropped, that is the
way to go and possibly worth investigating if this is a project- even if
the costs are beyond your budget. They extend battery life by delivering
just enough juice to do the job. Battery driven forktrucks and the like
use them.

Steve

 

It looks like for this project the absolute key is efficiency.
i.e. Minimum losses.

I don't know anything about electric motors except
that I am sure that they will not be equally efficient at all
speeds so gears will likely be necessary unless a constant
speed can be maintained and there is no wind and
the acceleration phase is not significant and ...
It will certainly be necessary to choose the
correct motor speed.

A google on [electric motor shunt torque series]
turns up quite a few decent looking results.
http://books.google.co.uk/books?id=...ig=KMd3sBeqKH4tiLapQKe-Vr_tiVY&hl=en#PPA31,M1

Page 30 has an efficiency graph.

If braking is required consider regenertive braking.
May be too hard to implement of course.

http://homepages.which.net/~paul.hills/Motors/MotorsBody.html
"Here is the efficiency graph for an Iskra shunt wound starter motor"
There are links here to performance graphs for a few kinds of motors
including efficiancy.

uk.rec.cycling may assist with some of the push-bike bits questions.

 

I suspect you'd be able to use a road bike cassette to get close ratio
gears - IIRC a strong cyclist can produce 300W of power...

 

Possibly a bit sophisticated for a school project, but I will pass it
on.
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| Pete Fisher at Home: |
| Voxan Roadster Gilera Nordwest * 2 Yamaha WR250Z |
| Gilera GFR * 2 Moto Morini 2C/375 Morini 350 "Forgotten Error" |
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In communiqué

As the event is won by distance covered rather than a race as such I
think the aim is probably constant speed. This is probably a lot easier
at Rockingham than Darley though.

Good thought.

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| Pete Fisher at Home: |
| Voxan Roadster Gilera Nordwest * 2 Yamaha WR250Z |
| Gilera GFR * 2 Moto Morini 2C/375 Morini 350 "Forgotten Error" |
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Excellent stuff Steve, many thanks.

Given modern kids' tendency to obesity, probably a significant
consideration. Though the lad is not alone in being like the proverbial
strip of wind at his junior school ( plus a fair few at the other
extreme).

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| Pete Fisher at Home: |
| Voxan Roadster Gilera Nordwest * 2 Yamaha WR250Z |
| Gilera GFR * 2 Moto Morini 2C/375 Morini 350 "Forgotten Error" |
+----------------------------------------------------------------+

 

Had a second look at this. Sheldon Brown (dead Bicycle mechanics guru) says
"The standard I.S.O. thread for freewheels is 1.375 x 24 TPI"

http://www.sheldonbrown.com/gloss_e-f.html#freewheel

TBH I'd be inclined to use bicycle components throughout - if there's no
regenerative braking option, you could use a freewheel to allow coasting. If
there is, you could use a fixed freewheel (not actually a freewheel, as used
by nutters and track bikes). However, remember that freewheels are threaded
so as to self tighten, and could eventually loosen given enough regenerative
braking / pedal backpressure. If you're allowed gears, you could use a road
cassette with a road derailleur, and have indexed (ie, one click up, one
click down, straight into gear) close ratio gears, and everything's nice and
light, with off the shelf and relatively cheap parts.

The other option would be to use a freehub as opposed to a freewheel, but
you'll not find an off the shelf BMX wheel that'll take a freehub. A freehub
takes a cassette of cogs via a system of splines and a locknut. You can see
a freewheel setup on the freewheel link... Again, single sprocket and fixed
options are available - on-one.co.uk used to supply such kit, and can
probably tell you who can if they don't any more.

Mounting the lot up will be fairly simple - make up or hack off a bike a set
of drop outs:
http://www.sheldonbrown.com/gloss_dr-z.html#dropout

Mount them onto your chassis at the correct spacing, and you're away. I'd be
inclined to use a single rear wheel setup for aero and simplicity if it's
purely about endurance / distance travelled, or even 2 closely spaced rear
wheels with one idling... OTOH you could look into what arrangement is used
on trikes. You'll easily get the kind of grip you need for power
transmission from a decent bike slick IMO, and very low rolling resistance.

If you need rear brakes, remember that you won't be getting discs to fit
onto a BMX hub easily, though you could build a mountain bike hub into a 20"
BMX rim - failing that, you'll be using rim brakes - cable v-brakes or
magura hydraulics are the only ones I'd consider fitting. I'd go for 32 or
36 spokes - bear in mind that BMX wheels are built for serious hammer. If
you have to have 2 rear wheels, it'd be simplest to put a 6 bolt iso disc
mount on the axle - Shimano and Magura brakes are available fairly cheaply
these days.

Cheap places for bike bits online include Merlin, Chain Reaction, Wiggle,
and SJS Cycles are handy for unusual parts and odds and ends - probably also
quite likely to have some knowledge of trikes too as they build frames.

 

And another thing. You won't need anything other than slicks if you use bike
tyres. Road slicks have the lowest rolling resistance and can be pumped up
to 100 PSI happily but are for big wheels (27" or so). Mountain bike wheels
are slightly smaller, heavier and IME the slicks for them are never as good
as proper road size ones. Don't know why, but they aren't. Slicks for BMX
wheels will tend to be fat, and made for grip on half pipes, not going fast
in straight lines, so you'll need lower pressures and get higher rolling
resistance. There's obviously going to be a comprimise between rolling
resistance, aero and inertia (though road rims and tyres are very light, a
bigger wheel will take more spinning up to speed).

The other thing is that bike tyres are crossplies and are designed to go
round corners by leaning, which isn't usual in 4 wheeled vehicles...

 

Dead? <googles> ****.

 

Indeed. The most remarkable thing about him was the way that he'd answer any
technical query with a lengthy and considered reply, despite the fact that
he must have got hundreds.

 

More very useful stuff. Many thanks.
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| Pete Fisher at Home: |
| Voxan Roadster Gilera Nordwest * 2 Yamaha WR250Z |
| Gilera GFR * 2 Moto Morini 2C/375 Morini 350 "Forgotten Error" |
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You make it sound as easy as FOAL (think logs not Fibre Optic Antenna
Links). I'll pass it on.
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| Pete Fisher at Home: |
| Voxan Roadster Gilera Nordwest * 2 Yamaha WR250Z |
| Gilera GFR * 2 Moto Morini 2C/375 Morini 350 "Forgotten Error" |
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