Ok this is going to sound like a strange request.

For my A2 DT project I'm designing and building a rolling road for a small electric car. It needs to be able to produce resistance approximately like the car will experience in a racing environment and allow long high load tests. The common method used in car rolling roads seems to be a water based resistance system, which sounds rather complicated or rarely an electric motor based system, which sounds more complicated.

A simple solution I've come up with would be to use air resistance trapping the air in bucket like things attached to some kind of hub. It would also have the advantage of offering progressive resistance, like drag does to the car in the real world. So I do a bit of googling and something smacks me across the face, most simple gym machines use air resistance to offer resistance and have some kind of system of changing the resistance.

Unfortunately google hasn't turned up anything much of a technical nature on the internals of gym equipment, even some photos of the buckets/blades and the system to vary resistance would be a start.

Or if anybody else has suggestions of an alternative system that would be very much appreciated as well.

I think one of the simpler ways is like an old stationary bike. There's a large wheel up front made up of wide(ish) paddles that provide the drag. You can then adjust the resistance through gearing.

Why not attach a wheel such as this http://www.hp-gramatke.net/pic ... physics/water_wheel_3.gif . Place this wheel in a container of water and then have different sized blades. The different sizes (larger/smaller area) will allow you to control the resistence on the system.

I can only think of the rower, which uses the wide paddles that Forbin mentions, with an adjustable cover on the blades to stop separate the internal air chamber from external air, thus reducing resistance. I don't know if they are angled at all.

I know Madman_CZ owns his own rower, I'll point him in the direction of this thread in case he can take a quick piccie for ya.

Quote from ans7812 :

Why not attach a wheel such as this http://www.hp-gramatke.net/pic ... physics/water_wheel_3.gif . Place this wheel in a container of water and then have different sized blades. The different sizes (larger/smaller area) will allow you to control the resistence on the system.

Much the same idea, I've gone through a number of different solutions and although water resistance may be more effective I think I can achieve what I'm wishing to do using a system of air resistance. This has several advantages, firstly it's less messy, I know a water system shouldn't leak or splash if well designed and built but it's one less thing to have to worry about, it's also lighter which will be part of the specification for this device.

What I'd really like is something somewhere giving me a ball park idea of what shape and size is going to get what kind of resistance without getting overly technical (as the actual engineering and physics element will only take more time and not help towards the project mark). If I could stumble across a 'how to build an air resistance exercise machine in your shed' guide or anything on working out roughly how much resistance to expect with a certain shape of paddle at different speeds that would be to convenient to be true. I'm considering using reduction gearing as an option to vary resistance but keeping it as a direct drive system would make it simpler (and cheaper).

I don't have much time for testing as part of my research but would ideally like to produce a product that is actually usable (because there is a genuine demand on this DT project for once)

Quote from ajp71 :

Much the same idea, I've gone through a number of different solutions and although water resistance may be more effective I think I can achieve what I'm wishing to do using a system of air resistance. This has several advantages, firstly it's less messy, I know a water system shouldn't leak or splash if well designed and built but it's one less thing to have to worry about, it's also lighter which will be part of the specification for this device.

What I'd really like is something somewhere giving me a ball park idea of what shape and size is going to get what kind of resistance without getting overly technical (as the actual engineering and physics element will only take more time and not help towards the project mark). If I could stumble across a 'how to build an air resistance exercise machine in your shed' guide or anything on working out roughly how much resistance to expect with a certain shape of paddle at different speeds that would be to convenient to be true. I'm considering using reduction gearing as an option to vary resistance but keeping it as a direct drive system would make it simpler (and cheaper).

I don't have much time for testing as part of my research but would ideally like to produce a product that is actually usable (because there is a genuine demand on this DT project for once)

Reduction gearing sounds like a great idea actually. If you are able to get it up and running without serious problems, i bet it will work great, be easy to change resistance and also look really neat.

Edit: I don't know if this helps or not but i remember i once took apart an old Coo Coo clock. Inside i found a menagerie of gears, big to small. In total, i would estimate around 10-12. For a small little clock, it was very interesting. I'm not sure how big your design is but you said its for en electric car so i'm guessing it isn't too large. If you need gears that fit into a 4 inch by 1.5 inch housing, a coo coo clock might be the way to go

Quote from Bob Smith :

I know Madman_CZ owns his own rower, I'll point him in the direction of this thread in case he can take a quick piccie for ya.

I am afraid that my rower is not working with air resistance. It is a magnetic rower so i am afraid I cannot help.

Quote from ajp71 :

Or if anybody else has suggestions of an alternative system that would be very much appreciated as well.

This is simple to make - and an online source for magnets. Another way would be to hook up an electric generator/motor and get back part of the energy being consumed. Talk about fancy.

I do photography for my local gym, but unfortunately I'm not allowed to start pulling the resistance kit apart in order to photograph the internals of them. If I could, I'd have gotten some detailed shots for you.

One thing that springs to mind is the resistance system on spinning bikes - a freely-turning flywheel with brake pads to provide the resistance. Problem is then of course that you have to engineer a system to provide adjustable brake pressure, which is a simple screw turn on spinning bikes, but it's all pretty substantial stuff.

Here's the clearest shots I've got of how spinning bikes get their resistance:


(They're crops by the way, not just really bad photos... )

Quote from STROBE :

I do photography for my local gym, but unfortunately I'm not allowed to start pulling the resistance kit apart in order to photograph the internals of them. If I could, I'd have gotten some detailed shots for you.

No need to pull those apart - the flywheel/magnet system is clealy visible - if that's what it is in the first pic, the adjuster in the middle appears to regulate the distance of the magnets on the caliper below.

Oh no, the pictures aren't the resistance kit I was talking about. The rowers, cross trainers, etc are all air resistance, and you can't see inside them. For obvious reasons the whirry bits are all enclosed either in solid plastic or such fine mesh that you can't see through.

The pictures above are examples of a spinning bike (in the studio, for classes) which uses friction resistance. There's no magnets, just a set of brake pads which you screw in and out with the adjuster next to the red lever (the red lever being the emergency brake, of sorts). Sorry for any confusion.

Quote from STROBE :

There's no magnets, just a set of brake pads

Oh - d'oh. And here I was wondering why the magnets were at the side and how the worked in providing any resistance given their apparent size... good old friction... chew on that, Magneto.

Quote from ajp71 :

Ok this is going to sound like a strange request.

Yep just the other day I went down to a gym equipment breakers yard to do a bit a seat sniffing. I got so excited I whipped out my camera & started snapping like crazy. OMG I was in heaven.

(seriously though good luck with your project)

Cheers Strobe, nice simple system, certainly useful for my research. Not sure about its suitability though because I'd be restricted by the size of plate I could force into the lathe

Quote from ajp71 :

For my A2 DT project I'm designing and building a rolling road for a small electric car. It needs to be able to produce resistance approximately like the car will experience in a racing environment and allow long high load tests. The common method used in car rolling roads seems to be a water based resistance system, which sounds rather complicated or rarely an electric motor based system, which sounds more complicated.

From what I can remember the water system is a simple friction brake with water cooling of the braking parts. I did an experiment at university with an old (1904, I think!) single-cylinder gas engine attached to such a dyno...if you know the water flow rate and the temperature rise of the water flowing through the braking system you can calculate the power being dissipated

I'll have a look through some books at work tomorrow...our rolling road is a fully transient AC electric system but we've got some old textbooks in our 'library' which describe some other dyno systems.

I'm not an expert in dyno hardware but I would imagine that any sort of electric motor would be able to absorb power from your rolling road. Connecting a high-power, low value resistor across the motor terminals should build up quite a lot of resistance when the motor shaft is turned. Add in a voltmeter and current meter and you should be able to calculate the power being absorbed. Don't quote me on that though

Quote from ajp71 :

Cheers Strobe, nice simple system, certainly useful for my research.

No probs. I'd offer more shots but they're all largely the same in terms of the view offered of the mechanism. Needless to say, the camera was focussed on the girl riding the bike.

Quote from STROBE :

Needless to say, the camera was focussed on the girl riding the bike.

Which was sadly cropped out for the sake of the scientific interests of this thread.

Quote from xaotik :

Which was sadly cropped out for the sake of the scientific interests of this thread.

Yep, sorry, the instructor is far too lovely for me to start splashing her pictures around on the internet.

I want pictures when this is completed.

Quote from xaotik :

Another way would be to hook up an electric generator/motor and get back part of the energy being consumed. Talk about fancy.

Yeah, a generator, and then you could make the power go through many resistors, and have a fan to cool the resistors, just like on a diesel-electric locomotive.