Scottish Elises

Okay this is very not lotus

Shrek my faithful old landrover is about to get an upgrade as in fiddle brakes.

Fiddle brakes are basically a handbrake for either of the rear wheels. So you pull the left one the left wheel is brake pull the right one and the right one is braked. These have two uses. Firstly you can lock the inside wheel and turn more tightly which improves the turning circle.

The other use is because i am running an open diff is that if one wheel starts spinning i loose drive from the rear axle. So if your left wheel is spinning you pull the fiddle brake to stop the wheel spinning and the diff feeds power to the other wheel.

Now the problem is how do you know which wheel is spinning?

So what i was thinking about was some sort of pick-up that sees wheel bolts or something and then each time it sees the bolt a light flashes. Then in the cab have a light for each wheel and you brake the one that is flashing.

So over to the electrical geniuses to tell me how to do the electrical side as the mechanical side is easy, well once i have fitted larger brake calipers and moved a shock absorber mount and replumbed the brake system.

Paul,

Low tech suggestion here - could you not stick an extension piece on to the tyres' valve as a witness marker and then look in the mirror?


Mac

How about two speedometers? You can then match the speed of the wheels with the brakes - better than just locking up the spinning wheel

mac wrote:Paul,

Low tech suggestion here - could you not stick an extension piece on to the tyres' valve as a witness marker and then look in the mirror?


Mac

Mirrors?

Whats behind me isn't important

But it has a nice low tech feel to it

I don't want anything that sticks out too far as you have to squeeze through gates and anything that makes it wider is bad.

David wrote:How about two speedometers? You can then match the speed of the wheels with the brakes - better than just locking up the spinning wheel

Then i would need to work out how to get a speedo on each wheel which brings me back to the same problem and that would cost money.

I did consider getting a couple of bike speedos but i wouldn't want to be trying to read them while bouncing around like a mental thing hence the low tech flashy light idea. As small hard to read numbers while you are trying to drive up a very steep bumpy bank and keep the thing upright and in a straightish line and somewhere near the ground could be hard.

How about just creating a flash on a light for each axle everytime each one does a revolution. That way you can tell when ones slipping as it's flashing faster.

You could create the flash with either a photo sensor (white spot or mirror on rotating bit) or magnetic sensor (magnet on rotating bit) to produce a pulse and show it on an LED.

maybe you could use a couple of these? Have something rotating that breaks the beam?
http://www.maplin.co.uk/Module.aspx?ModuleNo=27513

Simple answer...

Just pull both brakes, will brake the spinning wheel more and transfer torque to the stopped one...

Powered variable reluctance sensors are probably the way ahead if you really want an electronic solution. They come in industrial form factors; if your rear shafts are bolted onto flanges at the diffs then that would be the best place to point them at rather than the wheel nuts, simply because the wheel nuts are a bit more exposed (though everything relative when you're talking about shrek trekkin').

You can supply the power from the car electrics easily enough; output can drive a reasonably bright LED directly - if you put one LED on the left and one on the right of the dash, you'll see which wheel is spinning easily - might be worth putting the LEDs so that you need to move the handle towards the flashing LED to cancel the wheel spin, but that's entirely up to you.

Something like this will work, though these are a tad expensive:

http://uk.farnell.com/omron-industrial- ... dp/1608308

Cheers,
Robin

I have been playing about left foot braking in the DC2, basically just trailing it through the corners to stop one wheel spinning up.....it really does make quite a difference to traction even on a car with a LSD

I used these on the Elise for TC/LC through my data logger. 3 wire configuration, power (12v) earth and signal. They actually have a little LED built in so you can see if you have them positioned close enough to what ever you use as the trigger.

Thats what i was thinking of.

They look dead easy to mount and all i would need is to drill some holes in the brake disc so it sees metal/no metal going past it.

Now are they capable on their own of running an LED or do i need someone with a knowledge of white mans magic to design a circuit to flash lights at me?

alicrozier wrote:Simple answer...

Just pull both brakes, will brake the spinning wheel more and transfer torque to the stopped one...

Tried that with left foot braking and it is only marginally successful.

thinfourth wrote:Thats what i was thinking of.

They look dead easy to mount and all i would need is to drill some holes in the brake disc so it sees metal/no metal going past it.

Now are they capable on their own of running an LED or do i need someone with a knowledge of white mans magic to design a circuit to flash lights at me?

Before drilling disk check that the disk is the type of material that will trigger the sensor, and maybe use an old disk to work out what diameter hole is required to ensure the sensor un-triggers when the hole comes flying by.

They will drive a LED directly with a series limiting resistor. Depends on what colour you want, but led's just say it's 'king bright red like this one http://uk.rs-online.com/web/search/sear ... &R=2471909

This has a forward voltage at 20mA of 5.5v. That means you need to make sure that once the current flowing reaches 20mA you're series limiting resistor needs to make sure the supply voltage is dropped to 5.5v.

Now you're supply is 14v when the car is running, so you need to drop 8.5v at 20mA; V=IR, R=V/I, R=8.5/0.02=425 ohms. Using a 470 ohm resistor (standard value) you'll get 18mA, which gives you a bit of headroom.

But the light will stay on the whole time the sensor is triggered - in other words when you're wheel is stopped you'll get either a solid on (wheel nut in front of sensor) or a solid off (space in front of sensor).

You really want it to pulse on for each trigger from the wheel, so that the light will blink for the rotating wheel and deffo be off for the stalled wheel.

To make this work you can use either the classic 555 timer or something more intelligent (e.g. feed both signals into a microcontroller which can then compute differential wheel rotation speeds and only trigger one LED or the other when the differential rate exceeds some threshold).

So phase 1 would be wire up a couple of sensors to plain LEDs with 470 ohm resistors (you'll need 1/4 watt or larger) and then see what happens to the LEDs in various circumstances. Then work out what you actually want the LEDs to do and splice in the relative electronics or software to achieve it later.

Cheers,
Robin

Slightly O/T, but related - BMW had kind of the inverse of this system on the M3 GTR endurance racer. Brake/steering feel was so bad that you couldn't tell if you locked a wheel, so it had lights on the dash telling you when you'd locked a front.

/font of all useless knowledge faction

Have a look at these

http://www.greengauges.com/help/faq4.asp

its the speedometer solution in an off the shelf pack (admittedly at £140+vat+pp each) where you epoxy magnets onto your rotating bit (fnar) and then calibrate the supplied sensor.

Right solution, maybe just a little dear..



EDIT: Brain wave...How about just buying some cheap pedal bicycle speedos?

here you go, the whole solution for under a tenner.
http://www.bestofferbuy.com/Sunding-Ele ... rrency=GBP