Differentials by Ian Oddie

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Differentials by Ian Oddie

Postby Lone_Ranger » Mon Nov 26, 2007 1:29 am

Admin Post: The info posted here has been written by Ian Oddie.

The diff is one of the most important items of a large scale model car that has direct influence on the cars characteristics and lap times.

The problem with only rear wheel drive as we have is that drive out of the corners can easily be lost, and having an engine screaming on the exit of a corner may sound 'fast', it's actualy slow and isn't doing anything for tyre wear either. Some car settings can have adverse effects on the requirements of the diff as they provoke diff out despite having a beneficial effect on the set-up and driving charateristics of the car. Obvious ones are running stiff rear roll bar, but theres lot's more such as front hubs with large king pin off sets or useing lots of castor angle. Both create jacking of the car on lock, unloading the inside rear wheel.

Theres many many different versions of diff's, and really there are only 5 types ever used in large scale model cars. in particular order:-

1, Standard basic gear diff, no locking effect what so ever and even filling it with thick silicone has negligeable effect on the track. Easy to drive though in any condition and on tricky surfaces (changing, patchy or wet) can be very good. If the track has huge amounts of grip it can also be ok, but that only happens at some world championships.

2, 4 plates or taper cones and 2 plates behind the planet gears. Depends on the materials used to how aggresive the locking effect is and also the angle of the tapers. The latest diff's i do of this type have 2 flat plates and two tapers of 15 degrees (other manufacturers use 25/30/35 degrees because they were the angles being used when they copied the mods ). Can be very very good, but the track has to be suitable, medium to high grip, fairly smooth and don't run the curbs. Because the diff locks with the torque through the diff, any bumps/curbs unsettle the car and can make it very difficult to control. No good in the wet.

3, True scaled down lsd (ZF type). There's only a few of these around, Jozeph (rs5) built a handfull and i built 3 around 10 years ago. Very complicated which would equal very expensive. They can work very well though, but still very track dependent and best on very high grip. As they work with torque on a cage around the 4 planet gears, the locking effect is governed by the angle of the V between the cages and the oil/grease inside. They can also have a different V angle (locking effect) for acceleration to deceleration which no other large scale diff can do. Not generaly available, but also not a general purpose diff either. No good in the wet.

4, Viscous diff, originaly designed by an Austrian i believe for model cars. Can be very good in the right conditions with the right grade oil/amount inside and the right amount of plates. Generaly better on good to high grip, but even then can sometimes make the car unpredictable especially on bumps. Of the two makes(fg, lauterbacher), i found in testing that the lauterbacher viscous diff was generaly better. No good in average, poor, or wet track conditions.

5, Mag diff, made by Harm. Totaly different concept to the others, with 12 magnets fitted in rings on the output gears. These generate power in the case which is a dead short so the locking effect directly proportional to the diff out. The bonus is that it is a smooth locking effect no matter what the surface. Mods for them are fitting 2 mild flat friction plates and that's now standard on new ones from Harm. The latest mod i do is to convert it into the EL diff, this has a metal former fitted in the centre of the diff with two copper coils wound on it. This gives a massive increase in drive/locking effect but is still perfectly smooth in it's operation on the track. It is effectively an electric motor in reverse, as a brake but only for diffing. Works very well in all conditions, wet to very high grip.

I used the mag diff in the worlds in Lostallo in the wet, and have used one ever since including at the worlds in Australia which was high grip. 1st and 3rd respectively

That should do you Daz

Ian

Thank you very much Ian, on behalf of all members, much appreciated!

Edit: Added by Daz. For the context, see This Post.

:mrgreen: :mrgreen: :mrgreen: :mrgreen: :mrgreen:
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Postby RcTek » Thu Nov 29, 2007 11:05 am

Hi,

Ian Oddie has kindly provided some images of a prototype magnetic differential that are linked from this page and discussed in this topic.

Many thanks to Ian.

Daz

Edit by Daz to update link to pictures.
Last edited by RcTek on Tue Dec 09, 2008 7:43 pm, edited 1 time in total.
Hmm...
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Postby Thomas Vennekens » Sun Jan 13, 2008 10:57 pm

i always wondered how magnets can lock aluminum ???
can someone give some explanation on the locking effect of magnets rotating in an all aluminum casing

this is what i found on magnets and aluminum :

Paramagnetic materials are metals that are weakly attracted to magnets. Aluminum and copper are such metals. These materials can become very weak magnets, but their attractive force can only be measured with sensitive instruments.

Temperature can affect the magnetic properties of a material. Paramagnetic materials like aluminum, uranium and platinum become more magnetic when they are very cold.

The force of a ferromagnetic magnet is about a million times that of a magnet made with a paramagnetic material. Since the attractive force is so small, paramagnetic materials are typically considered nonmagnetic.

for the record i have one myself and got dutch national champion with it :mrblue:
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Postby Oddified » Wed Jan 16, 2008 10:56 am

Electric current is generated in the aluminium case which is an electrical conductor, similar to useing copper (which is also not magnetic) wires in a motor/generator :idea:

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Postby Thomas Vennekens » Wed Jan 16, 2008 9:58 pm

but the case can be seen as one wire thats shorts everything ?
i still not convinced because when i turn one wheel ,the other turns the opposite direction without any lock feeling .

:-?
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Postby tony1703 » Wed Jan 16, 2008 10:44 pm

Turning by hand is not fast enough. Start the motor, with the car off the ground hold one of the back wheels and rev the motor, then you will feel the effect.

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Postby Thomas Vennekens » Wed Jan 16, 2008 10:52 pm

i also tried that one , but thats more the drive train inertia that you feel at that moment , the other wheel will spin twice the speed of the diff
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Postby Oddified » Thu Jan 17, 2008 9:23 am

Yes the case is effectively a dead short, the extra coil system i fit inside is also connected as a dead short. The locking effect of the mag diff can only be felt when the diff is 'diffing' at far more rpm than can be done by hand, and by the way it works the locking effect is directly proportional to the rpm.

Ian
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mag diff

Postby Thomas Vennekens » Mon Jan 21, 2008 6:40 am

its like the eddy current effect

on youtube you have many movies about that effect

thomas
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Postby Tony Turbo » Wed Jan 23, 2008 4:53 pm

This diff sounds too god to be true.... Where can I get one? 8) 8) 8)
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