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Ross Crawford / projects / champgb

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2-speed gearbox for Silver Champion

Well, I finally forked out for a Silver Champion, after figuring I wouldn’t find one for much less than the AU$195 I paid, and I’ve gotta say, a couple of things disappoint me a bit. Number 1 is the amount of play in the suspension - those springs aren’t really doing much at all most of the time. Number 2 is the steering - with the steering wheel straight, the wheels point slightly to one side, also the geometry means the outside wheel turns further than the inside - not good for high speed cornering. And to be really picky, you’re not gonna see much out of those wing mirrors - check the view you get!

But the one thing I really miss in this set is a gearbox - it’s the only large technic car since the 70s without one! So I figured I’d add one. And here’s the result.

Update 9 Jan 2002

I changed it to use a new-style diff. The actual gearbox is moved 1 stud towards the rear, but otherwise un-changed. The final drive is pretty much pinched from the 8466 4x4 set. Here’s the photo (sorry no dat file).

Using new differential

Initially, I considered using a crash gearbox, like in the 8860. This would probably allow me 3 speeds, even with the space restrictions (see below), but I decided against it for 2 reasons:

The gear selector in the cockpit would need too much travel to be “nice”
Ever seen a Formula One car with a crash gearbox?

So I pretty quickly decided on a 2-speed, based on the more recent technic-type gearbox parts.

Once that was decided, I started looking at the space I had available. My conclusion - there aint much! Basically, there’s only a cross-section of 4 studs by about 3 bricks, so realisticly only room for 2 axles one above the other at the standard 2-stud separation. Also, the largest gear which can be used directly under the engine is a 12-tooth. This meant all the selection mechanism (for which we’re forced to use 16-tooth gears) must fit under the suspension, between the rear axle & the engine.

I quickly realised I’d have to replace the diff with an older style one, there simply wasn’t room to get the 90 degree conversion & the gearbox all behind the engine otherwise. And believe me, it’s not trivial replacing the diff on this car!

But once that was done, I had to work out the gearbox mechanism. And this is what I came up with.

CAD drawing

Most of it’s fairly self-explanatory, but the bit that isn’t is the connection between the right hand 16 tooth clutch gear & the 20 tooth bevel gear. There’s a #2 axle sticking out of the axle joiner, which is hidden. The clutch gear rotates freely on it. The dogs on the right side of the clutch gear mesh with the 1/2 bush dogs, thus forcing the clutch gear to rotate with the 20 tooth gear. See instructions below. Well OK, it’s not really clear in the instructions, either 8?( I’ll re-do them if I get a chance.

So in low gear (shown), the drive goes through the 12 tooth to the 20 tooth, along the #6 axle (bottom left), and to the rear via the 2 16 tooth gears. In top gear, power is transmitted through the centre 16 tooth gear to the other clutch gear.

Now, even with the mechanism worked out, I still had to fit it all in. Turns out the main problem was finding a way to slide the gear selector. A normal selector brick just wouldn’t fit anywhere. What I ended up using (not immediately obvious from the instructions) is two 4-stud lengths of flex tube, with the bottom one wrapped in 2 3/4 technic pins. When held by the 2 3x3 liftarms (yellow), these worked really well.

So here’s some pictures of the actual implementation.



This is a close-up view of the gearbox from underneath. You can see how tight the space is. You can also see the 3/4 pins on the flex hose.	Here’s a full under-side view. You can see the linkage, and the bottom of the gear lever (red) beside the driver’s seat. You can also see I had to remove a 1x6 technic brick to fit in the gear lever.	This picture shows the slight modification to the engine bay to allow room for the linkage.	Here’s what the driver sees - just a red gear lever. Nuetral is in the centre, back for low gear, forward for high gear.	This picture shows the reduction in ground clearance. It’s about halved in the normal standing position.

So there it is. Obviously there’s some negatives:

It’s only 2 speed
It requires an old-style differential (now overcome!)
It reduces the ground clearance a fair bit (but not prohibitively so)

But balancing that with the positives:

It’s a gearbox!
It’s 2 speed!
It works!
It requires no major chassis changes

I think I’d class the project as successful!

Anyway, here’s the instructions:

Site last modified November 4, 2004 Home

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