Invicta Military-Vehicle Preservation Society


Jeep Disc Brake Conversion Update

By Nigel Hudson – Weald Group

Back in the Spring of 2016, I converted the front axle of my Hotchkiss jeep from the original 9″ drum brakes to a Suzuki Vitara calliper disc brake set up and shared my findings with the ImPrint readers (Summer 2016 edition). Gareth Wear has posted it up on the IMPs website now too in the ‘Under the Hood’ section. 

Although pleased with the extra braking power provided by just a front axle conversion, I’ve been thinking more of changing the rear axle over too and will be proceeding with that project as soon as I can source some callipers in a decent condition. Once complete I’ll finally be done messing with brake adjustments for good. On the subject of sourcing Vitara brake callipers, in hindsight I would have sought out the Tokico variety of calliper rather than the Girling/Lucas variant. The Tokico ones seem easier to come by with quite a few aftermarket suppliers manufacturing their own replacement ones (should you not trust to a set taken off a salvage vehicle). As it is I’m still looking for a pair of Lucas callipers just for standardisation and also so that I only need to source 1 type of brake pad (as the 2 varieties use different pad designs and aren’t compatible with each other).

My current set up just on the front axle does require a similar push on the brake pedal as with the drums, the difference being it has a massively increased effect. The old drums just used to stop you when they stopped you, no amount of extra pressure ever seemed to make the slightest difference.

Further to my original article, here are a couple of updates.

  1. In my original conversion, I transplanted the old 1″ front brake cylinders to the rear axle to give an improvement in the rear braking action. However, even with the Adjustable Proportioning Valve dialled out to reduce the hydraulic pressure as much as possible, the rear sometimes gives the impression of locking up before the front. With this in mind, if I intended to stick with the rear drum set up, the only 2 ways to cure this would be to fit another APV in series to the rear to further lower the pressure or go back to the original 3/4″ brake cylinders. As I will be converting the rear to discs in the near future, I mention this only as advice to anyone thinking of converting just the front axle, to possibly keep with the original 3/4″ rear brake cylinders.
  2. When tightening the wheel nuts to secure the front wheels, the inner profile of the Hotchkiss non-combat wheel means that it rests upon the edge of the inner disc assembly leaving a space behind the wheel nuts.

 I have rested a steel rule in image 1 to better illustrate the gap I’m referring to. The edge of the raised central section of the disc sits against the top of the semi circular ridge in the wheel indicated just below the 2 1/2″ mark on the rule. If you were to imagine the disc having the same profile as a straw boater hat and you placed the hat upside down on the wheel in place of the ruler shown in pic 1 this may help you to picture what I’m trying to describe.

To prevent this, I temporarily packed this out with a couple of washers over the wheel lugs between the disc and the wheel so the wheel now tightens against the washers not the disc edge. Don’t be fooled by the counter-sinking around the wheel lug holes, this view really is that of the innermost side of the wheel. Not an ideal solution I know but this was an unexpected issue and one I soon rectified by replacing the washers with a universal fit 4 X 4  alloy 5mm thick spacer (5 hole fitment, 5.5″/139mm)

1 The Gap
2 Hotchkiss wheel plus 5mm spacer

Last Autumn I was asked about the suitability of this conversion by someone wishing to use the Wartime Combat Split rims. I had read in the US forums where I got most of my information for my original project that a 1/2″ spacer would be required to prevent the wheel from hitting the calliper body. With my curiosity roused, I went to see Gareth Wear to ask him if I could try fitting the spare wheel off his combat wheel equipped Willys onto my front axle and here’s what I found.

Firstly, I tried using 2 of my 5mm spacers. With the wheel nuts only done up finger tight and spinning the wheel, you can see from the picture above that it took off a small chip of paint from the wheel (sorry Gareth!) so clearly a 10mm spacer will not provide enough clearance. It’s also apparent that the spacer is necessary not to clear the split rim bolts but to provide clearance from contact with the rim itself. So then I tried using three 5mm spacers and that gave ample clearance.

3 Combat rim with 10mm spacer
4 Combat rim with 15mm spacer

So it seems that the jeep forum advice to use 1/2″ (12.7mm) spacers is correct. However that’s not quite the end of the matter. If you look at the profile of the combat rim in pic 5 you can see that as well as a semi-circular ridge around its edge, the central opening also has a 90 degree lip (which the Hotchkiss rims don’t have)

These both sit against the original brake drum which means that the rim is not supported immediately around the wheel lugs as they are tightened down. A bit of a design flaw if you ask me although one that has been rectified on the Hotchkiss (where there is no void between the wheel rim and the drum assembly). Due to the profile of the combat rim it would be very tricky to machine a spacer to take up this void, especially as the rims I looked at were quite bent out of shape. Bear in mind that the 1/2″ spacing is measured from the top of the lip and semi circular ridge to the disc central body surface (upon which it bears) not from the lower wheel rim surface. This means that the 1/2″ spacer would need a small enough inner diameter to bear on the inner rim lip and a large enough outer diameter to bear on the semi-circular projection. The universal spacer I show for my Hotchkiss wheel is not really suitable as it’s too small (and obviously not thick enough).

5 Combat rim and a 5mm spacer.
6 Universal Spacer dimensions are too small to use on Combat rim.

Unfortunately, you’re going to need a set of spacers specially machined up. Basically if you took the dimensions of the disc central body, that would be ideal for the spacer. In other words, use the disc as a spacer template. And have it machined out of aluminium as a 1/2″ steel spacer will add quite a bit more weight to your wheels.

Hope this helps any of you attempting this brake conversion. I’ll be sure to write up an update of the rear axle conversion once it’s done.

Nigel Hudson

7 Use disc as spacer template (inner diameter 107mm, outer 190mm)

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