1974 Unibody Triumph Spit6 (with some detours inbetween)

[biturbo228]

So, I started playing around with the handbrake pivot to try and find the least-worst option. Modified a 19mm deep socket so I could compress the belleville washers to swap the Peugeot pivots about:



One of those times when I was very glad to have bought an absolute monster of a vice (Record 25) as it only just fit between the jaws!



I tried swapping over the pivot arms side-to-side, but they get too close to the flexi union:



Initially I thought welding on the old pivot arms like this might work:



Before my brain finally caught up and realised that this would be pivoting the wrong way...

So I tried an underslung option which might work ok...



...before deciding on this arrangement:



The reason I prefer this one is because of GT6M mentioning having to keep the cable tensioned tight enough to make the brakes drag to get the handbrake to work at all. By setting the arm back at a shallow angle the effective lever arm is very short for the initial bit of travel, helping take up slack in the cable quickly. The effective lever arm then increases as you apply the handbrake, helping the last few clicks of the handbrake to put more force on the cylinder. When you look at OEM handbrake mechanisms this is usually the arrangement they have. Pic showing the lever arm after the handbrake's been applied:



The downside to this setup is I'll have to mount the caliper upside-down to get the cable orientation right, but I figure you use the handbrake more often than you bleed the brakes, so the compromise is in the right direction.

So with that decided I set about copying the shape onto the other side. Bent up another piece of 7mm plate and drilled some holes to align it up:



Bit of work with a flap disc and we end up with this




Weldy weldy:




Goes on something like this:

[biturbo228]

One of the good things about the new house's living is a reasonable proximity to a place that does zinc plating. Means I can make things and turn them a pretty yellow colour



Needed to tickle the original handbrake cable mount with a hacksaw to get the arms to fit properly across their whole travel:



Fiddly old job getting all the various widgets back into place. Special cut up socket, some needle-nose pliers, a small screwdriver and a lot of delicate fernangling got everything in the right place at the right time:




Done and looking shiny



Hopefully these should work fine with the stock handbrake cables (maybe with some light modification). The action pulls directly back into the centre of the car:



I'm hoping these arms will alleviate some of the issues Markus had with his iteration of these calipers. He said he had to keep the handbrake cables wound so tight they would bind to have any workable handbrake at all. By having quite an oblique angle to the arm, the first few millimetres of travel on the cable will turn the pin a lot of degrees, taking up the slack quite quickly. Then, as it actually starts applying pressure the effective lever arm should get longer, applying more torque for a given cable travel (up to 90 degrees of rotation).

Time will tell if it's a bit too aggressive, but the whole thing wasn't too difficult if I need to make them again.

[biturbo228]

Next step is to actually try a trial fit with the correct disc before I pull the trigger on a machined caliper bracket. Too many different things measured here (some of them in quite challenging ways) to be confident it's precise. I'd at least like to make a wooden mockup to test things.

The disc I'll be using is a front disc from a base spec 205/306. It's 247mm wide so fractionally larger diameter than the MGF disc, and has a 66mm centrebore which should locate onto the MGF hub spigot, but the top-hat section is shorter. That makes the whole thing just a smidge lighter (disc and caliper bracket) which is nice.

However, the PCD needs redrilling. As this is something I might have to do a few times I've been designing a tool for it:



The PCDs covered (in order) are:

• 4x95.25
• 4x98
• 4x100
• 4x101.6
• 4x108
• 4x114.3
• 4x130

The holes are 12mm and it's 20mm thick, so it should guide an M12 drill acceptably. There's 60-degree chamfers on the top surface to match wheel bolts so it should locate in a similar manner to lug-centric wheels (i.e. well, but with a bit of care). For brake discs I'm planning on buying a set of M12 wheel bolts and M12 wheel nuts to bolt through the disc with chamfers on each side to locate them. It'll be made in steel to prevent wear from the sides of the drill.

As a singular piece I've had it quoted at about £170, but if there's a couple of people who want to buy in then it gets cheaper.

If there's 5 other folks who want it then it gets down to £60 each (plus onward postage). If there's 10 then it's £50 (plus onward postage). These are cost figures, I'm not trying to make a profit.

[biturbo228]

So I've been fettling with the design of the PCD adaptor tool, and found that 3D printing it was a lot more affordable so ended up ordering a prototype. I went for nylon, which reading about it isn't the best choice of material for this, but hey ho I'd already ordered it. I use Xometry for machining, mainly because it's easy to use and finding local machine shops that are in any way interested in one-off private work is difficult. Same reason I want to be able to redrill discs myself rather than giving them to a machine shop. I can't find one nearby!

This is what I ended up with:



Learnt a lot from the prototype. Firstly, I'd ordered it before the wheel nuts I'm using to attach it turned up and it turns out the taper on their seat doesn't start until ~16mm out so it only just contacts the locating portion of the tool. It still worked, but could be better.



Second issue was that the tool would need to be much taller (and made out of steel as original) to locate the drill properly. It just about worked for marking the starting point and then drilling the rest by hand, but the tip did wander a bit and required fettling and correction with a centre punch (which was a bit of a stab in the dark, though aided slightly by the ability to bolt the tool back on to check concentricity).

Pretty proud of managing to get it so that with 12mm holes it slipped over the studs of the M12 MGF hubs, though it took a lot more care, effort and luck than I'd like.



I've since discovered that transfer punches are a thing. I'd been searching in vain for ages trying to find a straight-shanked centre punch with an OD I could trust and found nothing. Was even looking at the cost of having something made up. Then, I was idly talking to my mate about this and he said 'what, like a transfer punch?'. Lo and behold I search for that and tons come up for affordable prices! There's a set on order for doing disc number 2.

Anyway, I opened up the holes to 13mm and they slipped on fine. Turns out the MGF disc uses the hub OD for centreing not the spigot so the 66mm centrebore of the Pug discs does nothing, but I can get a set of spigot ring adaptors to sort that out.

That allowed me to trial the whole setup together. Disc, calipers and pads. Just as well I did! Turns out I'd made the assumption that the disc sits central in the machined cutout in the Peugeot calipers, but it really doesn't!



Luckily for me, that actually moves it in a much better direction. It means I can use a single flat plate to mount the calipers, which is much easier to make (and thus cheaper if you're not making it yourself). Ideally it would want to be spaced away from the upright at its base by 1.5mm to get the sliders in a central position, but I can do that with shim washers if necessary.



Last challenge is I'd got the measurements wrong for how far away the pad sits from the lug chord, so the whole thing needs to move in by a couple of mm. That means it's back to being very close to the upright where I clearanced it earlier. I've drawn up another CAD model that's skewed 3deg upwards to try and remedy this, so I'll transfer that to an MDF mockup and test if that fits.

Goes to show that it's worth mocking up the entire assembly before ordering anything, rather than relying on measurements upon measurements. Not sure where the error crept in but crept in it did!

[biturbo228]

Oh, for reference if anyone wants to use these Peugeot 106/Citroen Saxo calipers for their brake conversions, the relevant dimensions are:

Peugeot caliper dimensions
Lug spacing: 82mm
Bolt size: M12 (needs to thread into the adapter)
Lug centreline chord to outside of pad: 57.5mm (take this dimension away from your disc radius to tell you how far out your lugs need to be)
Lug mounting face to back of disc (34mm high Peugeot 205 front disc, 6mm face thickness, sliders centred, full thickness pad): 17.8mm

GT6 upright dimensions
Drum plate mounting face to front of hub face: 45.8mm
Mounting bolt pattern: 4x101.6mm