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Project Rear Disc Conversion

  1. Starting Point
  2. August 22nd Update
  3. Master Cylinder Installation
  4. Disc Completed

One of the things I had to attend to first of all was to replace the right rear upper shock mount on the frame which had been damaged beyond simple repair by the previous owner. To do this I had to grind out the welded plug, and substitute a replacement from a donor frame. This took a couple of hours, but it can now be welded up and should be as good as new.

The other item I wanted to look after, was to update the rear wheel to be a disc brake rather than the stock drum brake as you see in the photo to the left. What I'm looking to create is an approximation of what a 1978 GT750 might have looked like, had one actually been built. Adding a rear disc brake is a logical assumption as Suzuki did introduce this with the four strokes that replaced the GT series. The donor bike frame I have is a 1977 GS750, and it shares many of the same components with the 1977 GT750 frame, although the actual frame itself is quite a different design due to the engine differences. As it turned out, the swing arm, with the disc brake fittings just drops into the GT750 frame. The lower shock mounts are different, as they are a clevis mount on the lower fitting, but they otherwise just bolt on. An immediate issue that became obvious was that as the GS swing arm is longer than the GT one a stock exhaust system was not going to fit, as the lower shock mounts would contact the inside of the stock exhaust system. For this reason an after market 'Gibson' exhaust system will be mounted.

Update January, 2015: were I to do this again, I would modify a GT swingarm to work by adding the required tabs for the torque arm and then use one of the new Delkovic replacement exhaust systems which became available after this project was completed.

The front forks and brakes likewise are just a bolt-on fitting using the GT750 triple tree, with the GS750 callipers under slung behind the forks, rather than in front of the forks as on the GT. At this point I haven't decided whether to go with a single front disc, or dual front discs.

So far so good - the interesting bit it where to put the master cylinder for the rear brake. With the GT frame, the oil tank is in the way as of course these were not required with the GS750 engines, space is at a premium. To check the clearances, I 'dry mounted' a scrap engine I keep for parts into the frame, with the Gibson expansion pipes mounted as well as the air box, oil reservoir tank and side covers. I want to use the round brake fluid reservoir, rather than go with a newer square style as those weren't common in 1977/1978, as again I'm going for a look that is a natural progression from the last real GT750 model, made in 1977. The mock-up can be seen to the right.

I realize that a lot of people remove the stock GT750 air box and just go with individual filters on each carburetter, but that isn't how they came from the factory, plus I already have a few new foam air filters that I sourced from Twin Air and Moto Air - interestingly, both are Dutch companies, but the Moto Air prices are better. Both companies offer replacement filters that are exact matches for the original factory foam filter elements, for less than what you pay at your friendly local Suzuki dealer, as filters by themselves are not readily available. From a dealer, you have to buy the filter mounted on the wire support frame which adds cost. Just changing the filter element and recycling the old wire frame makes more sense to me.

I double checked with a few colleagues at the Calgary Loyal Order of Water Buffalo sub-group of the Rocky Mountain Section of the Canadian Vintage Motorcycle Group (CVMG), to see what John and Fred thought about mounting options for the master cylinder. That was a very useful discussion, and I'm glad I waited to get some additional input before finalizing the design.

I was originally thinking of using a long link from the underside of a modified brake arm, to a remote mounted master cylinder. After discussing this with folks in the local Rocky Mountain CVMG chapter, and looking at another bike here in Calgary with a similar conversion, as well as searching the web for photos of other completed rear disc conversions on GT750's, its obvious there is no 'right' answer as each one has been done differently. For my conversion, I've decided to use a mechanical linkage from the existing stock GT rear brake pedal, to a bell crank which will be attached to the master cylinder. This installation has the benefit of preserving the same 'period look', preserves the stock mounting for the rear brake light switch, and also gives the same mechanical advantage to actuate the braking action as is found on the 1976 GS750 that I've been using for donor parts. Preserving the same mechanical advantage will ensure maximum brake pedal effect on the master cylinder, which is not a bad thing when you are trying to stop ! You can see the mocked up installation in the photo to the right.

The linkage is salvaged from an old CB350 Honda front brake assembly and the master cylinder and remote brake reservoir are off a 1980 Suzuki GS750EC, all of which I found at TJ's Cycle (which has closed since this article was first written), I cleaned up the surface rust on the linkage with that old standby - Coca Cola (traditional, not the diet version !) - and it now looks quite nice ! To ensure sufficient clearance between the master cylinder and the swing arm, I will need to modify the injector oil tank reservoir slightly, or use something off a different model of bike, or possibly just make a new one as I want everything to fit properly under the stock right side cover. The plastic used in the stock injector oil tank is soft - possibly ABS or perhaps a urethane. In any case, with a bit of luck (and a lot of cleaning !) I should be able to solvent weld it using one of the newer adhesives on the market used for automotive repairs, or possibly just deform it as required using a heat gun.

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August 22nd 2008 Update

After some additional discussion, I have engaged Cactus Machine to fabricate the bell crank rear disc actuator for me. Obviously this needs to be robust - being able to stop is kind of a nice thing after all ! Once they have completed the machine shop work, I will be able to get the welding work on the rest of the frame and the tank finished. I've looked at having the frame powder coated - in the end, I've decided to take a pass and just use regular Krylon Enamel paints as this will bike not be getting a lot of miles, and as well it keeps the costs down. I've had estimates of between $200 and $300 for powder coating and a few of the CVMG folks have suggested that 'just plain paint" is good enough, much as Mark Evans did for his Triumph in his 'A Bike is Born" series.

Master Cylinder Installation

The part from Cactus Machine is back - looks quite nice ! I've attached a photo of it to the left, and a shot to the right of how it looks mounted in the bush I had welded into the frame by The Welding Shop here in Calgary. I'm still debating whether to add a grease nipple to make maintenance a bit easier, but most probably will not bother. Really, if it is well packed with grease and the inner and outer thrust washers are a good fit, it should be fine.

The next step is to mount the master cylinder and recheck all the clearances with the swing arm in place and also to double check the lengths of the new brake lines, I'll need to install. The plan is to go with custom made braided stainless steel brake lines which I can source locally here in Calgary.

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Disc Completed

This part of the project is now complete. As can be seen in the photo to the right, I used a small brake fluid reservoir to ensure everything would fit under the right side cover, and on the advice of John in the local Calgary chapter of the CVMG, I decided to use a single stainless steel hose from the rear master cylinder through to the stock 1977 GS750 rear caliper. The bell crank that Cactus Machine made for me fits well, and I have plenty of available adjustment in the brake arm linkage should it be required. You will also notice, that I have changed out the spoked rear wheels for the 'snow flake' cast wheels used in the 1978 EC GS750. All the axle spacers, the axle itself and the drive hub are the same part numbers as are used for the GT750, so it is just a drop in fit. I used a hot air gun to heat up the injector oil tank and then deformed it slightly on the rear of the tank, so there is enough clearance between it and the rear brake master cylinder. With the side cover installed, it looks quite tidy.

The parts for rebuilding the rear caliper and master cylinder were readily available, although I did end up ordering all of it from the USA as the Canadian parts costs from Suzuki Canada were double the cost for the same parts in the USA. Even with the shipping, handling, exchange rate difference and import costs, it was still quite a saving over what I would have paid locally, and it is difficult to understand why there is such a difference in cost. Suzuki Canada has not returned my calls so I don't suppose I will ever get a straight answer. As I've mentioned previously, I've actually had parts people at Canadian dealers tell me to order Suzuki parts from the US rather than buy from them here in Calgary - its an odd situation.

Another odd thing is that OEM parts listed and available in the USA are sometimes listed as discontinued here in Canada. A case in point are the rear shock absorber bushes used in the rear wheel to cushion the rear drive sprocket - as per the dealer I called here in Calgary, these are discontinued, so I had no choice other than to order them from the Dillon Brothers PowerSportPro site in the USA.

Originally when I was told these shock bushes were not available, I went into my parts bin and checked three other rear wheels to see if I had any good used ones, and it was interesting to find that they all had the same failure mode in that the rubber bush itself had separated on the left lobe, which would be the compression side as per the photo to below. This has me wondering what shape the shock bushes are in on my other two GT's - something that will have to be checked I suppose.

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