Disc rear end

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Lumpy
SD900 Darmah
Posts: 324
Joined: Sun Mar 25, 2007 5:49 am

Disc rear end

Post by Lumpy » Mon Sep 07, 2015 10:56 pm

Well I`ve finally got around to giving my poor old retired 860GTS a make over and am going to ditch that good for nothing rear brake. I`ve just been trawling through the 1975 SS parts mannual on here and the wheel section seems to be missing. So for those of you in the know is the front and rear hub on a spoked wheel bevel twin the same item?? Thats is to say, just get a hub, disc carrier and cush drive and I`m away??
Next question, does anyone have or know of a hub for sale?

wdietz186
Cagiva Alazzura
Posts: 700
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Re: Disc rear end

Post by wdietz186 » Thu Sep 10, 2015 4:53 am

The hub is the same and I think the bearing carrier on the disc side is too. There is some variation in the cush drive side that bolts to the wheel. The cast wheel cush drives are a bit thinner I think. The parts are available, but that crappy old drum brake might improve once you price the parts to convert to a disc.

Ray O'Donohue
Parallel Twin
Posts: 119
Joined: Tue Oct 13, 2009 8:11 am

Re: Disc rear end

Post by Ray O'Donohue » Thu Sep 10, 2015 5:40 am

Just a suggestion: Set up that drum carefully,and put your efforts into other issues. (It being a Ducati,you do have other issues...). There is little to be gained-arguably,nothing-by the use of a more powerful rear brake. You may also be increasing unsprung weight back there with conversion to a disc.(?)

Lumpy
SD900 Darmah
Posts: 324
Joined: Sun Mar 25, 2007 5:49 am

Re: Disc rear end

Post by Lumpy » Mon Sep 14, 2015 4:26 am

Had this bike a looooooooong time. Tried new cables, new bonded shoes. Even messed with the cam. Could`nt make it work well. Riding other things now but would like to build her into the Duc I always wanted but still very usable. Slightly 750s but different. Other bits and bobs to come with the rear end. But brakes are important I reckon.

Ray O'Donohue
Parallel Twin
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Joined: Tue Oct 13, 2009 8:11 am

Re: Disc rear end

Post by Ray O'Donohue » Mon Sep 14, 2015 5:30 am

Did you look into indexing/grinding the new shoes in the old-fashioned way that a good auto brake shop used to do with drum brake linings? Also,check the hub/liner for roundness? Probably finding a replacement rear hub would be not too expensive,due to all the "upgrades" that have been done over the years?

Lumpy
SD900 Darmah
Posts: 324
Joined: Sun Mar 25, 2007 5:49 am

Re: Disc rear end

Post by Lumpy » Mon Sep 14, 2015 6:03 pm

It was quite some years since I had new shoes bonded to the old girl. The mob that did it were a dedicated brake shop specializing in drum brakes. They took the drum as well and I`m assuming they matched it all up. They recommended a softer shoe material and in short there was little or no difference. The mods I am doing to her will include rear-sets and when I look at the length of the standard pedal and leverage applied by it I can`t see rear sets being able to cut it. I have ridden various other GTS`s and found the rear brakes on them all quite poor in comparrison to an SS and her disc rear end. Expensive mod??? Yes indeed. I have collected the master cylinder and the caliper over time as they come up. The hub, carrier and cush drive were hard to find and will set me back some $1500. Yowch!!! But there is no time limit on this build and I`m collecting what I need as I can afford/find the bits. Next bit to get thrown over my left shoulder will be that crappy single step advance Ducati Electronica ignition system.

Would I be to assume Ray that you have been able to get satisfactory braking from a GTS drum rear brake?? I have noted some degree of flex in the standard brake pedal and also seen that the cable will absorb some tension. That is, it will straighten the curves of the outer lining and absorb some tension that way. I toyed with the idea of using a straight rod instead of cable commonly used on a lot of Japanese bikes with what I considered good drum rear brakes but to acheive that on a RH brake would mean cross-overs and it all seemed a little complex and with the old drum in place the out come not guaranteed.

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Craig in France
Cagiva Gran Canyon
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Re: Disc rear end

Post by Craig in France » Tue Sep 15, 2015 12:14 am

Lumpy wrote: <snip> Would I be (right) to assume, Ray, that you have been able to get satisfactory braking from a GTS drum rear brake??
FWIW, Lumpy, I can't say I ever found the rear drum brake on my 860GT to be lacking. And I can't remember this having come up before, either in road tests of the time or subsequently amongst current day owners.

That said, I guess a sense of under-performance will depend, to some extent, on how you ride/what you expect ...

Craig

Ray O'Donohue
Parallel Twin
Posts: 119
Joined: Tue Oct 13, 2009 8:11 am

Re: Disc rear end

Post by Ray O'Donohue » Tue Sep 15, 2015 6:34 am

Part of this may be that many or most riders don't really care how powerful the rear brake is,indeed want it not to be powerful,and there are riders who don't use it at all-Freddie Spencer being one example.That said,maybe in stop and go riding the rear brake becomes more of an asset.

Ray O'Donohue
Parallel Twin
Posts: 119
Joined: Tue Oct 13, 2009 8:11 am

Re: Disc rear end

Post by Ray O'Donohue » Tue Sep 15, 2015 6:56 am

I raced a GT750 with that rear drum,and it was not an issue one way or the other.. I probably should have used it less than I did.

Lumpy
SD900 Darmah
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Re: Disc rear end

Post by Lumpy » Thu Sep 17, 2015 4:01 pm

That`s interesting Ray. While I don`t ride overly hard I have always, on all bikes, used a combination of front and rear brakes to stop. I find back brakes particularly useful in day to day use if traction is of concern. eg white painted lines that are wet, gravel on the road or those frightening rainbows on the road indicating fossil fuels about!! Vast majority of my braking is done by the front but the rear is always in play, just to a greater degree in situations mentioned previously.

To illustrate how pathetic the rear brake is on my GTS, I would expect that at a very low speed, say 10km/h, on a loose surface to press hard on my rear brake pedal I think I should be able to lock the rear wheel. This is not even close to the case on my GTS. A friend who has the exact same year GTS has the same issue.

My dirt bike is an old XR500RE Honda. Much smaller rear drum yet this drum is powerful and progressive. There is however no cable, rod used instead and no flex of the brake pedal. The GTS brake pedal is quite long as the pivot is behind the foot peg.
It`s also worth mentioning that the brake material used on the original shoes was incredably hard. I did some 60,000 kms on those shoes and they had plenty left on them. Early attempts to improve performance included scuffing the shoes with sandpaper. Scuffing the drum with sand paper by hand and spraying the whole thing with brake clean solvents. No change. The softer compound was recommended by the brake place that re-bonded and matched to the drum. I was amazed that it made virtually no difference to the braking performance.

I guess as you say different riding styles and expectations come into play.

Ray O'Donohue
Parallel Twin
Posts: 119
Joined: Tue Oct 13, 2009 8:11 am

Re: Disc rear end

Post by Ray O'Donohue » Mon Sep 21, 2015 4:25 am

No,I think you are right! You ought to be able to easily lock it up in that situation. But what is the cause.....?

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BevHevSteve
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Re: Disc rear end

Post by BevHevSteve » Mon Sep 21, 2015 2:35 pm

Tighten up the cable so there is lass pedal throw ...?
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Lumpy
SD900 Darmah
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Re: Disc rear end

Post by Lumpy » Mon Sep 21, 2015 3:18 pm

Tried that one Steve. Very early in all this. Tightened adjustment up so tight that the drum got very hot on one occasion and I had to back off adjustment on the road. As I mentioned earlier when you press the pedal hard you can see that the long pedal does absorb some pressure by flexing. The cable will try to straighten the outer casing absorbing some more tension and who knows???..........that cross-over shaft maybe be having a twist as well but I`m not sure on that one. That`s my slant on it anyway.

I am satisfied I`ve exhausted all my options with the original equipment and having ridden other GTS`s and found their rear brakes poor I`m going to run with the disc. Sadly I seem to have lost email contact with the gentleman who had a rear hub assembly for sale. No answer has been the stern reply................hopefully he`s just on holidays...............

Ray O'Donohue
Parallel Twin
Posts: 119
Joined: Tue Oct 13, 2009 8:11 am

Re: Disc rear end

Post by Ray O'Donohue » Wed Sep 23, 2015 5:19 am

How about: the brake arm/spline/index is one tooth off? I seem to remember some sort of incident from my mechanic days where something like this happened....?

ducadini
750 Sport [BEVEL]
Posts: 246
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Re: Disc rear end

Post by ducadini » Fri Sep 25, 2015 12:58 pm

A friend used to put the shoes for an hour in boiling water. Wether is was the grease melting or the lining getting a heated-water-treatment, we couldn't figure it out, but it worked.

When getting together a disc setup : The Original 860 rim has 36 spokes, the disc hub is drilled for 40.
You can fit the 280 mm disc carrier at the rear, but wil have to make a distance piece to fit between the bearing of the disc-carrier and the cush-drive.
Going for the small SS disc setup ? Then all parts and measurements are readily available.
You will have to make some distance-pieces to fit between swingarm/caliper-plate/cush-drive.
You can turn the sprocketcarrier on a lathe to the correct thickness and fit Original bearings.

Ever considered converting the rear drum to hydraulics ? No more losses over the cable.
Needs a bit of tinkering and experimenting though :-)

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ducadini
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