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Modify / "upgrade" 72 Sherpa T


rootsman2
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I bought a parts bike that a local guru put together. I think it's a 1970/72 hybrid. Shifter on the right, brake on the left. The brakes are simply a nod to breaking - they are very close to useless. The motor purrs, it handles great, and the clutch is solid. I was thinking - and this is where I really need you all's advice - I'd like to bore it out and fix the brakes. If there's a clutch trick I'm game. It's already got a CDI ignition and OKO  carb. I've attached photos if that helps. I love this bike but I need brakes and more power couldn't hurt. So I was hoping to get some concrete advice on "how to" and / or "where to"

 

 

 

Thank You!

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I have an M98 "Sherpina" for vintage trials and an M206 Pursang for vintage motocross. My running joke is when Bultacos were new brakes were optional, but they came with a couple extra neutrals. First thing to do is make sure the hard chrome is still present in your brake drums. If it's peeling or gone they can be repaired. Some shops will install a steel liner and others will flame-spray metal into the hub then turn it on a lathe. Neither repair is cheap and both have their positives and negatives. I'd also recommend using grooved brake shoes and having the shoes fitted to the drums. You can find instructions on how to do this all over the Internet or pay (big $) to have a shop do it for you. It is labor intensive.

 

As far as more power goes there's not a whole lot that can be done short of putting a 325/350 motor in the chassis.

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That hub did not come with a chrome lining. Later Bultaco hubs did have a chrome lining

Like every other drum brake, just because the pieces are all in place, it doesn't necessarily mean it will perform well

Even if everything is perfectly set up perfectly in that particular hub, it will not work as well as many other twinshock trials bike front brakes, so don't get excited about it ever working strongly.

 

As with all drum brakes, here are the things that need to be done right for it to work as well as it can:

The drum surface needs to be smooth (shiny)

The drum surface needs to be concentric with the axle

The drum surface needs to be parallel with the axle

The drum surface needs to be clean

The curvature of the show friction material needs to perfectly match the curvature of the drum

The outer surface of the friction material needs to be concentric with the drum surface

The friction material needs to be of modern composition and designed for the purpose

The friction material needs to be bedded in (smooth) and clean

The wheel bearing races need to have minimal running clearance

The bearings need to be solidly mounted in the hub

The axle needs to be installed so that the friction material is centralised in the hub

The camshaft needs to have minimal clearance in its bearing

The cam needs to be almost flat against the shoe pads as the friction material touches the drum

The camshaft arm needs to be at 90 degrees to the cable inner as the shoes touch the drum

The cable outer needs to be kink-free and clean inside

The cable inner needs to be kink-free and lubricated

The camshaft lever needs to be the right length

The handlebar lever needs to have the correct leverage ratio and be free-moving

 

There are businesses that can do the relining and machining of the shoes and drum for you

 

A common mod for Sherpa T front brakes is to fit the front hub or front wheel from an Alpina from M115 onwards or from a Frontera. They have bigger brakes that work well, but being bigger, do have a slight weight penalty. Alpina front wheels are commonly available on eBay. They look like the later model Sherpa T front hub. Most people would not tell the difference unless they were side-by-side. I can post a photo if you want

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As for power you can take it to about 270cc with the standard sleeve. Some people fit reed valves.

In my experience that particular motor is a dream to use in completely standard tune running well so maybe you just need to ride it some more and it might grow on you

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Hi,

I need better brakes on my 199A. Engine is superb but the brakes :o.......

Feetupfun, Do you know if the Alpina front hub you mention got a steel or chrome liner?

/Carl

The standard Alpina drum has a chrome liner, but like the Sherpa and Pursang drum, the chrome sometimes lifts and people get a steel sleeve fitted

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Thanks,

I assume that a hub with steel sleeve would be better brakes as chrome has lower coefficient of friction than steel

I have a sheet of this friction material that I will try later

People are sometimes using it for classic motorcycles.

/Carl

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Edited by carl ekblom
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