Front Brake Shoe Prep Question

[454MRW]

I searched for the proceedure of preparing front brake shoes on my 1977 MC5 250 Project Drag Bike and I couldn't find the previous post about sanding the shoes and spinning the wheel and clamping the front brake lever before tightening the axle. I have purchase a set of NOS KTM brake shoes and these are fairly rough semi-metallic pinned shoes. What grit sand paper should I use, or how smooth should I start out with so that they don't wear into the brake drum excessively before they are seated. I understand about riding for a while and then sanding off the high spots until they are uniformly seated, but how smooth should they be to start with? Thanks, Mike

***Also I have installed an original speedometer drive. Does anyone know how tight the axle spacer tube nut should be up against the speedo drive. I don't want to damage the speedo drive, or make it too tight. Thanks, Mike

Michael R. Winter
1974 250 Harescrambler
1975 250 HS Project
1977 250 MC5 Project
1978 250 MX6  
1979 250 KTM Project
1980 250 MX KTM

[Big Mac]

OK Mike, no smarter guy than I has responded so will give it a try. Not sure about seating brake shoes exactly, but have read articles on it from back in the day. Seems the best approach is to put something like talcum powder on the brake surfaces, spin the wheel and give it a few good grabs. Then when you pull off the wheel, the powder left behind should indicate low spots. Then sand or file off the high spots and try again. I believe you're going to be using some pretty coarse paper or a file since the material is pretty hard. On final finish, should be uniform but still rough enough for stopping power. Not sure how that's measured, just know if you have very smooth or glazed surfaces, the trick is to rough it up and break the glaze to get best stopping power.

The trick on getting the backing plate centered is to simply loosen the front axle, spin the wheel, then clamp down quick and hard with the front brake, holding it there (I use a velcro stappy) while you tighten up the axle and pinch bolts snug. Should center the backing plate up right.

The only thing that keeps the speedo drive from spinning on the axle along with the wheel is the clamping pressure between the outside axle nut's shoulder and the inside spacer in the wheel hub. The whole works should be cinched up tight enough to prevent it from turning, don't think there's a way to overdue it since it essentially acts as a spacer along the axle-in place of a true spacer that's used when the speedo drive is removed. If all is in the right places, the drive should sit about 2mm away from the actual hub it bumps up against, with the "ears" catching as need be to count the spins.

Two things I've learned: 1) Sometimes the axle spacer it buts to on the hub side is a bit too short (or gone)...I've found that the leftover bits when you last cut 1" off the width of your 7/8" handlebars (save everything) work just about perfect for replacement spacers on the I.D. and the O.D. 2) The speedo drive can be taken fully apart and cleaned/regreased, since their usually gunked up pretty bad. Have found a couple that had striped worm-drive gears that gave oddball (jumping needle) and wrong speed readings. If you install the speedo drive with no cable, the internal worm gear can ride up in the housing and bind, grab and have lots of ill effects. The cable housing cinched down in the housing prevents the drive from trying to escape out the hatch. Don't run the drive w/ no cable in it or your likely waste the internals. And if the "ears" on the outside plate get sheared off due to a bound-up drive, it's easy enough to take the tin snips out and cut two more ears, bend them over and go.3

I guess that's more than 2 things, but hopefully my trials, errors and mistakes will save you time and headaches.  Mac

[454MRW]

Mac,
Mac, Thanks for the help. I have it together, but the brake hub is not going in far enough to be flush with the outer ring of the wheel hub. There is at least 1/8" protruding where the brake hub should set flush inside the wheel hub leaving a small air gap, unlike my 1978 front wheel/hub which has no gap. The front wheel and hub is the original one from my 1974 250 HS. It seems like the spacer bushing between the wheel bearing dust cover and the inside of the brake hub is too thick and holding it out too far. The mechanic that trued my wheel and mounted the tire told me that it was in between the hub and the bearing dust cover. Should I thin the spacer down, or am I overlooking something, like the new brake shoes hanging up on the worn lip of the old shoes in the wheel hub and not allowing it to go together far enough, or is this normal? I don't think it ever was flush if I remember correctly. Mike

Michael R. Winter
1974 250 Harescrambler
1975 250 HS Project
1977 250 MC5 Project
1978 250 MX6  
1979 250 KTM Project
1980 250 MX KTM

[Big Mac]

Mike, if it never was flush, it may be that someone mixed a backing plate and hub off different bikes at one time. Can't recall off the top of my head the order of things, but the backing plate should be held out there in place by a spacer (or is it a shoulder?) on the axle, so no crime to grind down the length of the spacer to make it fit right. Should be that the backing plate mates up with the wheel hub pretty flush with a just a tiny gap of clearance.

If it's the fault of the shoe width, you'd surely hear something grinding and gnashing as you spun the tire. Seems like there's inner and outer spacers(?) or the backing plate has a nipple that seats on the axle shoulder, can't recall. I'd be inclined to try different spacers or try a chunk of 7/8" bar end as needed to get clearances you want, but would go sparingly on grinding anything off parts you can't replace easily. Hope this helps a bit.

[454MRW]

I was going to look for a similar sized piece of plastic pipe and substitute it in place of the spacer and trim it 1/8" thinner, and if it worked without binding the shoes, then thin down the metal spacer. I can't imagine that it was ever changed since I have had the 74 HS since 1979 or so. Thanks again. Mike

Michael R. Winter
1974 250 Harescrambler
1975 250 HS Project
1977 250 MC5 Project
1978 250 MX6  
1979 250 KTM Project
1980 250 MX KTM