Sticking front wheel

The bike is a recently acquired ‘76 R90S. I have not had a chance to actually ride the motorcycle, but I have managed to address a couple of minor issues. While rolling the bike around the garage this afternoon and checking tire pressure, I noticed the front wheel was extremely difficult to turn. No obvious cause. I have done nothing with the brakes. Any suggestions for where I should start?

Rickt

r65lsk75c said:

The bike is a recently acquired ‘76 R90S. I have not had a chance to actually ride the motorcycle, but I have managed to address a couple of minor issues. While rolling the bike around the garage this afternoon and checking tire pressure, I noticed the front wheel was extremely to turn. No obvious cause. I have done nothing with the brakes. Any suggestions for where I should start?

Rickt

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Hopefully there is a service history with the bike. When were the brakes last serviced, fluid changed, how old are the pads? Make sure the bike is properly sorted before you actually take it for a ride. If you are not familiar with the particulars of the bike, find somebody to help you. Have you joined the Airheads Beemer Club yet? www.airheads.org

Friedle

Could be a number of things brake related; stuck pad, corroded piston, contaminated brake fluid, etc. Unlikely, but wheel bearings might be knackered.

... pull the wheel and take a look.

stuck wheel

r65lsk75c said:

The bike is a recently acquired ‘76 R90S. I have not had a chance to actually ride the motorcycle, but I have managed to address a couple of minor issues. While rolling the bike around the garage this afternoon and checking tire pressure, I noticed the front wheel was extremely to turn. No obvious cause. I have done nothing with the brakes. Any suggestions for where I should start?

Rickt

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Best to put the bike on the centerstand and remove both calipers. That's an easy way to determine if the brakes or wheel bearings are the culprit.

Worked on a R90s a few years ago that had the exact same problem. The caliper pistons were frozen.

That's why it's best to flush brake fluid annually.

RickR90s

Probably not the issue, but you might try cracking the bleeder(s) on the calipers to see if there's trapped pressure. Likely it would have dissipated over time. But if there's a blockage in the return hole in the master cylinder or the brake lines are original rubber, they could prevent fluid returning or artificially keep pressure applied to the pistons.

r65lsk75c said:

The bike is a recently acquired ‘76 R90S. I have not had a chance to actually ride the motorcycle, but I have managed to address a couple of minor issues. While rolling the bike around the garage this afternoon and checking tire pressure, I noticed the front wheel was extremely to turn. No obvious cause. I have done nothing with the brakes. Any suggestions for where I should start?

Rickt

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Is the bolt that holds the caliper eccentric ? I recall that you rotate the bolt to adjust the angle of the caliper housing to the brake disk prior to securing the end cap on that bolt.

If the angle is offset, then the pad(s) drag on the disk. It is even more pronounced with dual brakes.

Brake master cylinders have a tiny return hole that allows the fluid to bleed back into the reservoir when the handle is released. If that is blocked, it can prevent the pressure in the calipers from releasing the pads. That having been said, I'd agree with the thought that it's likely that the calipers are seized.

Try rebuilding the master cylinder....Good luck

To diagnose this, remove the calipers from the disc and pull them back holding up with a wire to keep tension off of the brake lines. Now roll the bike, if it rolls fine. The calipers more than likely need rebuilt. This is very typical on the disc brakes.

(Please don't use dot 5 silicone based fluid in your rebuilt ATE system).

ATE brake calipers and master cylinders suffer from lack of proper service more than most brake systems I've dealt with. They really need serviced yearly. Use only DOT 3 or 4 fluid and bleed the brakes at least once a year when you get them working again of course. The amount of money you are about to spend on a dual disc set up will ensure your attention to this service in the future - I always say The commentary above by others has pretty much nailed the issues really. Sounds like they each know from experience. Water condenses and settles in low places causing rust pits to form in master cylinder bore and on the caliper piston. The resulting coarse surface destroys seals causing failures in the system one of which is a 'stuck piston' clamping a brake disc. Several folks in the US repair ATE master cylinders by replacing the damaged bore with stainless steel inserts which are then fit to your new M/C piston. The calipers themselves are probably fine but there may be damage to the pistons.
I'd make it stop first. Then go. Start fresh with new BMW hoses and brake pads. You might even consider some contemporary modifications to improve the performance of the ATE system.

Na Cl K9 said:

ATE brake calipers and master cylinders suffer from lack of proper service more than most brake systems I've dealt with. They really need serviced yearly. Use only DOT 3 or 4 fluid and bleed the brakes at least once a year when you get them working again of course. The amount of money you are about to spend on a dual disc set up will ensure your attention to this service in the future - I always say The commentary above by others has pretty much nailed the issues really. Sounds like they each know from experience. Water condenses and settles in low places causing rust pits to form in master cylinder bore and on the caliper piston. The resulting coarse surface destroys seals causing failures in the system one of which is a 'stuck piston' clamping a brake disc. Several folks in the US repair ATE master cylinders by replacing the damaged bore with stainless steel inserts which are then fit to your new M/C piston. The calipers themselves are probably fine but there may be damage to the pistons.
I'd make it stop first. Then go. Start fresh with new BMW hoses and brake pads. You might even consider some contemporary modifications to improve the performance of the ATE system.

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Ditto to all of the above. And even after I did everything listed above, my R90S always had a bit of brake drag on the stand. Not noticeable while riding.

I had exactly the same problem with my R90S and R100S. The R90S sat for over 20 years, during which time brake fluid leaked front the right caliper down to the rim. The pads were frozen against the rotors on both sides. R100S was a little better but when I removed the wheel I saw that one rotor was actually blue from heat caused by a sticking caliper and the brake pads crumbled when I removed them. If your R90S was not properly maintained, I suggest you consider a complete brake rebuild starting with the master cylinder.Remove the gas tank and look for rust on the frame below the master cylinder. This is indicative of brake fluid leaking from the mc. If this is the case you will either have to buy a new mc or have the existing one bored and sleeved, then rebuilt. (Apple Hydraulics)

Regarding the calipers, remove them from the bike and inspect the seals. if they appear wrinkled, this indicates that they have failed and contaminants may have gotten into the calipers.Even if they appear intact, plan on a rebuild. Remove the pistons with compressed air and inspect the bores and pistons. If the pistons are rusted and pitted, they will have to be replaced. The calipers on both my R90S and R100S had minor corrosion which I carefully removed with 600 wet or dry.The pistons were throwaways.

At the end of the day, I installed a rebuilt master, new brake lines, brake pipes, brake cable, rebuilt calipers(new pistons and seals) and new rotors on the R100S. Essentially, the entire system is brand new. It wasn't cheap but considering the brake system was 40 years old and the bike still had the original rotors, I felt it was best way to go. Very high Peace of Mind Factor.

Take a Sharpie marker, and draw a line on the brake rotor from the outer edge to the inner edge on both sides of the rotor. Then rotate the wheel. You will see where the rotor is binding by how much of the mark is rubbed off. I did this on my '87 R80, and found that the outside of one of the caliper pistons was not retracting. I rebuilt the caliper (cheap kit from Brembo, only $30 or so... easy job), bled the system, and no more binding.

Sticking pistons

Regarding the calipers, remove them from the bike and inspect the seals. if they appear wrinkled, this indicates that they have failed and contaminants may have gotten into the calipers.Even if they appear intact, plan on a rebuild. Remove the pistons with compressed air and inspect the bores and pistons. If the pistons are rusted and pitted, they will have to be replaced. The calipers on both my R90S and R100S had minor corrosion which I carefully removed with 600 wet or dry.The pistons were throwaways.

At the end of the day, I installed a rebuilt master, new brake lines, brake pipes, brake cable, rebuilt calipers(new pistons and seals) and new rotors on the R100S. Essentially, the entire system is brand new. It wasn't cheap but considering the brake system was 40 years old and the bike still had the original rotors, I felt it was best way to go. Very high Peace of Mind Factor.[/QUOTE] ruzneb

So much good information here. I would just offer a word of caution regarding the use of typical 120 +/- psi shop air. It is a good idea to put a rag between the piston and the caliper as a cushion. I find the ideal tool to use is a rubber tipped air pistol held to the caliper port. Just a tickle of air is often enough to dislodge a stubborn piston. Too much air and the piston will try to depart with amazing force. Keep your fingers clear!
Russ

Removing brake calipers

In my effort to find the cause of a sticking front wheel on my '76 R90S I got as far as inserting the 8 mm x 1.25 x 50 mm bolt into the bearing pin. I assume the bearing pin must be removed. Showing my loss of mental capability, I am unsure how to proceed. Does the bearing pin need to be removed from the caliper to release it from the disc? The master cylinder and the surrounding are clean and dry. The calipers show no sign of brake fluid leakage. The front wheel can be turned by hand but there is much more resistance than I would expect.

Recommendations will be appreciated.
rickt

rickt -

I usually don't remember what the specs on the bolt is but remember that the pinch bolts on the lower fork slider is a perfect size. Once that large cap is removed, the bolts is hand threaded into the end of the eccentric pin. Then take a pair of pliers or maybe a crescent wrench and loosely grab the shank of the bolt and lightly tap the side of the wrench with a hammer. The eccentric should drop out. If not you may have some corrosion inside which may require squirting some penetrant into it. And yes, once the pin is removed, then the caliper is freed up to be removed...once the brake lines are removed.

If you're just trying to free up the wheel so it turns easier, there could be a variety of reasons...so I wouldn't remove the eccentric pin just yet. If the brakes haven't been bled, that should be done. Possibly the original rubber brake line should be replaced. If those things don't free up the brake pads from dragging on the disk, then an overhaul might be in order. What is supposed to pull the pads back from the disk is the square o-ring that is inside the caliper bore. When the brakes are activated, the piston distorts the o-ring and when the brake lever is released, the o-ring tries to "undistort" back and it pulls the piston with it. Could be the o-ring has lost it mo-jo and/or there's internal corrosion. That's when an overhaul is needed.

Success on the left not so much on the right

20774 said:

rickt -

I usually don't remember what the specs on the bolt is but remember that the pinch bolts on the lower fork slider is a perfect size. Once that large cap is removed, the bolts is hand threaded into the end of the eccentric pin. Then take a pair of pliers or maybe a crescent wrench and loosely grab the shank of the bolt and lightly tap the side of the wrench with a hammer. The eccentric should drop out. If not you may have some corrosion inside which may require squirting some penetrant into it. And yes, once the pin is removed, then the caliper is freed up to be removed...once the brake lines are removed.

If you're just trying to free up the wheel so it turns easier, there could be a variety of reasons...so I wouldn't remove the eccentric pin just yet. If the brakes haven't been bled, that should be done. Possibly the original rubber brake line should be replaced. If those things don't free up the brake pads from dragging on the disk, then an overhaul might be in order. What is supposed to pull the pads back from the disk is the square o-ring that is inside the caliper bore. When the brakes are activated, the piston distorts the o-ring and when the brake lever is released, the o-ring tries to "undistort" back and it pulls the piston with it. Could be the o-ring has lost it mo-jo and/or there's internal corrosion. That's when an overhaul is needed.

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Following your instructions the pin on the left came out quite easily. Taking the same approach on the right afforded much frustration. Pounding on the bolt using a crescent wrench then a chisel on the lip of the bolt produced no movement. Several lubricant sprays also yielded failure. I suspect this caliper is the problem as the wheel can barely be turned by hand with the left caliber off the bike.

Any other suggestions for freeing the problematic pin?

rickt

Try heat. Careful use of a propane torch.

I think its time to get the brake disc out of the way. It sounds like the eccentric pin is being held in place by the tension between the frozen brake caliper piston with pads tight against the brake disc and the position of the eccentric pin in the bore of the caliper. Put the bike on its center stand using a 1 x 12 x12 or 2 x 12 x 12 under it to get a little more height at the front wheel and pull the front axle. Lower the wheel to the ground and roll it forward enough to extract the brake disc from between the pads. The wheel cant be completely removed until both calipers are loose and able to be rotated out of the way, You will or should be able to rotate the caliper in its mount and that should free up the eccentric so it can be pulled. The eccentric pins ride in brass bushings inside the mounting flanges on the sliders so it's not frozen in the mounts except in this case by trapped tension. Once the caliper is off the slider you can pull the wheel completely or put it back so you'll be able to roll the bike around your shop. No heating required. Just wiggle the caliper back and forth as you pull the eccentric down and out of its bore.