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|Bike Profile - Posted 10th March 2009|
"Ineffective", "Utterly awful", "Needs beefing up". Familiar phrases to anyone who's had the pleasure of using BMW's single piston ATE front brakes. The one on Martin Gelder's R80/7 was even worse than that...
Roxanne the 1977 BMW R80/7 is fitted with her original issue single piston ATE calliper front brake. It's rubbish.BMW ATE 40mm Swinging Calliper.
Part of the problem is that motorcycle brake performance has improved dramatically over the last 30 years. I had a similarly equipped R60/7 in 1980 and the front brake didn't seem too bad compared to the other bikes I'd ridden back then, but over the years I've been spoilt rotten with the state of the art brakes on most of the bikes I've ridden.
Truthfully though, the BMW's brake is pathetic. It seems I've mentioned this in nearly every write-up I've posted:
June 2006: "Graham was right about that front brake; it's pretty ineffective."
It worked after a fashion, of course, and it passed every MOT it was put in for, so I suppose I'd just accepted that 'bad' was as good as it got. Forearm-straining pressure on the lever would eventually slow the bike, and repeated hard stops would improve the feel slightly, as though the pads and disc were finally getting up to temperature. It almost felt like there was a disc / pad compound mismatch, as though someone had fitted "race" pads that didn't work with the PFM aftermarket cast iron disc. And at low speeds after braking hard, the brake would groan and squeak as the calliper finally got hold of the disc. Rubbish.
Having ruled out the possibility of spillage by a stray squirrel raiding my stash of Fully Synthetic 10W40 - don't laugh, I've just unearthed a rodent nest in the battery box of my Kawasaki ZX10; perhaps whatever has been kipping in there is planning to do a track day or two when the hibernation season is over - it was obvious I had a brake fluid leak. I cleaned the disc, wheel and calliper with a handy tin of brake cleaner that the squirrels hadn't stolen yet and pumped the brake lever a few times, hard, to try and see where the leak was coming from. There was no sign of fresh fluid flowing out.
The sensible thing to do at this point would have been to put the bike back in the shed and use alternative transport until the entire braking system had been rebuilt with new parts and then bled to within an inch of its life using fresh DOT 5.1 brake fluid from an unopened bottle. Only an utter fool would think, "It's not leaking that much, perhaps a spin round the block will reveal where the fluid is getting out".
Returning from my spin round the block, I couldn't help noticing that the brake performance was actually no worse than before. In fact, with a freshly cleaned disc, it actually seemed a little better. Could the brake have been leaking very, very slightly for all these years? If that was the case, it made sense that it had taken a long period of being parked up for fluid to collect in a noticeable quantity. A drop or two smeared onto the disc during every ride wouldn't be visible, but it would certainly affect brake performance as it soaked into the pads.
I cleaned the calliper and disc again, and left the bike for a couple of days to see if the leak could be traced. Sure enough, there was a distinct moistness where the bleed nipple and brake line connected to the calliper. The lower part of the BMW's brake line is a solid pipe rather than the more common flexible hose. The swaged end of this pipe helps form the seal with the calliper body and over 30 years it would be no surprise for things to get a bit loose down there despite the fastening nut being properly nipped up. The bleed nipple was also looking a bit tired and corroded, and its flats were starting to round off.Crisp and straigt new brake line at the bottom, old and kinky old one at the top.
Both these parts are available off the shelf by mail order from Motorworks and within 48 hours they had dropped through my letterbox, along with a fresh set of Ferodo pads recommended for use with the cast iron disc. The new pipe was a slightly different shape to the original and I was worried that it wouldn't line up but in fact it was the old pipe that had been deformed at some stage in its long life. The new one fitted perfectly. Lovely.
I also took the chance to clean the disc thoroughly and to give the pads a buff up with some emery cloth laid on a flat surface. This got rid of the oily deposits on their surface, leaving a much more effective looking pad material. With new fluid pumped through the system - I wasn't joking about using an unopened bottle, by the way; brake fluid absorbs moisture from the air over time, so never use an old bottle of brake fluid that's been kicking round the shed since the last time you fitted new pads - and all the bubbles bled out, it was once more around the block to see if there was any improvement.Pads with oily coating on the left, and after being cleaned and rubbed down on the right.
And there was. The brakes had proper 'bite' for the first time and while not up to modern standards, they were much better than I could remember them being in the last few years. With no signs of any leaks after a decent length bimble later that day, I declared the brakes Officially Fixed. Cakes all round in celebration.
Or not. Another couple of days standing in the shed left the BMW with a tell-tale drip of brake fluid at the bottom of the calliper mounting. Obviously the ill-fitting brake pipe hadn't been the only leaky component.THe calliper after removal of the old piston. Note the rubber swarf.
Armed with a calliper rebuild kit, I did battle with the brakes once more. Pumping the old calliper piston out revealed a little corrosion on its body and while the sealing ring itself didn't look too bad, the inside of the calliper itself contained a small collection of rubber "swarf" granules. The calliper dust seal also disintegrated while it was being removed.Calliper rebuild kit on the right, old bits on the left.
So. More fresh fluid, more bleeding, more cleaning of discs and pads, and finally we're getting somewhere. I held back on replacing the pads until I was certain that all the leaks were plugged but everything remained oil tight over the next three or four days. If anyone spotted me grovelling around on my knees in one of Cambridge's bike parks it was because I was obsessively checking for dribbles of oil, not praying for the successful and safe completion of my journey home.
Fitting new pads to one of BMW's ATE brakes means that the alignment of the calliper and pads in relation to the disc must be checked. The calliper has only one piston, which pushes one of the pads against the disc. The other pad, on the other side of the calliper, is pulled onto the disc by the calliper body; it doesn't have its own piston. So that both pads stay in contact with the disc as they wear, the calliper body pivots - or 'swings' - on a vertical pin, hence the description of these callipers as 'swinging'.BMW ATE 40mm Swinging Calliper.
So that the pad faces can get maximum contact with the disc, the pin is eccentrically mounted and can be rotated to align the pads with the face of the disc. Getting this setting right is essential if you want to get the most out of these brakes, but checking it is simplicity itself. Using a non-permanent marker, draw a radial line across the inner and outer faces of the disc. If the pads are set properly, when the wheel is rotated while the brake is held very slightly on, the marks will be rubbed off across the width of the disc. If adjustment is needed, only part of the mark will be rubbed off.Calliper alignment. Red mark on left (at 10 o'clock position) has just been made on disc, mark on right (at 7 o'clock position) is after being 'swept' by the pads. Some very slight ajustment is still needed.
I've shown the outer face of the disc here because it's easier to photograph, but the critical factor seems to be getting the pads to sweep the full area of the disc on its inner face. I rotated the calliper pin using my favourite 'trial and error' technique until marks on both sides were being rubbed off with just the lightest pressure on the brake lever. This is one of those jobs where it's worth taking the time to get everything set just right, but at least it only needs doing when new pads are fitted.
So after all of this, is the front brake any better?
Yes, definitely. Before it was verging on the dangerous; now, although it needs a firm squeeze, the braking available is certainly adequate. There's a lot more feel for what's happening between the pads and disk when you squeeze the lever, and while performance still isn't on a par with modern systems it's much more confidence inspiring. To celebrate, I ventured into darkest Suffolk to hunt down a Norton or two.Norton, near Bury St Edmunds.
Although on the way home I couldn't help noticing that the bike was misfiring slightly under load, but that's another story...
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