Early coffin brake master seal replacement?

[Darkness]

Originally Posted by Mr Gazza

I used to watch the forks bend backwards on my Ajay after converting to a hydraulic twin leader with Mini shoes instead of the 7/8" single leader. But I was still a single sided drum with the torque anchor on one fork only and there was no twist or pulling to any side. So the brake was essentially only anchored to one fork leg like a single disc.... I've never seen forks twist under braking.

Was the Ajay still wearing a chunky mudguard?
70’s Yams and Hondas looked pretty with skinny Rickman style fibreglass front mudguards, but without a forkbrace, or the original mudguard bracket they’d twist when braking hard. A little wear in the bushes helped exaggerate this. Little trailies with high level mudguard also twisted all over the place.

Originally Posted by Mr Gazza

If you're serious about this statement.. "Callipers in front or behind the fork leg change the polar moment of inertia about the steering axis, so affects the castor shimmy of the forks." I will happily listen to your explanation..

Going from memory:

The geometry for standard front forks relies on a castor effect to be stable when moving forwards. The contact patch of the tyre on the ground is behind the steering axis, so self centres when going forwards. Going backwards it is unstable and wants to be on full lock instead.

There are harmonic frequencies when castor effect wants to wobble. Generally around 35 mph and again at around 120 mph. The slower one used to be fun: no hands on a Honda G5 at 40 with no throttle would allow a full lock tank slapper to develop, but fade away as speed dropped to 30ish with no real worries. Never felt confident enough to play with a tank slapper at high speed though!

Handling improvements try to shift the wobbles above the top speed of the bike so it didn't happen, or occurs at lower speeds where it’s less dramatic.

To come back to calliper positions, the caster effect of the front end is affected by suspension geometry, but also by mass. Spinning mass which gives gyroscopic stability, and inertia about the steering axis. The inertia is the mass of the components multiplied by their radial distance from the steering axis. Larger inertias take more force to damp out the caster wobble, so if a wobble occurs it is more violent.

At this point help would be welcome from anyone who’s recently read up on chassis theory?

Large handlebar mounted fairings change the handling adversely, whereas bigger frame mounted fairings don’t have quite the same effect.

[Mr Gazza]

Yes the Ajay did still have the deep steel mudguard with a lower stay to the fork bottoms, so I guess that would have been a pretty good brace come to think of it.

I understand what you mean about where weight is on the front end effecting the handling now... I thought you might have been winding me up a bit with some nonsense science! My apologies
I still don't believe that the calliper position can effect the braking characteristics though.

[Darkness]

Originally Posted by Mr Gazza

Yes the Ajay did still have the deep steel mudguard with a lower stay to the fork bottoms, so I guess that would have been a pretty good brace come to think of it.

I understand what you mean about where weight is on the front end effecting the handling now... I thought you might have been winding me up a bit with some nonsense science! My apologies
I still don't believe that the calliper position can effect the braking characteristics though.

Agreed: not the braking characteristic, unless the airflow is shielded and the Caliper overheats?

[mickj]

I was told by a mechanic many years ago that the reason most manufacturers changed to rear callipers from forward callipers was because they are less prone to seizing when behind, as the fork leg offers some protection from the weather. I know my Honda CB550 was in need of a brake strip down and clean every six months to prevent it from seizing.

[Darkness]

Originally Posted by mjgt

I know my Honda CB550 was in need of a brake strip down and clean every six months to prevent it from seizing.

But Honda did use a cheap single piston Caliper with swivel mount comprising a steel pin in alloy mounts with just a couple of o rings to try and keep the salt water out.

[mickj]

Originally Posted by Darkness

But Honda did use a cheap single piston Caliper with swivel mount comprising a steel pin in alloy mounts with just a couple of o rings to try and keep the salt water out.

Yep, that's the one, crap, even on a good day.

[Darren69]

Sorry for the threadjack but its been an interesting discussion on something that has bugged me for a while and I was curious what opinions were. So it seems that the caliper position may give better protection on the rear of the fork or maybe just down to fashion?

From the fork flexing prospective regarding mudguard stay mid pipe twanging it may be preferable to have the calipers on the rear so if there is flex then the forks flex outwards?

[Capo]

I recollect the advent of disc brakes, there was discussion then wether the mount should be fore or aft. Aft won out due to we were told effects on the steering

[350TSS]

I agree with Capo - when they were first fitted the disc calipers were heavy and being in front of the fork legs they were effectively outside the wheelbase and made tank slappers (more common in those days with spindly fork stanchions) more pronounced and less likely to be recovered

[Darkness]

Originally Posted by Darren69

......From the fork flexing prospective regarding mudguard stay mid pipe twanging it may be preferable to have the calipers on the rear so if there is flex then the forks flex outwards?

Not sure that they would flex outwards in either position?

There are two types of flexing from the braking.
If the brakes are mounted on both fork legs the braking force tries to bend the bottom of the forks back towards the engine, but the Caliper/drum tries to pull them forwards, a balanced couple that gives a bending moment: how big a bending moment, and how big the deflection depending on how good the brakes are versus how stiff the forks are.
With single sided disk or drum where all of the braking force is put into one fork leg, only that fork leg tries to move back towards the engine. So far as the front wheel is concerned, it doesn’t know the handlebars haven’t turned but only that the fork bottoms have. Same effect as a sharp pull back on the left handlebar to turn right!
How dramatic this is in the real world depends hugely on how stiff the forks are, plus how rigid the front axle and its mounts are, plus how well the front mudguard and fork brace keep the fork legs parallel.
Experience of small 70s trailies with spindly fork legs and the low factory mudguard and its bracing completely absent is not easily forgotten!

[utopia]

Originally Posted by 350TSS

...... when they were first fitted the disc calipers were heavy and being in front of the fork legs they were effectively outside the wheelbase and made tank slappers more pronounced and less likely to be recovered

I think this is due to their position relative to the steering axis rather than the wheelbase.
Front mounted calipers increase the polar inertia about the steering axis, like Darkness describes.

At something of a tangent (pun), I'm also thinking that front mounted calipers might lead to slightly less fork dive induced by the brake reaction .. but also that the effect would be tiny compared to the effect of weight transfer under braking.
This may have less practical relevance than other previously mentioned factors though.

No doubt the design of "upside down" forks greatly reduces leg flex, due not only to their bigger diameter these days but also to the greater proximity of the fork bushes to the wheel spindle .. and hence bracing across the mudguard mounts is less necessary.

On my old 1976 Honda TL trials bike, the mudguard mount brace was made out of spindly steel plate, only about 1mm thick.
On my 1996 Dommie trailie (also wth "old-style" forks), the brace is more like 3mm thick and much deeper too. It is massively more rigid.
When I first bought the Commando, it's fibreglass front mudguard was attached with jubilee clips to each fork leg (if I remember correctly). Not much bracing there then !!

I guess there may even be a case for saying that rear mounted calipers make front wheel removal easier .. a tiny point but one which I'm sure bike manufacturers will consider nevertheless.

"It's all linked though, innit ?"

[Darkness]

Originally Posted by utopia

......I'm also thinking that front mounted calipers might lead to slightly less fork dive induced by the brake reaction .. but also that the effect would be tiny compared to the effect of weight transfer under braking.
This may have less practical relevance than other previously mentioned factors though.

I almost agreed with you on this as the friction force between the Caliper pads and disk would be tangential to the swept area of the disk. That means the greater the offset of the Caliper pads from the fork leg, the more a force vector would slope down (If forward mounted), or up (If rear mounted).

Unfortunately you can’t make use of the axial components of the force as it is exactly countered by the reaction through the wheel axle. The brakes don’t actually produce a force, they produce a rotational couple or moment.

At the contact patch, the moment from the braking force is countered by that from weight transfer onto the front wheel.

If we can find a way to isolate forces from their reactions we can be rich as perpetual motion is the dream!

[Darkness]

I must dig out my old copy of Zen and the Art of Motorcycle Maintenance for a refresh as I’m sure that was the source of lessening of my general Ignorance!

[bobbertandsammy]

Originally Posted by Mr Gazza

To update this...

Last night I decided to get serious with this pesky little clip/ring.
I found an old pair of scissors in a first aid kit and ground the outer edges into fine hooks, with the intention of getting the hooks under the ring and pulling the scissors open and up.
This worked to some extent as it was successful in removing the remains of the rubber cap from under the ring, but I couldn't pull hard enough to shift the ring even with the scissors nicely located as designed.
I remembered Carl Harrison's advice of trying a little warmth and just prising it out. So I stripped everything out that might get damaged by heat.. The switch and plastic level window. I warmed her up and just levered the ring out with a screwdriver...Simple as!

Now for the bad news! I discovered that there is a little "pushrod" or "button" that slips in the master body and makes the connection from the lever to the microswitch, as the tiny little button on the switch will not reach the lever. Put it this way, I discovered it, due to working out that it was missing after removing the switch.
Although it must be a small black cylinder about 10mm long and 4mm round, I can't find it, even in my reasonably clean workshop... It could have gone in any direction unseen and unheard!!

So, a big ask of anyone who can help with this. Does anyone have a spare one of these little "buttons"? I will gladly pay your asking price if one is available.
I am initially thinking this would be an easy part to have 3D printed, possibly in multiples.
So if I could even have the loan of one it would be fantastic as I could measure and draw it, the rest is simply a matter of a few emails and these would be cheaply available to anyone.

Thanks in advance.

Got my clutch master dis assembled and going to clean it and re assemble tonight hopefully. How did you press in the crush washer when you put it back together? Did a socket work?

[Luddite]

Originally Posted by bobbertandsammy

Got my clutch master dis assembled and going to clean it and re assemble tonight hopefully. How did you press in the crush washer when you put it back together? Did a socket work?

I think Mr Gazza used his customary ingenuity (and thrift!) and fashioned his own tool...

Originally Posted by Mr Gazza

I made a drift out of a short length of 22mm copper pipe, with 6mm cut out of it length ways. Pythagoras told me that the circumference is 6.284 less for a 20mm pipe and he was right.
Simply held together with a jubilee clip and carefully de-burred, it worked a treat. It pushed the slippy assembly down nice and square and then just a light doink with a hammer and everything was nice and firm.