Front brake tuning!

I finally have my commando on the road (breaking in) and am now working through the “rideability”. I have a Norvil front disk, 4 pot Nissin caliper, new wheel bearings, new fork stanchions, and bushes, new head stock bearings, Lansdowne dampers and the fork legs have been sleeved to remove the slop. I have been out testing various things and have now balanced the front and rear wheels which makes the ride much smoother.

If I brake hard I can get the front tyre to scream at me and start to lock, so caliper and master cylinder seem well matched. At about 50% braking all seems quite smooth and predictable. 75%, forks start to spring and braking becomes uneven but managable, at 100%, not nice, forks springing around and the tyre is not gripping evenly pulsing with the fork twang – danger zone!

I am sure there are a dozen of things that can upset the “balance” and would really appreciate some help here. Tyres front and rear both new Road Riders, wheel alignment good. Disk does not appear/feel distorted but could be inconsistent surface grip?

Thanks

The first thing I would do is to talk to the man who made the Lansdowne dampers for you! Very difficult to diagnose at a distance, with the little information you have provided so far. The dampers are designed to 'damp' or modulate the springing, so it should not be "springing around". Are the surfaces you braked on smooth or bumpy?

Maybe try to be more precise in describing how the fork behaves. Does it dive significantly under braking (this is one serious Commando fault that the Lansdownes will deal with if set up right). If the springs are too soft and perhaps also inadequate compression damping, under hard braking, the front end will bottom and the tyre will tend to skip, which is unnerving. Does it 'pogo''? Are you experiencing brake judder from your Norvil disc? Are the forks nice and supple with minimal stiction when you pump them at a stand still? Are you using the standard springs of the right length? I think this is what JRB recommends.

John Bould will tell you exactly how much oil should be in each leg and its viscosity, depending on temperature. He will also give you compression and rebound damping settings to begin with, which you can then tweak a little to your own preference. I should really let him take over from here and he will guide you through things systematically. Over to you, John, if you're reading this.

It could be a result of the rubber trim fitted to the brake dish. Make sure the disc is flat and if so maybe a new dish - minus the rubber trim - will solve the problem. Do you know if you've got a Norvil 'Norvil' disc or a Mick Hemmings 'Norvil' disc?

A 4 pot caliper may be too much brake the this application and/or may requires larger diameter master to take some of the touchyness out of it. Too good of braking will take "Feel" away. Too much may be worse than too little.

You can also try to dial in more compression (and perhaps a dash of rebound) 1/4 turn at a time.

Thanks guys,

Disc has Norvil on it more than that I don't know. Damping seems to be great, perhaps a little too stiff - there is no dive, very well controlled. The forks are bending back and springing forward horizontally, vertically seems to be under control. I have to pull hard (4 fingers) to get it close to locking (about 70 mph) so even in panic mode should not give way on me unless it's wet of course. I am really trying to use everything the front end can deliver - if I need it to work 100 % then I need to do some serious improvement here. At the moment not up to the delivered performance.

There is no pulsing in the lever, which I would expect if the disc is running out, perhaps part of the disk is harder than the rest?

Ps road surface good but I will take a slow run over the test track next time to check.

Thanks

Nigeldtr said:

Thanks guys,

Disc has Norvil on it more than that I don't know. Damping seems to be great, perhaps a little too stiff - there is no dive, very well controlled. The forks are bending back and springing forward horizontally, vertically seems to be under control. I have to pull hard (4 fingers) to get it close to locking (about 70 mph) so even in panic mode should not give way on me unless it's wet of course. I am really trying to use everything the front end can deliver - if I need it to work 100 % then I need to do some serious improvement here. At the moment not up to the delivered performance.

There is no pulsing in the lever, which I would expect if the disc is running out, perhaps part of the disk is harder than the rest?

Ps road surface good but I will take a slow run over the test track next time to check.

Thanks

Click to expand...

The forks are bending back and springing forwards horizontally? Do you mean that you feel that the fork tubes are actually flexing that much? If they are, then you must be braking very hard and this could interfere with proper fork actuation. From talking to Dick Hunt, who supplies front ends to racers in the UK, stanchions can come in various wall thicknesses. Someone more knowledgeable than I may be able to elaborate on this as far as Commandos are concerned. Could a thinner wall stanchion flex more? - I don't know. The reason I say this is because Dick Hunt offered me cut down Commando stanchions for my Seeley (with Seeley forks) with either thinner and thicker walls, but I can't remember the ID figures now. He recommended thicker wall stanchions for my application because of the very hard braking I do on the track. I have a pair of used Commando stanchions with an internal diameter of just over 26 mm. My Seeley tubes, which are very like the Commando units, have the same ID and OD and don't flex under hard braking. Do you know what ID yours are, and are they good quality tubes that are absolutely round and straight?

Lastly, I assume that if you have gone to the trouble of getting your sliders sleeved, the fork bushes should be a smooth sliding fit with very little slack. Again, JRB will elaborate on this.

This above may or may not be relevant, but I guess no harm in mentioning it. I'd be interested to hear what our Commando gurus think.

Pete V - your point about having too much braking -.yes, we know that the standard Commando fork and frame have their limits!

Nigeldtr said:

Thanks guys,

Disc has Norvil on it more than that I don't know. Damping seems to be great, perhaps a little too stiff - there is no dive, very well controlled. The forks are bending back and springing forward horizontally, vertically seems to be under control. I have to pull hard (4 fingers) to get it close to locking (about 70 mph) so even in panic mode should not give way on me unless it's wet of course. I am really trying to use everything the front end can deliver - if I need it to work 100 % then I need to do some serious improvement here. At the moment not up to the delivered performance.

There is no pulsing in the lever, which I would expect if the disc is running out, perhaps part of the disk is harder than the rest?

Ps road surface good but I will take a slow run over the test track next time to check.

Thanks

Click to expand...

I know you have already checked it Nigel, but it may be worth a double check as its the easiest, quickest, cheapest option: an out of balance wheel can demonstrate the exact symptoms you describe.
Other possibilities that come to my mind are sloppy fork bushes or play between the sleeves you've fitted and the sliders.
And finally, I read your earlier post about the fitting of a rubber door seal to the disc mounting dish... Frankly Nigel, this sounds quite potentially dangerous to me and I strongly advise reverting to standard ASAP.

Fast Eddie said:

Nigeldtr said:

Thanks guys,

Disc has Norvil on it more than that I don't know. Damping seems to be great, perhaps a little too stiff - there is no dive, very well controlled. The forks are bending back and springing forward horizontally, vertically seems to be under control. I have to pull hard (4 fingers) to get it close to locking (about 70 mph) so even in panic mode should not give way on me unless it's wet of course. I am really trying to use everything the front end can deliver - if I need it to work 100 % then I need to do some serious improvement here. At the moment not up to the delivered performance.

There is no pulsing in the lever, which I would expect if the disc is running out, perhaps part of the disk is harder than the rest?

Ps road surface good but I will take a slow run over the test track next time to check.

Thanks

Click to expand...

I know you have already checked it Nigel, but it may be worth a double check as its the easiest, quickest, cheapest option: an out of balance wheel can demonstrate the exact symptoms you describe.
Other possibilities that come to my mind are sloppy fork bushes or play between the sleeves you've fitted and the sliders.
And finally, I read your earlier post about the fitting of a rubber door seal to the disc mounting dish... Frankly Nigel, this sounds quite potentially dangerous to me and I strongly advise reverting to standard ASAP.

Click to expand...

The fork legs have sleeves in (they were sloppy) and "thinner" bushes done by "Lansdowne" I will re-check the balance and have a check at the door strip (it's really a much better soln. than it sounds, the disk can still "float" but not slop about)PS I do I appreciate the concern here.

To be honest, I am now thinking about replacing the front end as it does'nt really seem up to it (too many cars with ABS) and I would prefer a disk on each side at the front (another topic though)

Regards

Nigeldtr said:

Fast Eddie said:

Nigeldtr said:

Thanks guys,

Disc has Norvil on it more than that I don't know. Damping seems to be great, perhaps a little too stiff - there is no dive, very well controlled. The forks are bending back and springing forward horizontally, vertically seems to be under control. I have to pull hard (4 fingers) to get it close to locking (about 70 mph) so even in panic mode should not give way on me unless it's wet of course. I am really trying to use everything the front end can deliver - if I need it to work 100 % then I need to do some serious improvement here. At the moment not up to the delivered performance.

There is no pulsing in the lever, which I would expect if the disc is running out, perhaps part of the disk is harder than the rest?

Ps road surface good but I will take a slow run over the test track next time to check.

Thanks

Click to expand...

I know you have already checked it Nigel, but it may be worth a double check as its the easiest, quickest, cheapest option: an out of balance wheel can demonstrate the exact symptoms you describe.
Other possibilities that come to my mind are sloppy fork bushes or play between the sleeves you've fitted and the sliders.
And finally, I read your earlier post about the fitting of a rubber door seal to the disc mounting dish... Frankly Nigel, this sounds quite potentially dangerous to me and I strongly advise reverting to standard ASAP.

Click to expand...

The fork legs have sleeves in (they were sloppy) and "thinner" bushes done by "Lansdowne" I will re-check the balance and have a check at the door strip (it's really a much better soln. than it sounds, the disk can still "float" but not slop about)PS I do I appreciate the concern here.

To be honest, I am now thinking about replacing the front end as it does'nt really seem up to it (too many cars with ABS) and I would prefer a disk on each side at the front (another topic though)

Regards

Click to expand...

Don't give up on it yet matey! Lots of guys race fast bikes with these forks without the problems you currently have, I'd put money on the fact that it is due to something being wrong, its not cos you're reaching the limits of the forks themselves.

My Avitar pic is a 906cc Weslake powered Dresda with Norton forks and a Norvil (PR) single disc. That was a fast bike and the forks and brakes were fine. The design, especially with Landsdowne bits in, should be well up to the task... IF everything is tip-top...

The stanchion tubes flexing are a design limit. You've reached it.

concours said:

The stanchion tubes flexing are a design limit. You've reached it.

Click to expand...

Reached the design limit by braking hard at 70mph ?!

If they're flexing, and if they're 'good uns' then I would agree. But I can't be alone in having braked hard from a tad more than 70mph without having these symptoms.
Hence, I still feel something else is amiss here.

From a handling perspective actually surprisingly good for an older machine (and older rider!). I stongly suspect the disc/rotor surface but I will check all the bearings and bushes again tomorrow and perhaps try a different damper setting - could this cause it?

The front RoadRider tyre seems to be nice and sticky and does'nt really want to lock so perhaps it is at, or close to the limit?

PS Caliper has been serviced and new pads fitted.

Hmm -maybe I'll put my Son's GoPro on the front and see if I can film what is happening

Thanks

Nigeldtr said:

From a handling perspective actually surprisingly good for an older machine (and older rider!). I stongly suspect the disc/rotor surface but I will check all the bearings and bushes again tomorrow and perhaps try a different damper setting - could this cause it?

The front RoadRider tyre seems to be nice and sticky and does'nt really want to lock so perhaps it is at, or close to the limit?

PS Caliper has been serviced and new pads fitted.

Hmm -maybe I'll put my Son's GoPro on the front and see if I can film what is happening

Thanks

Click to expand...

Please do keep us posted as to what / if anything, you find Nigel.

A few other things to check while you're at it:

tyre pressures
spoke tension and check for out of roundness of front wheel rim
wheel spindle clamp on slider: should be tight, but not over-tight. They have been known to fail when over-tightened
wheel bearings
headstock bearings: stand in front of the machine facing the headlamp and pull forward hard on the bars with the front brake applied
check the fork tubes are pulled up hard against their tapers in the top triple clamp by first loosening the lower triple clamp bolts

Have you looked closely at the frame to make sure there are no cracks around the headstock? Unlikely, but at this stage, leave no stone unturned.

Apologies if I am teaching my grannie to suck eggs here! Sometimes even the best of us can miss things like these when chasing a problem because our attention is on something else!

Look forward to your Go Pro movie...

Fast Eddie said:

concours said:

The stanchion tubes flexing are a design limit. You've reached it.

Click to expand...

Reached the design limit by braking hard at 70mph ?!

If they're flexing, and if they're 'good uns' then I would agree. But I can't be alone in having braked hard from a tad more than 70mph without having these symptoms.
Hence, I still feel something else is amiss here.

Click to expand...

I can make my forks tuck under at will, with just the standard disc brake, no MC sleeving. Sighting down the forks, I watch the axle move to and fro 1/2".
Yes, design limits. The stanchion tubes are spindly and you've upsized the brake, you mentioned changing the fork, that will work, and then the frame will be the next weak link.

A decent Roadholder Commando should brake confidently like racers state so with all the work on forks & brake might suspect rear lifting to shocks top out with rear wanting to swap ends so reflexly countering with some fork steering but isolastic frame bending-rebounding tends to side slap the front gap slack which in turn wiggles forks. I can induce this wallowing sense with squeal on plain Jane '72 Trixie if a 110 front fitted but not enough tire grip on a 100 so just noisely lack of slowing I will never put up with again after that 100 tire worn out. I have observed fro/aft horizontal oscillations that can blur axle but only moves like 1/8" ie: 1/16" each way beyond neutral d/t bush slack and another 1/8" total d/t stansion/fork slider springing. Trixie's forks don't have a fork brace so also tends to twist/untwist pretty fast out of rhythm with pilot corrections - on no choice short pull downs. All 3 of my cycles have more brake power than tire grip so don't be misled by thinking too much brake is causing trouble as all that does on our size tires and forks is make tire more difficult not to lock up in tough conditions. Fork braces about hardest thing to fit right on Roadholders and may not solve issue.

concours said:

I can make my forks tuck under at will, with just the standard disc brake, no MC sleeving. Sighting down the forks, I watch the axle move to and fro 1/2". Yes, design limits. The stanchion tubes are spindly and you've upsized the brake, you mentioned changing the fork, that will work, and then the frame will be the next weak link.

Click to expand...

I think you are mistakenly failing to consider the movement allowed within all the components including the yoke stem bearings, that TOGETHER are allowing the "tuck" yo describe. I highly doubt the stanchion tubes would allow more than an almost imperceptible amount of flex except under VERY extreme conditions.

There is an issue or issues in the assembled system that need to be carefully investigated in a methodical manner to eliminate the cause(s).

daveh said:

A few other things to check while you're at it:

tyre pressures
spoke tension and check for out of roundness of front wheel rim
wheel spindle clamp on slider: should be tight, but not over-tight. They have been known to fail when over-tightened
wheel bearings
headstock bearings: stand in front of the machine facing the headlamp and pull forward hard on the bars with the front brake applied
check the fork tubes are pulled up hard against their tapers in the top triple clamp by first loosening the lower triple clamp bolts

Have you looked closely at the frame to make sure there are no cracks around the headstock? Unlikely, but at this stage, leave no stone unturned.

Apologies if I am teaching my grannie to suck eggs here! Sometimes even the best of us can miss things like these when chasing a problem because our attention is on something else!

Look forward to your Go Pro movie...

Click to expand...

Thanks for all these, an excellent check list for me. It is far too easy for me to get focused on one thing and neglect the obvious! I' ll see if I can do some filming this evening.

concours said:

Fast Eddie said:

concours said:

The stanchion tubes flexing are a design limit. You've reached it.

Click to expand...

Reached the design limit by braking hard at 70mph ?!

If they're flexing, and if they're 'good uns' then I would agree. But I can't be alone in having braked hard from a tad more than 70mph without having these symptoms.
Hence, I still feel something else is amiss here.

Click to expand...

I can make my forks tuck under at will, with just the standard disc brake, no MC sleeving. Sighting down the forks, I watch the axle move to and fro 1/2".
Yes, design limits. The stanchion tubes are spindly and you've upsized the brake, you mentioned changing the fork, that will work, and then the frame will be the next weak link.

Click to expand...

This sounds very much like what is happening. I am just getting used to the Norton, been mainly on BSAs over the last few years (much more sedate). I feel very comfortable with the machine and love the animal "grunt" Almost totally rebuilt the bike. Traffic here is German is fast even on the country roads so if you want to keep up or like driving quick, like the norton does, then everything has to be in top working order otherwise something will fail or let me down just when I need it. equally, I need to understand where the limits are and hopefully avoid getting to close. Also, good fun getting to know the machine - I imagine a track day would be very good learning excercise

Thanks

grandpaul said:

concours said:

I can make my forks tuck under at will, with just the standard disc brake, no MC sleeving. Sighting down the forks, I watch the axle move to and fro 1/2". Yes, design limits. The stanchion tubes are spindly and you've upsized the brake, you mentioned changing the fork, that will work, and then the frame will be the next weak link.

Click to expand...

I think you are mistakenly failing to consider the movement allowed within all the components including the yoke stem bearings, that TOGETHER are allowing the "tuck" yo describe. I highly doubt the stanchion tubes would allow more than an almost imperceptible amount of flex except under VERY extreme conditions.

There is an issue or issues in the assembled system that need to be carefully investigated in a methodical manner to eliminate the cause(s).

Click to expand...

...ohhhh, where to start.... :roll:

"I think you are mistakenly failing to consider" I think you are failing by not stopping and considering that there is way more going on than you know about. Blissfully unaware. :|

" I highly doubt " based on what? Ever ridden with me? 42 years riding, off road and on, all terrain, 36 years in the metal trade, machinery repair, you think I don't know what I'm looking at? :shock:

"There is an issue or issues in the assembled system that need to be carefully investigated in a methodical manner to eliminate the cause(s)." The cause is as originally stated. Stanchion flexure. My forks and steering stem are freshly rebuilt, tight as a pin. Same flexure observed before refurbishing. :idea:

" except under VERY extreme conditions" You mean greater than parade speeds?

With all due respect gp, your are way in over your head making such comments. :mrgreen: