Front brake tuning!

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.

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Concours,

I reckon if your forks are moving back and forth by 1/2" then there is something wrong with them - I'm not talking about design limitations. Plus, if you were to bend a stanchion by 1/4" (I think you're suggesting this is how much your stanchions are bending in use) over the distance from the bottom yoke to the end of the stanchion, approx 14", it would probably stay bent. Fair enough, Commando's could benefit from stiffer forks, but it's not the fork design which is responsible for Nigeldtr's problems.

Nigel - having been used to riding modern road sports bikes, and riding my almost standard Commando at a good pace alongside modern bikes, it did not feel as planted and secure as a decent modern bike with a stiff chassis, sophisticated suspension and better brakes which the sturdier chassis can deal with. Commandos can be made to handle surprisingly well within their performance envelope, but you will find you reach a limit beyond which it starts to protest a bit!

Re track days - Much has been written on this forum about handling. If you decide to do a track day, one of the best and simplest mods to improve the handling is a decent after-market head steady. Maybe you have one already, but if you don't, get one. Once you do this, it will improve your confidence in the bike and you will enjoy track days better. Some people go further and fit rod links fore and aft of the engine/gearbox to stiffen the plot. All that said, your suspension also needs to work well, your wheels need to be aligned as precisely as you can get them and your isolastics need to be in good nick and adjusted properly, which is an art in itself.

daveh said:

Nigel - having been used to riding modern road sports bikes, and riding my almost standard Commando at a good pace alongside modern bikes, it did not feel as planted and secure as a decent modern bike with a stiff chassis, sophisticated suspension and better brakes which the sturdier chassis can deal with. Commandos can be made to handle surprisingly well within their performance envelope, but you will find you reach a limit beyond it starts to protest a bit!

Re track days - Much has been written on this forum about handling. If you decide to do a track day, one of the best and simplest mods to improve the handling is a decent after-market head steady. Maybe you have one already, but if you don't, get one. Once you do this, it will improve your confidence in the bike and you will enjoy track days better. Some people go further and fit rod links fore and aft of the engine/gearbox to stiffen the plot. All that said, your suspension also needs to work well, your wheels need to be aligned as precisely as you can get them and your isolastics need to be in good nick and adjusted properly, which is an art in itself.

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You'd expect some improvement in road holding over the last 40 years so no surprise the difference in feel between the modern bike and the Commando. I agree with wheel alignment and a decent head steady but forget rod-ends - time and money is better spent on chassis alignment. There's no secret to adjusting iso's, it's straight forward. Getting the thrust faces parallel to each other however requires some work. Commando's do have stiff chassis's (stiffer than a Featherbed apparently - every thing is relative) and they handle very well when chassis alignment is correct. I'm doing two more track days at Cadwell, 17th May and also the Classic Bike Burn-Up, 1st August. Will anyone else be there?

Back to the original post.

Previously there was a concern that the disc was warped if I remember correctly? The lack of pulsing at the lever may not prove anything either way - it just means the disc is the same thickness all the way round. if there was a definite kink, perhaps, but a warp may not be detectable by feel.

Have you measured the run-out with the front wheel jacked up?

If the brake bite is uneven around the disc, I wouldn't be surprised if the forks do twang backwards and forwards under harsh conditions.
It seems the focus is on the symptoms, not the cause.

I really would bin off that disc and start again - there's clearly already been doubt about its fitness for purpose, so eliminate it before trying to understand why the effects are so dramatic.
If nothing feels wrong until the brake is applied, then perhaps there is nothing else wrong?

The standard disc setup with a re-sleeved master cylinder is very capable, and although I did fit an RGM conversion to my 850, I wouldn't say it's an essential, just another 10-15% less effort on the lever. Both setups can squeal the tyre if I'm daft enough to try it.

I vote with those that say the upsetting fork stability on max pull downs ain't the upgraded front end. My factory stock Roadholder R brake Trixie tends to hop-skip-chatter front tire/forks on good surfaces trying for best effort, so have to back off some while gritting teeth between stopping in time or crashing to slide into objects. I am very scared of brakes so FORCE myself to work up nerve to both warm up my reflexes and tire and determine if I'm in crazy enough ego lucky mood to ride near limits that day. On the other hand Ms Peel with the tri-links was so stable I could pull an ~1" stoppie while sliding a locked front so many times so long - trying to get a real stoppie by various rates of lever action feathering - as I entered my village square - other bikers could see-hear us coming so would run up close yelling I had locked the front! No way can a modern pull down as sharply as Ms Peel which could out brake my ability to stay in saddle and not fly over bars with bike still mostly level. I practice lock up stops pulling into garage or up to gas pumps to have my modern shock me with unexpected=unwanted stoppies while Peel would make helmet strike bars if not stiff armed enough. I can't load poor ole stock Triixe that much even on 110 tire, which did please me more than 100. Tirixe as sloppy as she is can't hang with good brave moderns in our nice hwy turns or THE Hinge onsets but I get so bored with them on Ms Peel pretty much don't even think of road racing as thrilling anymore compared to the off road antics. Again this braking fork wiggle may be sign of Roadholder twist/rebound trying to resist the rest of the isolastic wiggle worm behind it as it angles forces on tire to grab and release at slight mis aligned loads that turn into fork=tire chattering fight more than just straight ahead steady pull down. i don't ever feel that confident braking hard so look up to those working up to it and wish for as much success as my Peel so improvements in handling and braking don't even occur to me any more, just my own limits to face.

Thanks for the comments and expertise! I tried filming this evening but light was failing and I seem to need to use a lower resolution but higher frame rate! The vibrating front just became blurred! I’ll try again in the morning, dashed clever stuff this GoPro.

I checked the front end through, tighten up the stanchion nuts (a little loose), checked front wheel balance, spokes, bearings etc etc and all good. I suspect I am probably expecting too much from a 40 year old bike, which in fairness is great to ride and bags of fun. I will seriously consider changing the disk/rotor if the consensus is this is the culprit. I am quite enjoying this little “challenge” as it is helping me to get familiar with the bike. The head steady is a Norvil type and was in a terrible state and misaligned. I serviced the front and head steady isolastics (rear also adjusted) when I put the motor back in 2 weeks ago – now have 300 miles on it.

Generally the bike rides and handles very well, it sits well on the road and has a solid and stable feel. My biggest and most surprising improvement to the ride and handling so far came through balancing the wheels a BIG difference, they were way out!

Back with a film tomorrow - hopefully.

Al-otment said:

I reckon if your forks are moving back and forth by 1/2" then there is something wrong with them - I'm not talking about design limitations. Plus, if you were to bend a stanchion by 1/4" (I think you're suggesting this is how much your stanchions are bending in use) over the distance from the bottom yoke to the end of the stanchion, approx 14", it would probably stay bent.

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That is more or less what I was implying, and check the response I got...

"What we have here is failure to communicate"

I'm thinking something is getting lost in translation in several sections of this discussion...

grandpaul said:

"What we have here is failure to communicate"

I'm thinking something is getting lost in translation in several sections of this discussion...

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Cool hand Luke...

Right?

Concours, If you look down the forks while braking, because of the geometry - as the forks compress they appear to tuck under. Perhaps that is what you are seeing ?
I suggest that if you are getting vibration or shaking, something is loose or cracked. A distorted disc is unlikely, you should feel it even when braking at low speed.

Al-otment 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...

Concours,

I reckon if your forks are moving back and forth by 1/2" then there is something wrong with them - I'm not talking about design limitations. Plus, if you were to bend a stanchion by 1/4" (I think you're suggesting this is how much your stanchions are bending in use) over the distance from the bottom yoke to the end of the stanchion, approx 14", it would probably stay bent. Fair enough, Commando's could benefit from stiffer forks, but it's not the fork design which is responsible for Nigeldtr's problems.

Click to expand...

Not quite accurate, the fork slider extends another 8" beyond the stanchion, less when compressed, so, the flex, projected out, .250" either way, is likely less than .200" at the end of the stanchion. This is WELL WITHIN it's ability to flex, no where near it's yield point.

I'm not proposing a stiffer fork, the OP mentioned it as possibly his next move. I'm simply citing that the spindly forks are overwhelmed by hard braking and I stand by my observations.

grandpaul said:

"What we have here is failure to communicate"

I'm thinking something is getting lost in translation in several sections of this discussion...

Click to expand...

Maybe a couple nights in the box would help..... :lol:

acotrel said:

Concours, If you look down the forks while braking, because of the geometry - as the forks compress they appear to tuck under. Perhaps that is what you are seeing ?
I suggest that if you are getting vibration or shaking, something is loose or cracked. A distorted disc is unlikely, you should feel it even when braking at low speed.

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No, not so much. I mentioned TO and FRO, slang for forward and backward.... this motion is generated as the braking force varies causing the fork stanchions to flex, first bending back, then unloading forward. (the exact same motion observed by the OP) Braking hard on bombed out broken pavement helps too.

Nigeldtr said:

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

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.


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.

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

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There is a reason modern inverted forks are such a HUGE improvement... the bane of telescopic forks has always been the flex of stanchions. They grew fatter and fatter trying to combat the problem. The Commando forks were not substandard, they were what was happening half a century ago.

Here is a brief summary (from the Yamaha website):
"Inverted forks are positioned on the motorcycle opposite or upside down when compared to conventional forks. The leverage forces that cause fork flex are greatest at the triple clamp area and weakest at the front axle. On inverted forks, the large outer tube of the fork is clamped in the bike’s triple clamps and the sliding inner tube holds the axle and front wheel. By locating the large diameter tubes in the triple clamp, the inverted or upside down fork have their largest and strongest parts combating the highest stress. This arrangement gives the forks high rigidity, which improves their response by reducing the side loading of the internal bushings (sliding surfaces). This kind of response is particularly important in high performance applications. Most inverted forks use cartridge-type damping systems."

Mush as the construction and concepts of motorcycles has advanced If ya read what pilots report scares them and see how they still get upset - just being completely strong enough is not enough so they further report trying to re-weaken some elements to be compliant enough then causes lost of control again or just breaks. We know the sort of brake you installed works well on others and Lansdowne forks raved on by racers and nothing makes sense so far, just keep in mind stuff behind the stem before resigning to disconcerting brake confidence. Best wishes on a Goldie Locks solution as the high may never leave ya.

B+Bogus said:

Back to the original post.

Previously there was a concern that the disc was warped if I remember correctly? The lack of pulsing at the lever may not prove anything either way - it just means the disc is the same thickness all the way round. if there was a definite kink, perhaps, but a warp may not be detectable by feel.

Have you measured the run-out with the front wheel jacked up?

If the brake bite is uneven around the disc, I wouldn't be surprised if the forks do twang backwards and forwards under harsh conditions.
It seems the focus is on the symptoms, not the cause.

I really would bin off that disc and start again - there's clearly already been doubt about its fitness for purpose, so eliminate it before trying to understand why the effects are so dramatic.
If nothing feels wrong until the brake is applied, then perhaps there is nothing else wrong?

The standard disc setup with a re-sleeved master cylinder is very capable, and although I did fit an RGM conversion to my 850, I wouldn't say it's an essential, just another 10-15% less effort on the lever. Both setups can squeal the tyre if I'm daft enough to try it.

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Nigel,

there's some confusing information in the above. Pulsing at the lever indicates a warped disc. It has the same effect as quickly applying and releasing pressure so could be the cause of the symptoms you describe but should be obvious going by the severity of fork action you describe under braking. I don't know how you'd get a kink in a cast iron disc without an obvious crack appearing.
I wouldn't bin £200 worth of disc without first checking it, I would change the DISH - this is an important part and is crucial for disc alignment. However, if the brake is from Emery's shop I wouldn't fit it in the first place.
The forks are well capable of coping with hard braking at speed and should not behave the way yours are. You do have a problem but it's not the fork design. Are the pads compatible with cast iron? - the caliper is probably off a bike fitted with stainless steel discs.

concours said:

Al-otment 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...

Concours,

I reckon if your forks are moving back and forth by 1/2" then there is something wrong with them - I'm not talking about design limitations. Plus, if you were to bend a stanchion by 1/4" (I think you're suggesting this is how much your stanchions are bending in use) over the distance from the bottom yoke to the end of the stanchion, approx 14", it would probably stay bent. Fair enough, Commando's could benefit from stiffer forks, but it's not the fork design which is responsible for Nigeldtr's problems.

Click to expand...

Not quite accurate, the fork slider extends another 8" beyond the stanchion, less when compressed, so, the flex, projected out, .250" either way, is likely less than .200" at the end of the stanchion. This is WELL WITHIN it's ability to flex, no where near it's yield point.

I'm not proposing a stiffer fork, the OP mentioned it as possibly his next move. I'm simply citing that the spindly forks are overwhelmed by hard braking and I stand by my observations.

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I assumed your forks are flexing by 1/2" under hard braking i.e bottomed out. If they're moving about as you describe under normal conditions then you might need new bushes, fork sliders and steering head bearings. Could you back up you're flexing theory with some figures? Typing words in capitals dosen't make what you say any more valid.
Roadholder (and other fork) stanchions do bend, permanently, under hard use. Next time the forks are apart put the tubes on V-blocks and measure the deflection with a dial gauge.

From a purely pragmatic view point, it seems to me that the forks must flex, they have no choice, the question is by how much and what determines mechanical/physical reaction. With the huge amount of force at work the backward bend energy under braking must be released in a spring loaded forward (equal and opposite) way, the speed at which this happens and the force pushing back again "could possibly" set up an undamped oscillation horizontal to the road surface :?: This is what I "think" could be happening, this is a complex equation as the tyre grips and then gives way a little (wheel starts to lock), the tensioned forks unload their stored energy in a forward direction and if still braking hard, the cycle starts again. At lower force levels this is probably all unnoticeable as it never gets into oscillation mode. At high speed, high force, sufficient energy is being stored and released fast enough to hit resonance which could get the forks singing like musical tuning forks. All a bit fanciful I know

Perhaps I'll try lowering the front tyre pressure a little and see if the increased contact area holds back the "twang" so to speak :?:

Nigeldtr said:

From a purely pragmatic view point, it seems to me that the forks must flex, they have no choice, the question is by how much and what determines mechanical/physical reaction. With the huge amount of force at work the backward bend energy under braking must be released in a spring loaded forward (equal and opposite) way, the speed at which this happens and the force pushing back again "could possibly" set up an undamped oscillation horizontal to the road surface :?: This is what I "think" could be happening, this is a complex equation as the tyre grips and then gives way a little (wheel starts to lock), the tensioned forks unload their stored energy in a forward direction and if still braking hard, the cycle starts again. At lower force levels this is probably all unnoticeable as it never gets into oscillation mode. At high speed, high force, sufficient energy is being stored and released fast enough to hit resonance which could get the forks singing like musical tuning forks. All a bit fanciful I know

Perhaps I'll try lowering the front tyre pressure a little and see if the increased contact area holds back the "twang" so to speak :?:

Click to expand...

If your still braking hard then there will be a horizontal force opposite to the direction of travel on the forks, even when the tyre loses grip there is still friction indicated by the tyre squealing and therefore a force applied to the forks.
I think you're over complicating matters. When the bike is moving forward there is always a force acting horizontally against the forks. This force fluctuates depending on speed, bumps in the road and under braking. I think you're suggesting your stanchions are acting more like springs than stiff tubes when subjected to this horizontal force. As has been said before, you need to go through everything methodically as something is wrong. The symptoms you are describing are not inherent with Roadholders. Mine don't do it and I've got the same design brake but Lockheed racing caliper and sleeved master cylinder. As Peter Williams didn't design the brake with car door trim remove it and replace the dish as a priority and then check everything else.

Nigeldtr said:

From a purely pragmatic view point, it seems to me that the forks must flex, they have no choice, the question is by how much and what determines mechanical/physical reaction. With the huge amount of force at work the backward bend energy under braking must be released in a spring loaded forward (equal and opposite) way, the speed at which this happens and the force pushing back again "could possibly" set up an undamped oscillation horizontal to the road surface :?: This is what I "think" could be happening, this is a complex equation as the tyre grips and then gives way a little (wheel starts to lock), the tensioned forks unload their stored energy in a forward direction and if still braking hard, the cycle starts again. At lower force levels this is probably all unnoticeable as it never gets into oscillation mode. At high speed, high force, sufficient energy is being stored and released fast enough to hit resonance which could get the forks singing like musical tuning forks. All a bit fanciful I know

Perhaps I'll try lowering the front tyre pressure a little and see if the increased contact area holds back the "twang" so to speak :?:

Click to expand...

I for one would not disagree that the forks must flex, and that they must surely flex a lot more than the latest 55-60mm diameter upside down jobs, as concours says, these will be stiffer, much stiffer (and better).
A debate about how much different forks flex, under what conditions, as a result of what causes, and what the effect is and when it becomes an issue vs when it is unnoticeable, would be a good debate to have, but it wasn't your original question / point. Your problem, if I'm correct, was some quite unsettling front end behaviour under hard braking at circa 70mph. This isn't common and does need correcting if you're going to continue enjoying your bike, safely!

Again, I would agree with anyone that the forks will flex in these conditions, although how much, and what the effect is, I've no idea.

My own opinion was and still is, that the flex occurring in correctly made stanchions made of the correct material and in good condition, to whatever degree this occurs, at this speed and in these conditions, is not the root cause of the symptoms you are trying to fix.

My opinion is based on the following logic:
1. I had the exact same forks and brake on a racer that had a better power-to-weight ratio than a Yamaha R6, and I didn't suffer your symptoms (ok, I did once, when I had an out of balance wheel).
2. Roadholder based forks are fitted to more more classic / historic racers than any other.
3. Similar basic designs with similar 'skinny' 35mm stanchions are fitted to such things as 100bhp 1000cc BSA / Triumph triple racers.
4. Whatever make the fork, many classic race classes limit the stanchion size to 35mm. Therefore, all such equipped bikes will be exposed to the same degree of inherent stanchion flex. And many of these bikes are ridden very, very, hard indeed.

So, whilst I agree your fork stanchions will flex, and I agree that modern USD forks are far stiffer (and generally better in every respect), I would still nevertheless say, that if they are functioning correctly (as per design standard), then they are not causing the symptoms that you describe. The cause, in my humble opinion, is something not functioning correctly, something that is 'away from design standard'. That is what needs finding and fixing.

A final point from me to all involved in this thread is this: This is a very interesting thread, it is very interesting indeed to read different people's opinions. My own opinions are only that, I make no claim to be Peter Williams! I therefore welcome anyone arguing against me with a counter opinion, we can then decide how much we agree with each other, or not.
But I would suggest that this forum will be far better, if such debate is done so without insults, accusations that poster are in over their head, etc!

ALL IMHO of course.