Front brake tuning!

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Al-otment said:
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.

Happens at less than full compression as well.
Follow along, a page ago I mentioned freshly rebuilt, everything has been inspected/measured.
Stanchions are straight.
The flexing I (and the OP, many others) observe is not my theory, but rather simple physics, properties of materials.
Put aside for a moment the "parts changer" mindset and try to fathom the HUGE (sorry) loads created by dynamic braking and suspension.
 
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 :D

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 :?:
That is exactly what is happening.
 
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 :D

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 :?:

Front end 'patter' is a well known problem under heavy braking. The tyre breaks traction, then grips, skipping across the road surface, and you're heading for the verge or the 'kitty litter'. I have had this a few times when I rode a CBR400 track day bike with suspension that was too soft and poorly damped. But this has nothing to do with the stanchions springing back and forth, as you suggest is happening. The suspension has bottomed out and is simply unable to react as it should to keep the tyre in permanent contact with the road surface.

I would like to substitute the disc for another and see what happens. Is the tyre totally round and seated properly on the rim? Is the tyre new and has it been bedded in?

Al-otment - agree about fork stanchions bending permanently from sustained heavy braking. An engineer over here has a service straightening and hard chroming fork tubes and races his CB350 Honda with great success, and he brakes very hard. He showed me his (uncrashed) 35 mm Ceriani stanchions and they were visibly bent.
 
I have a nasty suspicion there are too many variables here. I fitted Landsdowne dampers when rebuilding the front end and to be honest I can't feel them bottoming out They do seem to work really well. however, perhaps they could work even better and need fine tuning. Adjusting seems a bit hit and mis. I am confident about the tyre, rim etc because she rides so well under " normal" conditions. Another disk and mount is not so expensive and this is possibly the solution. So I will probably order a replacement. However, I really would like to be sure the dampers are optimised for my setup. How can I best tune the dampers, glue in an Alan key and drive up and down the round twiddling the adjuster :?: - anyone with some experience here?
 
How can I best tune the dampers, glue in an Alan key and drive up and down the round twiddling the adjuster - anyone with some experience here?

Why not? Mountainbike suspension has top adjusters for just that purpose...
Front brake tuning!
 
The Landsdowne dampers changes the original unadjustable fixed rate damping to a adjustable FIXED rate damping. You can vary the damping with the adjustment allowed before a ride but it only changes the single rate for each side eg compression and rebound, it does not add dual rates of damping based on speed of defection or add any anti dive.

So if you are setting the damping rates up at normal riding but are having trouble at extremes then you need to change the damping rates to match the extreme requirements and live with the consequences at normal riding.
 
Well, after writing the response, I thought exactly that - why not, so that's what I've been doing :!: I have increased the compression resistance so the ride is a bit firmer and it has greatly improved the braking under load. The brake bites hard and is constant in power and feel until about 2/3 of the speed is off and the tyre starts to scream and the front then starts hopping. I tried several different settings and now seems to be about right. Noticeably less springing at higher speed and good traction. Seems it has removed the "nasty" bit but is still "springy".

Ps I have a 90-19 front and 100-19 rear. Handles well and steers quickly into and out of the corners - would a "fatter" front make much difference?

Thanks
 
You could put a zip tie around a fork stanchion (not to tight) so you can see how much travel is being used,maybe the fork is being hammered up under braking until there is little travel left.
Even on a classic bike,static and loaded sag is worth checking.
 
Up the rebound damping a bit if the compression is getting high to increase overall damping, you can test to see if you have too much rebound by finding a set of close together bumps of similar size eg rumble strip or speed bumps. Go over these series of bumps and check the forks extend enough between each bump, if you go over on the rebound damping the forks will recover less over each bump and eventually bottom out as the available travel reduces after each bump.
 
Braking is by far my most vital feature on cycles and highly suggest 110 front size to distinctly lower the interval of fear of slowing on hopping skipping out 90 size. If your front wears the center flatter faster than the sides then sure sign not enough tire to take what ya can dish out to it. My Trixie with plane Jane factory Roadholders did not skip and hop on 110, just had to feather off on slowing to stay in squeal zone with fork action bother while on 100 i have to let go some instants to re-grip/settle forks down then re-grip brake with lesser intentions than the fear factor sense of grip level so desperately desired - so just don't go that fast on Trixie to need much brakes and first reflex is steer around stuff even if means lane spliting or going into ditch - it beats slamming down to impact a hazard on 2nd impact. I seem to sense the extra 110 size tire mass helps dampen the faster fork action that can harmonize with the skipping patch on lighter unsprung mass. I get some what better brake control by almost topping off fluid level some what short of hydro lock after air is pumped out of forks in few 100 yards of slight bumps. Anywho keep tire size in mind if all else fails. Fullauto says he likes 110 at both ends and so do I on ordinary powerful untamed iso Commando. FullAuto and others say better on one size up rim but I prefer big tires pitched rounder so happy with factory Combat set up. I tend to practice over powering tire on down hills to make the speed ups for next test a bit faster.
 
It seems my observations aren't too far off from what these folks found as well. Whaddayaknow. :mrgreen:


The Third Motion | Drawing the Line


By James Parker, Photography by Gold & Goose



Motorcyclist, February 24, 2012







?






James Parker Investiagates The Third Motion





Barring a few exceptions, the front wheel of your motorcycle is steered and suspended by a telescopic fork. Steering motion is a simple rotation around the steering axis, and suspension motion an equally simple sliding motion between two pairs of tubes.

That’s not to say that the forces and mechanisms involved are necessarily simple. Steering motion is constrained by steering geometry; by the forces generated from the rotation of the front tire, wheel and brakes; by the complex interactions of the tire against the pavement; and even by the rider’s hands in contact with the handlebar. To further complicate things, the steering motion can sometimes be damped by a hydraulic steering damper.

The suspension motion is controlled by springs that support the weight of the motorcycle, dampers that in turn control the spring motion, bushings that reduce friction to ease the sliding motion and oil to lubricate it all.

These two motions are so much a part of riding that we don’t think about them much. If they come to our attention at all, it’s probably because we’re experiencing problems: headshake or wobble in steering motion, or loss of traction over rough pavement.

But there is a third motion that isn’t talked about much. Though there are springs inside the fork, the fork itself is also a spring. It allows the axle to move back and forth, perpendicular to the fork’s sliding axis (and, to a lesser extent, side-to-side), because the fork tubes flex under load, bending as load conditions change. And this flexing and recovering motion is not damped. The extent of flex is controlled by the strength of the components, but the movement itself is not controlled.

Ride an unfaired vintage bike and look down at the front axle: It buzzes back and forth perceptibly. Newer forks do a better job, but still show some movement, although rarely enough to matter. As speeds and the forces generated by speed increase, however, the third motion of your fork might not be so innocent.


James Parker Investiagates The Third Motion

James Parker Investiagates The Third Motion Not only does a motorcycle’s fork compress under braking, as Nicky Hayden demonstrates her

Under heavy braking, where the rear suspension extends to the point that the rear tire literally lifts off the ground, the fork sees its highest loading. In that situation, the entire weight of the bike and rider is trying to bend the fork back toward the engine. That’s at 1G (1 x the force of gravity) braking. Add carbon brakes and exceptional tires, as in MotoGP, and braking forces may rise to 2G.

Anecdotal evidence from an AMA Superbike team sheds light on this third motion. Their bike’s front tire cleared the radiator under full suspension compression in the garage, but on the racetrack the tire contacted the radiator to the point that they had to move the radiator back more than 25mm. That’s an inch of flex.

As a bike enters a corner under braking, with the front suspension fully compressed and the rear wheel an inch off the ground, the trail measurement might be reduced from 100mm to about 70mm. Reducing trail generally reduces stability, so the bike may become unstable at a 30 percent reduction in trail. Ironically, fork flex comes to our assistance now, because if the fork flexes back 20mm, that adds 20mm of trail, giving us 90mm for better stability.

Let off the brakes and the fork can spring back over center, reducing trail. It can get pretty complex, mounting your front wheel on the end of a spring. In my view, a basic reason to have flex in the frame, and to work on tuning flex, is to provide some way of damping the third motion inherent in the fork. Compared to the fork, the typical aluminum frame is a relatively “soft” structure in which some of the energy imparted by fork flex and rebound can be dissipated as it travels through the frame members.

The MotoGP Ducati’s troubles with steering feel may be attributable to the fact that its steering-head structure is a short box which, being much stiffer than an aluminum beam frame, is unable to dissipate this fork-flex energy. Making the box from aluminum rather than carbon-fiber might not make much difference, as the box is stiff from its shape and size, regardless of material.

The telescopic fork is simple in concept but hardly simple in the details of its motions. The third motion, in particular, presents a complex mystery as the fork is pushed to its maximum performance limits.


Read more: http://www.motorcyclistonline.com/featu ... z302alM12e


link to full article here: http://www.motorcyclistonline.com/featu ... rd_motion/
 
hehe since when is lifting rear while leaned into a turn anything to do with harshly accelerating into turns the poor corner cripples hurting great pilots. If front tire can't hold the grip it will allow fork to spring forward on each tire chirp and back again on re-grit to oscillate like crazy. If tire can hold the brake loads howling it don't let forks oscillate as much or at all. My report to the world is one robust rump link and two complaint helper links in tri-angulated Watt's like geometry solves this 3rd level of loading so much for me I can't imagine better handling and still don't really know how much harsher Peel can take it as ran out of power to twist frame up even more for the wonder of the sling shot release w/o rebound but to total neutral. I call it the Goldie Lock's Porridge solution. I see most of Peel axle level blurring at limits of coutner steering d/t lightened front from acceleration causing it to slide limiting that way around, which can only evolve into a lazy flat tracker wide slide, so i shift into next higher energy handling to screw down and store more to release, then the next faster way but alas not enough power to explore the next way beyond that yet, except on THE Gravel.
 
This really is great stuff. The original reason for the thread was to improve and optimise my front brake and I think with what we have discussed and checked, have acheived a lot :!: My understanding of what is going on has gone for almost say 20% to 80% and I am amased how little I had thought about all of this. Interestingly, its the Commando, that simply whats to "get up an go", that encourages me to twist a bit more and brake a bit later - these machines are definately NOT for pottering around the country lanes :D
 
concours said:
It seems my observations aren't too far off from what these folks found as well. Whaddayaknow. :mrgreen:

The Third Motion | Drawing the Line
By James Parker, Photography by Gold & Goose
Motorcyclist, February 24, 2012

Is this James Parker the journalist looking for subscriptions and features to fill his on-line magazine or James Parker the mechanical engineer? I think the phrase to describe the feature is, 'baffling with bull-shit'. I doubt very much a 45+mm dia. fork tube would bend 25mm over it's unsupported length of approx. 300mm, let alone under full compression of the fork. I would be surprised if elasticity in the complete front fork assembly, including bending of the steering head and supporting frame tubes would allow this much movement at the axle. He's talking about a 2012 AMA superbike here I assume with USD forks. Crucially the evidence is anecdotal....bloke down the pub said...... He also makes a sweeping statement that all 'vintage' bike forks 'buzz' back and forth perceptibly - indicating what? I would suggest that they're f****d. Mine don't exhibit this when riding at a constant speed - under breaking I assume they do bend but I wouldn't know by how much as I'm looking where I'm going.

I don't claim to be a mechanical engineer, although I've engineering qualifications, just using logical reasoning. If I'm wrong, well good - at least I've learned something. Flex obviously occurs in just about any solid material or structure I can think of but if modern front forks, or even Roadholders, are bending by the amounts mentioned in this discussion then I think they would have been redesigned years ago so that they work properly.
 
A lot of this does seem possible and not surprising. If there is resonance then strange things will start to happen. It would be interesting to mount an end weighted fork leg in a jig and see what the resonant frequency is - any engineering students or mathematicians around :)
 
Nigeldtr said:
A lot of this does seem possible and not surprising. If there is resonance then strange things will start to happen. It would be interesting to mount an end weighted fork leg in a jig and see what the resonant frequency is - any engineering students or mathematicians around :)

Resonance occurs when a force applied to a body is at the same frequency as the natural frequency of that body. In the case of fork stanchions it could be vibrations from the road surface or bumps in the road equally spaced and with the bike traveling at such a speed that the frequency of the forks hitting the bumps equals the natural frequency of the stanchions and so you get resonance. I think the chances of it occurring in real world conditions on a motorcycle are small and not worth worrying about. How are you getting on with your forks?
 
Ani't nothing set in stone on fork frame or swing arm stiffness and give as depends a lot on what the pilot wants to feel. Articles on GP methods of customized flexure experiments per pilot feed back. There is a distinct element of fork twist/untwist axles blurring on hard braking that a fork brace helps tame by raising the flex frequency above the tire harmonics. If you are not hearing tires complain on powered leans and slowing - most of this is just academic. I learn't my lesions in public on very capable modern to never make tire noise any more as likely last noise before hard impact with so fast a shutter through whole cycle mostly lucky to recover in time to avoid next time.


http://www.insidemotorcycles.com/blogs/ ... otogp.html

http://motodna.com.au/flex-or-not-flex/
Norton’s 1950’s chassis was so poor it was nicknamed the ‘Garden gate’.
Reasons I call the rigid moderns corner cripples and floppy isolastics rubber baby buggies.

Braking Styles by Keith Code and more...
Braking, as it turns out, doesn’t just slow you down. A host of forces and often contrary changes come into play. To start, heavy braking can double front wheel load. The forward pitch of the bike steepens fork angle, for example, on a 1098 Ducati up to 4 degrees, reduces trail from 96.5mm to 72mm and shortens wheelbase. As the tire is squashed, tire height shrinks, also slightly decreasing trail. Balancing that out, the contact patch both broadens and lengthens rearward while it flattens. This can restore bike stabilizing trail by as much as 25mm. Trail is now close to where it started unless it locks up releasing the tire. At which point you’d have very little stability from the huge and instant reduction of trail.
http://www.motomom.ca/keith-code-on-tra ... interview/
 
Al-otment said:
Nigeldtr said:
A lot of this does seem possible and not surprising. If there is resonance then strange things will start to happen. It would be interesting to mount an end weighted fork leg in a jig and see what the resonant frequency is - any engineering students or mathematicians around :)

Resonance occurs when a force applied to a body is at the same frequency as the natural frequency of that body. In the case of fork stanchions it could be vibrations from the road surface or bumps in the road equally spaced and with the bike traveling at such a speed that the frequency of the forks hitting the bumps equals the natural frequency of the stanchions and so you get resonance. I think the chances of it occurring in real world conditions on a motorcycle are small and not worth worrying about. How are you getting on with your forks?

The setting of the dampers on the road made a big difference. It is now much more manageable right up to were the front starts to skip. The damping was way too soft and probably was bottoming out and then bouncing around on the uneven road. Anyway, a lot better. :!: This little exercise has put me and the bike to the test and we know each other a lot better now. I don't think there is any point changing the disk as really, me and the bike are at the limit here. Generally a lot happier with the bike, setup and brakes - needed doing and I think she is well sorted now. One other thing that came out of the wheel balancing, is my speedo drive doesn't slip round any more, horribly out of balance but this was another thread which I will update now.

Thanks
 
Nigeldtr said:
The damping was way too soft and probably was bottoming out and then bouncing around on the uneven road.

Time Warp said:
You could put a zip tie around a fork stanchion (not to tight) so you can see how much travel is being used,maybe the fork is being hammered up under braking until there is little travel left.
Even on a classic bike,static and loaded sag is worth checking.

You need to keep things simple then weed out the folks who read to many books. :wink:
You could still fine tune things given the added adjustability of the Landsdowne inserts,sag is always worth checking,modern or classic given the latter's limited spring options.
 
Time Warp said:
Nigeldtr said:
The damping was way too soft and probably was bottoming out and then bouncing around on the uneven road.

Time Warp said:
You could put a zip tie around a fork stanchion (not to tight) so you can see how much travel is being used,maybe the fork is being hammered up under braking until there is little travel left.
Even on a classic bike,static and loaded sag is worth checking.

You need to keep things simple then weed out the folks who read to many books. :wink:
You could still fine tune things given the added adjustability of the Landsdowne inserts,sag is always worth checking,modern or classic given the latter's limited spring options.

Hi Time Warp, sorry, I wanted to reply to your original post but got overtaken by events. I found the idea good but I have gaiters on the fork legs? I would love to be sure I have the best out of the dampers, they are better than I expected. What is sag, Don't recognise this?

Thanks
 
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