Structural Reasons for Fork Shudder on Bicycles with Cantilever Brake Systems

[KootK]

At the beginning of covid I acquired a version of the bicycle shown below. It's a 1992 Miyata 1000LT. It was the best, production touring bike that one could buy in 1992 for any amount of money. It is legendary for being a sweet, smooth, "steel is real" ride and the legend could not be truer. When I ride the bike, I feel so little discomfort in my body that it's almost as though someone chopped off my head and mounted it to the handle bars. I feel nothing but joy and exertion.

Performance wise, the only non-awesome thing about the bike is the cantilever brakes. As cantilever brakes are prone to doing, hard front braking will induce a pulsating front end shudder that is very disconcerting. Before attempting to fix the problem, I would like to accurately diagnose it. Everywhere this question is asked out in the online Bike-o-Verse, you get the same "bow and arrow" explanation described in detail here: Link. Looking at that explanation as a structural engineer, I find it to be utterly implausible. Some suggested remedies can also be found at that link but I deem them to be questionable because, in my opinion, they are based on a flawed theory of causation.

So my first question here is this: what do you think of the "bow and arrow" explanation? Do you buy it? Do you share my skepticism? I have reasons for my doubt, of course, but I'm going to withhold them for a spell here in the hope of not contaminating the thought pool. I've reproduced part of the "bow and arrow" explanation below for reference.

I've got my own theory about the cause of the fork shudder which we'll get into in more detail on later. The short version is that I think that it's a function of torsional fork twist rather than the front to back fork flexing that the "bow and arrow" theory suggests.

Why does shudder occur?

Brake shudder is widespread because it’s built into the design of almost all ’cross bikes; it’s inherent to the design of a center-pull cantilever brake. To understand the reason why it happens and why reduced pad size, lots of toe-in, and a tight headset help take a look at the chart titled “Brake Shudder in cantilever brakes.”

As the brake is applied, the ground applies a force directed backward on the tire as shown, causing the fork to flex backward. Problem is, the brake cable is fixed at one end at the brake caliper and at the other end at the cable stop above the headset (as you can see in my case, at a cable hanger attached to a bolt on the stem face plate).

Think “bow and arrow” and imagine the fork between the cantilever bosses and the top of the headset is like the bow, and the cable is like the string. As the fork flexes back due to braking, the cable tightens like the string in the bow, because its two ends – the cable hanger and the brake calipers, have moved further apart. So even though you may have pulled the brake lever carefully enough to modulate it properly, as soon as the pad slows the wheel down, the fork flexes back and tightens the cable, which in turn pulls the pads harder against the rim. This in turn flexes the fork back further, which tightens the cable more, which pulls the pads harder against the rim, and so on.

Eventually, something has to give: Either the tire must slip on the ground, the rider must go over the handlebars, or the pads must break free from the rim. It is the latter that creates the shudder, the pads bind and release, bind and release, each time allowing the fork to flex back and forth and the tire to roll and stop, roll and stop. This is why the problem goes away in mud and wet sand, because the pad can break free smoothly. It is also why smaller pads with more toe-in help.

If the headset is loose, the problem is greater, because the length change between the brake posts and the cable stop atop the headset is greater as the fork moves back when the brake is applied.

 

[LittleWheels]

A big component of the bow string and slip/ stick discussion is the flexibility of the fork steerer, which has not been considered in this thread.

The OP's bike uses a fairly long 1" dia. steerer which does not have a huge amount of bending stiffness compared to more modern bikes with a 1 1/8" steerer or more recently a tapered steerer with 1 1/2" at the fork crown to 1 1 1/8" at the top headset bearing.

 

[LittleInch]

I think there are so many different inputs into this that it is difficult to get to any one explanation. Basically any play or flexing in any of the component parts or structure will generate juddering.

Myself I've usually found that reducing any play on the brake arms always helps as does getting new pads or roughening up the ones you've got. Rim type brake blocks need to be soft enough to have a good CoF, yet hard enough that they don't wear away too much in use. They often seem to get "glazed" and a very hard surface develops which generates more of the stick slip phenomena.

Have you investigated a retrofit to disc brakes? They are just superb and might make your bike even better....





Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LittleWheels]

A fork with a 1" steerer and fork blades designed for rim brakes is not suitable for the greater eccentric loading imposed by a disc brake. Replacement disc brake forks for 1" steerer are pretty rare and, together with the replacement equipment required to run a disc brake, are cost prohibitive for an old touring bike.

A fork crown cable stop for cantilever brakes removes steerer flex as a contributor to front brake chatter.

https://www.bikehugger.com/posts/cyclocross-tidbits-tektro-crown-mounted-cable-hanger/

 

[KootK]

Have you investigated a retrofit to disc brakes? They are just superb and might make your bike even better....

I have, that's actually where I started when I got the bike. When I was a child, I was flying down a hill when the front cantilever brake straddle wire on my bike hopped the cable guide. The straddle wire then fell on top of the knobby tire, caught, and brought the bike to an almost instantaneous stop. I flew over the handlebars and collided mouth first into the tailgate of a parked F150. The fake front tooth that I earned that day still causes me difficulty and cost me $$$.

So... I'm not a fan of cantilever brakes. To an almost pathological degree really. I inspect them before every ride and probably spend way too much time inspecting them while I ride. Part of the move to the CX50's is that the straddle wire setup should nearly eliminate the possibility of my experiencing that same nightmare a second time.

Switching to disc brakes would be awesome. However, it would be impossible at the rear because the frame mounting hardware for that is non existent. Additionally, I've only got 130 mm between the dropouts there which is probably not enough. I could potentially go to disks at the front but it would require me replacing the fork with one having a 7/8" steerer tube and, preferably, a quill stem setup. That's a pretty rare setup that would be tough to come by. Additionally:

1) The asymmetry of disk on the front and rim brakes on the back would bother me immensely.

2) The best thing about the bike is it's compliant steel frame. I'm hesitant to mess with that, even at the fork.

3) I've come to have a begrudging respect for cantilever brakes. And I believe that I can make them safe with an improved setup and proper maintenance. When I was a kid neither I nor my parents was really paying any attention to the maintenance of my bikes.

Surprisingly, even switching to V-brakes is not an option without a fork replacement. The cantilever brake post spacing on the vintage bikes is very narrow by modern standards. 2.5" vs a more common 4"+. That doesn't sound like much but it results the the V-brake arms being flared out at 45 degrees and the angles of everything being all wrong with respect to leverage and shoe contact angle. The folks on the bike forums suck at physics on average but in this they gave me good advice: the brakes need to stay cantilevers.

The pic below is somebody else's attempt at the V-brake. Even at that, you need a very particular model to get it done at all.

 

[XR250]

Can't say that I have ever run into the shudder on any of my bikes but they all have 1 1/8" steerer tubes.I did have a lot of squealing problems on my 1994 bike. I fixed that by replacing the brake arms as mine were very worn out. I tried to put V's on instead but ran into the same issue you had - the post spacing was off. They worked in the front, however, so I have V in the front and canti in the back. My mountain bike has disk brakes and it is a game changer.

 

[KootK]

As a novice cyclist I haven't really taken the time to consider parts of my bike in a rigorous engineering sense.

If anything, that makes your input here more valuable to me. Like anything, bike mechanics is saddled with it's own, established dogma that can be an impediment to fresh thinking.

Unless it's coming from imperfections in the rim and minor out-of-roundness.

Thanks for that suggestion. I hadn't considered the wheel being out of true as contributing to the problem. While I'm not convinced that it is the problem, I'm also unable to rule it out logically. Happily, that's something that I can test easily. When spring finally arrives here, I'll swap front wheels with my precision road bike and see if that makes a difference. Two things mitigate this possibility in my opinion:

1) I true my own wheels on a semi-pro truing stand. My front wheel is within 2mm of true everywhere and closer to 1mm most places.

2) One of the perceived advantages of the cantilever brake setups was that the straddle wire can slide left to right, allowing one side of the braking system to come into contact with the rim before the other side does with out it being a problem. It's sort of self equibrillating. That said, at a very high wheel RPM, that may not happen fast enough to avoid a "grab" problem.

That makes me wonder about your number 2...it seems the greater mechanical advantage offered by V-brakes would help to overcome that.

In a way, I'm thinking along similar lines. I agree with compositepro, human909, and dold that this absolutely is a stick and slip problem. But the larger questions is why it's a stick and slip problem for this particular setup. I view the braking mechanism as similar to concrete shear friction in that, along with the shear resistance, there is always a propensity to kick the joint apart. Here, that means throwing the pads off of the rim momentarily. I speculate that the greater mechanical advantage of V-brakes tends to keep the brake pad in proper contact with the rim further into the braking load history. It's like a kick-ass clamping for in shear friction.

 

[jayrod12]

Stalking does fit his profile. Watch out for some guy in yellow crocks next time you're out for a ride.

Whoa whoa whoa, you think he's going out in his dress crocs stalking? Come on, you should know that's when he wears the camo coloured crocs.

 

[KootK]

Stalking does fit his profile. Watch out for some guy in yellow crocks next time you're out for a ride.

You're right, it does fit my profile. On this forum, I mostly only get to know folks by way of their ideas. At some point, when I've found enough of someone's ideas interesting, I start to want to know more about their background in order to better understand their ideas. Where are they from, what kind of work do they do, how old are they, do they have kids... I find that, over time, I can decipher a fair bit by just paying attention.

 

[dauwerda]

human909 and dold seem to have a very plausible theory in my opinion. I don't buy the "bow and arrow" theory at all.

 

[KootK]

A big component of the bow string and slip/ stick discussion is the flexibility of the fork steerer, which has not been considered in this thread.

Can't say that I have ever run into the shudder on any of my bikes but they all have 1 1/8" steerer tubes.

While I'm grateful for the suggestions on the steerer tube, I'm pretty set against that as a potential source of the problem. Here's why:

1) I suspect that the stiffness of my 1" steel steerer tube is actually greater than that of your average 1.125" aluminum steerer tube once the difference in elastic moduli is considered. The thing is basically a plumbing pipe of old.

2) More than anything, I can say with considerable confidence that caliper brakes do not produce the shudder problem. A picture of one of my high end vintage calipers is shown below. As shown in the sketch, I believe that caliper brakes and cantilever brakes both exert identical forces to the steerer tube. But only the cantilever brakes exhibit the shudder. That leads me to believe that the steerer tube is not the difference.

It was in examining the caliper brake below that I came to my "aha" moment on this. The same forces shown in red that would torque the forks on a cantilever setup are also present in the caliper setup. The big difference, I feel, is that the caliper setup rectifies the torsion within the brake assembly itself rather than passing it through the fork legs. That's how I've come to believe that torsion occurring between the brake pads and the fork is the culprit.

 

[KootK]

I'd be happy to hear the elaboration.

It's bascially the situation shown below with braking applied only at the rear. It's an unimpressive endo of sorts where your back wheel never leaves the ground. The reduction in contact pressure between the back wheel and the ground means that any serious braking will result in the back wheel breaking into a skid and all of the braking power coming from the front wheel.

Might also help me understand the dynamics going on it KootK's problem a little better.

It certainly helps me understand it better. I think that a logical question to ask here is this: "why is shudder always a front brake phenomenon and never a rear brake phenomenon?". The proponents of the bow theory would say that it is because the fork is flexy and the rear brake is mounted to a stiff, triangulated frame. I don't buy that because I don't buy the bow theory. Rather, I feel that shudder doesn't happen with rear braking simply because the physics of it make it such that you never really do any serious rear breaking. You know, at least not unless you get shot out of a cannon up a 45 degree slope and try to stop in a hurry.

In fact, if I had the ovaries required to ride my bike backwards at 40 km/h, I speculate that I could make my rear brake produce more shudder than the front brake does. That, because the seat stay tubes that the rear brakes are attached to are of a significantly smaller diameter than are the front fork legs. Thus, the possess much less torsional stiffness.

 

[KootK]

I was referring to the 100% effectiveness of the front brake. It scared me...

I may be able to offer some meaningful help with regard to your cycling technique.

Firstly, in a lot of situations, rear braking is safer than front braking. That basically includes serious cornering or any time that you're on a surface where you might break traction at the front wheel (ice, rain, leaves, gravel). The reason is simple enough: a front wheel that is skidding sucks pretty badly at steering. And steering can be pretty important. So there's logic in rear braking. The real weakness with rear braking is that it simply won't get the job done in a high speed emergency situation when you need to stop quickly. That, because of the endo weight transfer thing that we discussed above.

Secondly, I've read some interesting stuff suggesting that our common perception of what happens when we front brake and fly over the handle bars is deeply flawed. What everybody perceives is that they hit the front brake hard and then flip over the front wheel with their bodies and their bikes traveling together as a rigid body of sorts. Apparently, what really happens is this:

1) You hit the front brake hard and your bike slows fast. Faster than your body.

2) Because your arms are poorly braced, your body flies over the handle bars separately from your bike.

3) Your bike only flips over because you drag it along with you. This is especially the case if your shoes are clipped in.

4) You and your bike wind up in a heap of pain and shame so quickly that it's almost impossible for you to accurately parse out what really happened.

The moral of the story is that, from a physics perspective, it is nearly impossible for you to flip over the front wheel, along with your bike, as a combined rigid body. You'll invariably start skidding the front wheel before that happens. So, from a technique perspective, what you need to do is to simply brace your arms as stiffly as you can before you hit the front brake. This encourages you and your bike to travel as a rigid body.

I practice this technique a few times at the beginning of each cycling season just to get reacquainted with it. I'll do it in an empty parking lot and can indeed lock up my front caliper brakes without flipping over. It's kind of fun really.

 

[EdStainless]

With rim brakes (side pull, center pull, or cants) I have always been able to fully lock the fronts.
Having a feel for that is one key to good bike handling.
I almost never use the rear brake (one of my bikes doesn't have a rear brake).
I have one bike that likes to shudder a lot when braking.
It has a very flexible fork which only makes it worse.
And it is a very steep frame (not touring at all).
I replace the pads at the beginning of each season.
Wash the rim sidewalls with solvent (after dismounting the tires).
And make sure to use a bit more toe-in on the pads than is typical.
Brake flex is part of this, and that can't be changed.
I have tried three different sets of brakes on this particular frame, and they all do it to some extent.
This is only an issue on front brakes because the brake loads on the rear are not enough to cause this to happen.
If you to-out the rear pads you can cause it to happen a little bit.
But usually when they bite it is enough to lock the rear.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[XR250]

You can def. flip a mountain bike over with disk brakes as a rigid body (squishy fork, sticky tires and a high CG). For maximum braking, they teach us to drop out heels and our CG as much as possible and put your weight back. You can then use both brakes more effectively. Having a dropper seatpost that is already down helps even more.

 

[KootK]

Yeah, mountain bikers are always very resistant to the concept. I suspect that there are many complicating factors on typical mountain bike ride where one flips over:

1) Maybe downhill bumping up and down.

2) Front wheel may catch in a rut, against a root, or something that changes the physics a bit.

3) Riding stiff armed is pretty much anathema to comfort on a mountain bike.

My hardtail is a Marlin 6 with hydraulic breaks and some sticky, Schwalbe Big Apples on the rims (basically giant road tires). I can lock that up on pavement with just the front brake without going over. The stiff arming basically encourages the weight shift to the rear.

 

[TLHS]

My instincts align with a lot of what's already been said. I had a pretty solid laugh though as I scrolled to the bottom and started seeing diagrams getting thrown out.

But now I have other opinions!

(a) Disc breaks are wonderful. I thought it was probably overblown until I started using them.

(b) Yes, general wisdom is that front braking isn't a thing you should do while cornering to avoid skids. However, it's generally an important part of the whole stopping plan in any other situation.

(c) I would amend Koot's braking technique suggestion of just bracing your arms as hard as you can. On top of that, you should lift your butt of the saddle and shift back as far as you're comfortable. Being off the saddle gives yourself the ability to play with your center of gravity and the angle of force applied to the pedals. You'll feel more stable, you'll be able to take a bit more of the braking force into your legs, your center of gravity will shift down and back. People have told me that the primary benefit of this is the shifted center of gravity. Personally, I think it's more about the fact that you're getting the angle of your legs closer to the angle of the resultant force. I picture it as standing on the pedals as that resultant moves.

 

[phamENG]

I really like the diagram dold put together.

Thanks for the cycling tips. I'll have to put them to the test next week. Now, if you see me posting between 11am and 1pm Eastern time next week - yell at me for not going riding. This whole "wake up, work, eat, work, eat, hang out at home (or work some more), sleep, repeat" thing is taking a serious toll.

Had a prof that was an avid mountain biker. He did the rigid body over the handle bars thing once. Disappeared from the university for a couple weeks, showed back up in a neck brace. Went head first into the side of a mountain. Doc said a quarter degree entrance angle up or down and he would have been dead. Instead, he hit straight on and creating a textbook compression split of one of the vertebrae in his neck. By himself, he rode the remaining 10 miles of the coarse down the mountain to finish the race and went to the hospital...

 

[KootK]

I think it's more about the fact that you're getting the angle of your legs closer to the angle of the resultant force.

Yes, and I would argue that is another version of bracing yourself to your bike to encourage rigid body motion of the two of you together. Just with your legs instead of your arms.

 

[KootK]

I really like the diagram dold put together.

Me too, especially because it saves me the effort of putting a similar diagram together myself.

I agree that the stick and slip is the front end of the problem but I feel that the lager question is why that's a particular issue with the cantilever brakes. Dold's suggestion that it's about the cable pull line of action doesn't speak to me for a couple of reasons:

1) Different models of cantilever brakes and v-brakes actually reverse the positions of the pads vs the cables.

2) I feel as though the main arms of both brake types are pretty solidly moment connected to the braze on mounting post. I don't see the cable offset forcing a rotation through that.

When you look at the diagram below, I feel that it is the transition from "clamped" to "bites" that is of most importance. And, just as dold showed it, that would seem to be a study in torsional load versus torsional stiffness. To the extent that a system lacks torsional stiffness, that gets you to the bite faster. With regard to potential flexibility, here's what I see for options.

1) Lateral flex in the brake assembly. I don't feel that this is a major contributor because:

a) I've held them in my hand and they are pretty sturdy.

b) The flex that there is feels, to me, mostly like the take up of slop. And that happens very early on in the breaking process and would be common to all of the non-disc brake styles including calipers which never produce shudder.

2) Flex in the plates of the little braze on boxes to which the cantilever post is welded. On older bikes like mine, the boxes are three sided as shown below rather than a larger diameter tube which would be better. The walls on my bike's "box" look to be about half as thick as the ones shown.

3) Flex in the wall of the fork which is very thin.

4) Twist in the fork leg which is of a fairly small diameter and thickness.

In contrast, caliper brakes eliminate all of those things but #1. And caliper brakes can deliver a very strong braking force without brake shudder.

 

[3DDave]

If you don't already own them, it's time for a Fright Harbor trip to get some inexpensive dial indicators and indicator stands to measure twist in the pad holders when the brake is applied.