Author Topic: Vector diagram for disc/rim front brake fork loads.  (Read 993 times)

Vector diagram for disc/rim front brake fork loads.
« on: September 29, 2020, 06:28:28 am »
As any fule kno disc brake forks have to be more rigid with dropouts to resist vertical loads. I was thinking about how to show that diagrammatically. Has it been done already?

zigzag

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Re: Vector diagram for disc/rim front brake fork loads.
« Reply #1 on: September 29, 2020, 09:04:12 am »
is it vertical loads or loads perpendicular to a fork leg at the brake mount and also counter clockwise twisting forces?

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #2 on: September 29, 2020, 09:56:05 am »
I did a very rough crummy diagram a while ago. I think it is roughly correct

https://photos.app.goo.gl/vyHRDiRrMtYqvEfP6

This is an attempt to show the torque
<i>Marmite slave</i>

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #3 on: September 29, 2020, 11:09:22 am »

(from cyclingtips)

shows why lawyers lips and/or forward facing dropouts are a good idea with disc brakes. Both combat the risk of wheel ejection under braking.

cheers

zigzag

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Re: Vector diagram for disc/rim front brake fork loads.
« Reply #4 on: September 29, 2020, 12:49:35 pm »
in the above diagram (drawn by a non-cyclist, by the looks of it) the wheel will want to rotate around the brake pad (during braking), so the force against the dropout will mostly be back, rather than down and back.

Tim Hall

  • I want to eat the fleeting shade of your lashes
Re: Vector diagram for disc/rim front brake fork loads.
« Reply #5 on: September 29, 2020, 01:18:34 pm »

(from cyclingtips)

shows why lawyers lips and/or forward facing dropouts are a good idea with disc brakes. Both combat the risk of wheel ejection under braking.

cheers

Does that also show why putting the caliper on the back face of the fork is a Bad idea? It looks like a caliper mounted on the front  face would direct the wheel up into the dropout. Or, as most things, is it not that simple?
There are two ways you can get exercise out of a bicycle: you can
"overhaul" it, or you can ride it.  (Jerome K Jerome)

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #6 on: September 29, 2020, 01:46:47 pm »
With the caliper on the back face, braking force is pushing the caliper into the fork.  Put it on the front, and it's pulling away from the fork, putting all the load on the 2 little bolts and the glue that holds the nuts in.

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #7 on: September 29, 2020, 01:52:21 pm »
in the above diagram (drawn by a non-cyclist, by the looks of it) the wheel will want to rotate around the brake pad (during braking), so the force against the dropout will mostly be back, rather than down and back.

The diagram is not perfect but it is the best one I have found to date. Plus one that is 'correct' would include more detail than most people can cope with.

Because the disc is 1/5th the diameter of the wheel, the forces generated by the brake are potentially about x5 larger than the other loads on the wheel. This means that they are plenty large enough to drag the wheel out of the bottom of a conventional fork, and the only thing stopping it is usually the lawyer's lips.    I don't recommend that you demonstrate this whilst riding the bike, but it has been observed to happen in reality, not just theory, and the consequences ain't pretty.    The idea that the forces are 'mostly backwards' is not correct.

cheers

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #8 on: September 29, 2020, 02:01:13 pm »

Does that also show why putting the caliper on the back face of the fork is a Bad idea? It looks like a caliper mounted on the front  face would direct the wheel up into the dropout. Or, as most things, is it not that simple?

it arguably would have been a better idea to put the caliper on the front of the fork, from the POV of wheel ejection.  But there are other issues to do with the loads on the caliper mountings, the consequences of the caliper mounting failing etc.

In practice it isn't even that good an idea to mount some calipers the 'wrong way round' on the front of the RH fork leg; oftentimes the caliper body  opening is wider going one way than the other, which means that (for example) worn pads can escape (or see abnormal loading) when the disc is going 'backwards' through the caliper.

But a major element in the choice of position must surely have been how it looks; front facing calipers on motorbikes have not been widely used in the last forty years; not since various Brembo-equipped models from BMW, Ducati and Moto Guzzi.   If they had put the calipers on the front of the forks they would have lost the irrational/subconscious appeal..."they must be good because they are like motorbike brakes"

cheers

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #9 on: September 29, 2020, 04:00:43 pm »
in the above diagram (drawn by a non-cyclist, by the looks of it) the wheel will want to rotate around the brake pad (during braking), so the force against the dropout will mostly be back, rather than down and back.

The diagram is not perfect but it is the best one I have found to date. Plus one that is 'correct' would include more detail than most people can cope with.

Because the disc is 1/5th the diameter of the wheel, the forces generated by the brake are potentially about x5 larger than the other loads on the wheel. This means that they are plenty large enough to drag the wheel out of the bottom of a conventional fork, and the only thing stopping it is usually the lawyer's lips.    I don't recommend that you demonstrate this whilst riding the bike, but it has been observed to happen in reality, not just theory, and the consequences ain't pretty.    The idea that the forces are 'mostly backwards' is not correct.

cheers

I remember seeing a video about 10-12 years ago, when disc brakes were coming in, where someone did loose their front wheel when braking on a fast descent.  At the time many people (including me after I'd seen the video!) argued that quick release + disc brakes were not compatible.

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #10 on: September 29, 2020, 04:13:20 pm »
With the caliper on the back face, braking force is pushing the caliper into the fork.  Put it on the front, and it's pulling away from the fork, putting all the load on the 2 little bolts and the glue that holds the nuts in.
The calipers on my bike (and also on cars with disk brakes) are held on by bolts that are loaded in shear. The axis of the bolts are parallel to the wheel axis, so it doesn't matter whether the caliper is being pushed towards the fork or away. The bolts are equally strong in either direction.
Quote from: Kim
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Re: Vector diagram for disc/rim front brake fork loads.
« Reply #11 on: September 29, 2020, 04:27:08 pm »
With the caliper on the back face, braking force is pushing the caliper into the fork.  Put it on the front, and it's pulling away from the fork, putting all the load on the 2 little bolts and the glue that holds the nuts in.
The calipers on my bike (and also on cars with disk brakes) are held on by bolts that are loaded in shear. The axis of the bolts are parallel to the wheel axis, so it doesn't matter whether the caliper is being pushed towards the fork or away. The bolts are equally strong in either direction.
How do your brakes mount to your fork? Flat mount just bolt in like this:

As you can see, the bolts holding the caliper onto the fork are not taking any braking load when travelling forwards. If you were to apply them while travelling backwards then you would be putting the entire braking load through the threads of the bolts.
The ones on my trike are used in shear (with an adapter onto the brake caliper itself), but that comes with it's own alignment problems and is going to be really tricky to achieve with a carbon fork. I think this is the best picture I have of that:
2016-08-24_09-04-17 by duncancmartin, on Flickr

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #12 on: September 29, 2020, 04:33:16 pm »
The force between the caliper and the disk will be circumferential as that is the direction of the sliding between the pads and the disk.

The force has to be opposed by the force between the axle and the fork.
(It also has to be opposed by the forces where the caliper meets the fork and the disk meets the hub, and the hub meets the axle, but we assume that those joints are OK)

The torque applied to the wheel adds up to nearly zero as it's not slowing down quickly. The caliper applies force at a small diameter compared to the diameter at which the road applies force, but the torques are equal and opposite. The force between the caliper and the disk is therefore large compared to the force between the wheel and the road.

As a first approximation, we can then ignore the force between the wheel and the road. Therefore the force between the axle and the fork will be in very nearly the opposite direction to the force between the caliper and the disk.

If the centre-line of the fork drop-outs point to the middle of the caliper, the braking force will be neutral in terms of forcing the axle out of.

Most bikes have the drop-outs near vertical, and the caliper behind the fork, so the braking force tries to push the axle out of the dropouts.
Quote from: Kim
Paging Diver300.  Diver300 to the GSM Trimphone, please...

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #13 on: September 29, 2020, 04:35:25 pm »
Place the caliper to the front of the fork, but mount it on a bracket which passes round to the back of the fork and bolts in there.

This way you're not relying on just the threads to keep the caliper in place and the rotational forces wouldn't eject the wheel.

Possible issue with flexibility and weight though.
Rust never sleeps

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #14 on: September 29, 2020, 04:35:50 pm »
in the above diagram (drawn by a non-cyclist, by the looks of it) the wheel will want to rotate around the brake pad (during braking), so the force against the dropout will mostly be back, rather than down and back.

The diagram is not perfect but it is the best one I have found to date. Plus one that is 'correct' would include more detail than most people can cope with.

Because the disc is 1/5th the diameter of the wheel, the forces generated by the brake are potentially about x5 larger than the other loads on the wheel. This means that they are plenty large enough to drag the wheel out of the bottom of a conventional fork, and the only thing stopping it is usually the lawyer's lips.    I don't recommend that you demonstrate this whilst riding the bike, but it has been observed to happen in reality, not just theory, and the consequences ain't pretty.    The idea that the forces are 'mostly backwards' is not correct.

cheers

I remember seeing a video about 10-12 years ago, when disc brakes were coming in, where someone did loose their front wheel when braking on a fast descent.  At the time many people (including me after I'd seen the video!) argued that quick release + disc brakes were not compatible.

Disc brakes with QR were around somewhat longer ago than 10-12 years ago. Newish for road bikes maybe but been on mtn bikes for at least another 10 years on top of that.

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #15 on: September 29, 2020, 05:32:44 pm »
With the caliper on the back face, braking force is pushing the caliper into the fork.  Put it on the front, and it's pulling away from the fork, putting all the load on the 2 little bolts and the glue that holds the nuts in.
The calipers on my bike (and also on cars with disk brakes) are held on by bolts that are loaded in shear. The axis of the bolts are parallel to the wheel axis, so it doesn't matter whether the caliper is being pushed towards the fork or away. The bolts are equally strong in either direction.
How do your brakes mount to your fork? Flat mount just bolt in like this:

As you can see, the bolts holding the caliper onto the fork are not taking any braking load when travelling forwards. If you were to apply them while travelling backwards then you would be putting the entire braking load through the threads of the bolts.
The ones on my trike are used in shear (with an adapter onto the brake caliper itself), but that comes with it's own alignment problems and is going to be really tricky to achieve with a carbon fork. I think this is the best picture I have of that:
2016-08-24_09-04-17 by duncancmartin, on Flickr

Although mine are suspension forks, and the caliper mounting flange is cast into the fork, the bolt direction is this:-
http://www.bobjacksoncycles.co.uk/product/reynolds-853-disc-brake-front-fork/

I agree that alignment is a fiddle. Mine has a selection of washers that pack out around 1 mm between the caliper and the flange.

Mine are quite old, and I think that many have moved on to flat mount.

However, on flat mount, the bolts should still be strong enough to take the braking force. Bolts are supposed to be tightened to the point where there is always tension in the bolt, to reduce the likelihood of the bolts coming loose and fatigue failures.

It's not a big deal. The worst grade of M5 steel bolts is good for about 1/2 tonne in tension (https://www.engineeringtoolbox.com/metric-bolts-minimum-ultimate-tensile-proof-loads-d_2026.html) so a couple of those will hold the caliper on fine.

Actually, I think that the flat mount calipers can still put the bolts in tension when braking is applied. The mounting face of the caliper is a couple of cm to the left of the disk, so there is a torque trying to twist the caliper around a vertical axis. That is being resisted by the quite narrow mounting face of the caliper, which will apply more pressure nearer the disk and less pressure further away from the disk. The tension in the bolts will stop it turning in that direction.

I don't think it's a problem. Larger bosses could easily be used if it was an issue. But I also don't believe that it would be a problem to put the caliper in front of the disk, where the twisting forces would be exactly the same, so the strain on the bosses would be very similar to what they are now. I think that the calipers are behind the forks to keep them out of the way.
Quote from: Kim
Paging Diver300.  Diver300 to the GSM Trimphone, please...

FifeingEejit

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Re: Vector diagram for disc/rim front brake fork loads.
« Reply #16 on: September 29, 2020, 05:51:27 pm »
it arguably would have been a better idea to put the caliper on the front of the fork, from the POV of wheel ejection.  But there are other issues to do with the loads on the caliper mountings, the consequences of the caliper mounting failing etc.

In practice it isn't even that good an idea to mount some calipers the 'wrong way round' on the front of the RH fork leg; oftentimes the caliper body  opening is wider going one way than the other, which means that (for example) worn pads can escape (or see abnormal loading) when the disc is going 'backwards' through the caliper.

Some manufacturers did do that though

I remember seeing a video about 10-12 years ago, when disc brakes were coming in, where someone did loose their front wheel when braking on a fast descent.  At the time many people (including me after I'd seen the video!) argued that quick release + disc brakes were not compatible.

Despite having been standard equipment in MTBing for a while and particularly downhill where huge 200mm rotors were common already.
Thru Axles were invented around about then though.


An IS caliper would be mounted in Shear; but post-mount won the day in MTB forks and that fed into Flat mount.

The wheels continuing rotation is pushing the pads into the caliper and so the caliper into the fork, there's a bit of a haul back when you come to a halt, you can usually hear when you've got loose bolts because of that...


zigzag

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Re: Vector diagram for disc/rim front brake fork loads.
« Reply #17 on: September 29, 2020, 05:53:26 pm »
in the above diagram (drawn by a non-cyclist, by the looks of it) the wheel will want to rotate around the brake pad (during braking), so the force against the dropout will mostly be back, rather than down and back.

The diagram is not perfect but it is the best one I have found to date. Plus one that is 'correct' would include more detail than most people can cope with.

Because the disc is 1/5th the diameter of the wheel, the forces generated by the brake are potentially about x5 larger than the other loads on the wheel. This means that they are plenty large enough to drag the wheel out of the bottom of a conventional fork, and the only thing stopping it is usually the lawyer's lips.    I don't recommend that you demonstrate this whilst riding the bike, but it has been observed to happen in reality, not just theory, and the consequences ain't pretty.    The idea that the forces are 'mostly backwards' is not correct.

cheers

I remember seeing a video about 10-12 years ago, when disc brakes were coming in, where someone did loose their front wheel when braking on a fast descent.  At the time many people (including me after I'd seen the video!) argued that quick release + disc brakes were not compatible.

my guess is that the skewer has not been done up properly, or a poor quality skewer or some other similar "fault" (or a combination of). if that was a real issue, we would hear about it more often, as there were millions(?) such bikes made and sold in the recent decades. my gravel/winter bike has such forks too and the axle never budged - offroad (e.g. 700k bikepacking race), alpine descents (e.g. alpi4000, 6+6 isole audaxes), super steep roads (e.g. on tinat audax), emergency braking in london. no rubbing, squeaks or other untoward behaviour, pretty much flawless.

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #18 on: September 29, 2020, 07:07:08 pm »

my guess is that the skewer has not been done up properly, or a poor quality skewer or some other similar "fault" (or a combination of). if that was a real issue, we would hear about it more often...

IMHO the main reason why it is not a daily occurrence with QR/disc brake wheels  is that forks are fitted with lawyers lips.  Folk often rely on these without even knowing it.

cheers

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #19 on: September 29, 2020, 07:11:06 pm »
In practice it isn't even that good an idea to mount some calipers the 'wrong way round' on the front of the RH fork leg; oftentimes the caliper body  opening is wider going one way than the other, which means that (for example) worn pads can escape (or see abnormal loading) when the disc is going 'backwards' through the caliper.

Some manufacturers did do that though...

they probably chose their caliper with care, and if you plan to do likewise, so should you.

cheers

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #20 on: September 29, 2020, 07:19:48 pm »

my guess is that the skewer has not been done up properly, or a poor quality skewer or some other similar "fault" (or a combination of). if that was a real issue, we would hear about it more often...

IMHO the main reason why it is not a daily occurrence with QR/disc brake wheels  is that forks are fitted with lawyers lips.  Folk often rely on these without even knowing it.

cheers

I’m inclined to agree with this. I have had a rear wheel twist in the dropouts when climbing in a low gear, as a consequence of a skewer that was unable to provide sufficient load for the titanium dropouts on that frame. Replaced with Shimano skewers and all was fine, but disc brakes can (I suspect) generate considerably more force at the dropout than I can through the pedals.

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #21 on: September 30, 2020, 06:47:54 am »
Hope's original disc brake was mounted in front of the fork leg.

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #22 on: September 30, 2020, 09:22:49 am »

Disc brakes with QR were around somewhat longer ago than 10-12 years ago. Newish for road bikes maybe but been on mtn bikes for at least another 10 years on top of that.

It may have been longer ago, and the video might not have been new at the time.  But the content certainly stuck in my memory!

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #23 on: September 30, 2020, 10:08:06 am »
Discs on mtb started to become popular in the late nineties, Julie's year 2000 F700 cannondale is still on the road, headshok is going to be fixed today (slowly loosing air) . Seals and Magura shock oil, looked after they refuse to die. Never had an issue with the front wheel.

Re: Vector diagram for disc/rim front brake fork loads.
« Reply #24 on: September 30, 2020, 10:18:55 am »
Isn't this problem is why closed dropouts and through axles were invented?