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Scary spoke setup on a disc brake wheel?
Call me a fusspot but wondered if anyone else has thought about this: Knowing that with disc brakes, it's all down to the spokes, to carry the braking forces from the rotor/hub out to the rim, I took a look at my giant defy's 20 spoke front disc brake wheel. It's radially spoked on the non rotor side and cross laced on the rotor side. I feel sure that for disc braking, radial spokes are a just a little bit better than a hairnet in a rainstorm, as all they'd do is have a vague and spongy effect. So on my wheel, of the 10 remaining, crossed laced spokes we're down to just the 5 forward facing spokes that I can count on for braking. Ie with the bike in the garage, when I push the bike forwards against the brake, the forward spokes go up in pitch when played like a harp whereas the back facing spokes go down in pitch. So when I'm just about to brake on a nasty downhill, should I be scared, or am I just paranoid? For the record, the rear wheel has no radial spokes, and I'd argue, why for the sake of a few saved grams, weaken things with radials on the front, where to my mind the major braking forces are. All comments and reassurances highly appreciated!
If you're new please join in and if you have questions pop them below and the forum regulars will answer as best we can.
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18 comments
radial vs crossed spoking on different sides of the same wheel is an attempt to equalise spoke tension given the differences in spoke length and rim/flange interface angle caused by dishing. this is because a disc brake takes up hub width in much the same way a freehub body does.
nothing to do with one side transmitting torque.
The standard wheels on my Giant Revolt 1 came like this. That wasn't the reason I ditched them though - I managed to wear the camembert-based hubs out (back & front) in about 5000km (I am 1.96m & 90kg after all)... The hub/spoke/rim geometry remained intact and true despite my heavy braking on descents - the internals of the hubs, not so much.
Ride it like you stole it and replace the knackered bits when you need to.
Yes WB my brain is hurting even more, now that you've got me thinking, does it matter where on the rim the brake shoes actually are. Forgetting for a while then about disc brakes, I'm thinking of this:
1) Imagine a hub hanging from a tree, by each side of its axle...then imagine someone grabbing the wheel and hanging from the 3 o'clock and 9 o clock positions. That to me would be like the brake going on in use, the two positions now being the ground and the brake at the top of the fork.
In both cases the wheel is being pushed/pulled in a particular direction.
In the tree hanging case I'd be picturing the spokes below the hub coming under greater tension in stopping the wheel from being pulled downwards, and the ones going upward, under less tension.
So this would mean that for rim brakes, assuming the brake is right at the rim top, it's 50% of spokes that act in braking, all of the ones behind the hub.
2) Now I'm considering again what you said, does it matter where (on the rim) the brakes are:
I'm now imagining the complete opposite to 1) ie say it were possible to have the brake blocks where the ground is, e.g. at the end of a bar going down from the frame.
To me this would be like again in the garage, having one person gripping the rim and pulling one way, whilst another person, gripping at the same point and pulling the other way. The net result, nothing..no spokes involved.
3) Back to disc brakes in the garage. Imagine the hub somehow in a vice but where someone could try to rotate the wheel. Imagine the World's Strongest Man taking the wheel and turning it. Something would happen...Before the wheel ended up looking like a crashed daddy long legs, the spokes facing away from direction of rotation would slacken, the radial spokes would do nothing but the the ones facing direction of rotation would tighten. Only these spokes would be doing anything.
So to my mind, it does make a difference to spoke action, as to where on a rim the brakes are, and also whether braking is at the rim or at the hub.
Right, my brain is resting now......
Thanks for all the comments. Yes, I do agree that if there ever was going to be a problem then with all those wheels out in use already, it would have happened by now.
I just can't help myself, the nerd coming out in me!
It was really more of a mind exercise. The life long engineer in me, delving into the workings.
Yes Welshboy I do agree that with rim brakes as well, it's still the spokes taking the braking load.
But still, when I visualise it there.....in the garage again pushing the bike against the brake, I can see that it's the top of the rim (brake) and the bottom of the rim (ground) that are jointly pushing the wheel backwards against the hub. So to me, all the spokes that lie behind the hub are being stretched, albeit to differing amounts depending on angle, and all those in front are being pushed. So at any split second, I visualise at least half the spokes on rim brakes in use under braking, ie the ones being stretched.
But with a disc brake doing the same thing...to me it seems pure rotational force. When I push the bike against the brake, the radials don't change pitch at all, the back facing crossed spokes go down in pitch and the forward ones go up.
So that is confirming to me that only the forward facing crossed spokes are doing anything on a disc brake wheel when slowing it down.
I do agree though that it must be ok and so it just confirms another marvel of modern engineering and materials I guess, and I am actually fan of discs, which is why I have the Defy.
Someone please come and rescue me out of this hole!
Dave, i am playing the same mind games and i am thinking of the Venge with the back brake fitted on the rear of the seat tube or manu other bikes with botton bracket mounted brakes. Does this positioning put different loading on a wheel compared with a conventionally mounted brake?
My brain hurts!
In Roger Musson's wheel building book it says it is fine to have radial lacing on the side of the wheel that doesn't transmit torque.
Your wheel should be okay, based on this.
(You cannot transmit torque through the hub body, its way too weak for that!)
Unless the hub twists when you pedal, surly both flanges (ie sides of the wheel) transmit torque equally.
I think there is a lot of pseudo science and "my mate says..." stuff being spouted about this topic.
@Welsh Boy, not only that, but it's all rotational, too. Imagine what force has to go through that 18 radially spoked wheel to lift your rear wheel off the ground under braking.
Exactly the same force as lifting my back wheel with a rim brake. Which my 18 spoke radial wheel can deal with.
"Knowing that with disc brakes, it's all down to the spokes, to carry the braking forces from the rotor/hub out to the rim"
How is that any different from a rim brake? With a rim brake the mass of the bike is connected to the hub by spokes so this too means that the spokes carry all the braking force, they must carry all the braking force because there is nothing else between the hub and rim.
The forces involved in rim braking don't involve transmitting torque from/to the hub. Yes to other forces, and the hub flange needs to be designed (read stronger) for a radial spoke pattern...
[/quote] The forces involved in rim braking don't involve transmitting torque from/to the hub. Yes to other forces, and the hub flange needs to be designed (read stronger) for a radial spoke pattern...[/quote]
Surely braking involves stopping a rotational force (rim and tyre in contact with the road) to which the mass of the bike/rider is attached by means of spokes. If a disc brake doesnt do that please explain to me how it does stop my 70kg mass from moving forwards.
i am not being funny, i really cant see how stopping a (spoked) wheel rotating does not rely on the spokes to do transmit the same forces.
The forces involved in rim braking don't involve transmitting torque from/to the hub. Yes to other forces, and the hub flange needs to be designed (read stronger) for a radial spoke pattern...[/quote]
Surely braking involves stopping a rotational force (rim and tyre in contact with the road) to which the mass of the bike/rider is attached by means of spokes. If a disc brake doesnt do that please explain to me how it does stop my 70kg mass from moving forwards.
i am not being funny, i really cant see how stopping a (spoked) wheel rotating does not rely on the spokes to do transmit the same forces.
[/quote]
I believe the difference with rim brakes is that the braking forces are transmitted through the rim (which should easily be strong enough) and then through the brake blockes to the front fork. The spokes shouldn't have anything to do with it.
If you like, a rim-braked wheel could be built without a hub or spokes (e.g. http://bicycledesign.net/2010/03/a-hubless-wheel-from-the-past-the-black-hole/) and the forces would be the same. However, as you know, having the braking transferred to the hub (and brake disc) would necessitate the forces going through the spokes. I would guess that the manufacturer would figure that into the design, so it depends on whether you trust your wheel builder or not.
If I were you, I'd go for a different wheel so that you can feel confident. It's tricky to push yourself if you don't have faith in your steed even if everyone else has had no problems with disk brakes and fewer spokes.
That makes sense, it is the best explanation i have heard yet, thanks.
Stephen
Not heard of any giant defy front wheels exploding would have thought that would have been all over the internet seeing as Giant are one of the most popular bike brands in the world. If you really are worried about it take it to your LBS
I would suggest you buy a different wheel with more spokes or build one to your specifications to satisfy your worry. There is no reason to have a concern when options are available
In theory you can't have a radially-spoked drive side rear wheel because of the pedalling forces but I've seen it done plenty of times. You just need to use a specialh in that can transfer the stresses.
It'll be the same here, the wheel will work together as a unit rather than a couple of spokes.
Others may have heard different, but I am not aware of any reports of collapsing front wheels on disc brake equipped bikes. It's not even on the standard list of make believe problems that the disc haters trot out every time there's a discussion about them.
Not to say that any wheel cannot fail under extreme circumstances or manufacturing defect, but basically you would be safe to assume that the physics is sound, that the design engineers understood what they were doing and that the manufacturer produced a safe wheel of good quality.
Even if the wheel does collapse, you (or your next of kin) will be filthy rich from the product liability claim, so stop fretting.