Spoke tension underbraking..

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matata
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Spoke tension underbraking..

Postby matata » 12 Feb 2018, 4:11am

I need to settle a question raised in the pub... When under braking with the same rate of retardation, is the spoke tension greater, less or equal comparing rim brakes and disc brakes and why? I need a techy physics answer not just anecdotal stories please! Nik

Samuel D
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Re: Spoke tension underbraking..

Postby Samuel D » 12 Feb 2018, 9:27am

The main difference in the two cases is that torque must be transmitted between hub and rim with disc brakes whereas that is not necessary with rim brakes.

Since spokes are flexible wire and anyway free to pivot at the hub, their only way to transmit force is by changes in tension. Under braking load, the pulling spokes in the disc-brake wheel increase in tension while the pushing spokes decrease in tension by the same amount (the same because the distance between pulling and pushing spokes does not change at the hub or rim, so a given hub wind-up causes equal tension change in all spokes except for sign). Therefore the average spoke tension remains the same, and also the same as in the rim-brake wheel without braking torque. All the same, the disc brake stresses its spokes and rim more.

There is another consideration that might be beyond the intended scope of the question: under braking, all of the spokes dynamically change tension from the road pushing back at the bottom of the wheel and the rider pushing forward at the hub while the wheel spins. With a rim brake, forces are also transmitted to the top of the wheel. I will not attempt to describe the resulting changes in spoke tension except to say that they are complex and small.

hamster
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Re: Spoke tension underbraking..

Postby hamster » 12 Feb 2018, 9:49am

In short, a disc-braked wheel under braking is like a back wheel. We all know that spoke loadings are worse for back wheels - try an all-radial-spoked rear wheel for example!

Airsporter1st
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Re: Spoke tension underbraking..

Postby Airsporter1st » 12 Feb 2018, 10:06am

What I have difficulty with is understanding why there should be any difference between discs and rim brakes.

In the fact that rim brakes effectively stop the part in contact with the road, i.e. the tyre, still puts tension on the spokes because the braking force is transmitted through the spokes to the frame. In the case of discs, they are effectively fixed to the frame and thus braking force is transmitted through the spokes to the rim/tyre.

To me its all the same, but I am not daft enough to believe that's the case. I would therefore love to see a clear and concise explanation of the differences.

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andrew_s
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Re: Spoke tension underbraking..

Postby andrew_s » 12 Feb 2018, 10:27am

Airsporter1st wrote:What I have difficulty with is understanding why there should be any difference between discs and rim brakes.

With disc brakes, the load is transferred from the contact patch to the rim, all round the rim, down (all) the spokes, out to the disc, through the brake, and up the fork blades to the headset.
With rim brakes, the load is transferred from the contact patch to the rim, round the rim to the brake calipers, and from the brake to the fork crown & headset.

Brucey
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Re: Spoke tension underbraking..

Postby Brucey » 12 Feb 2018, 10:37am

disc brakes do indeed create very high stresses in the spokes when the brakes are used hard. These do vary but are always larger than the stresses in otherwise similar rim-braked wheels. Depending on the design of the front hub the torque is or isn't shared appreciably between the flanges, and depending on the size of the flanges the net change in leading tension vs trailing tension can be large (small flanges) or smaller (large flanges).

Disc braked front wheels are dished, too, which makes them quite a bit weaker than rim braked wheels. There are also much greater loads on the disc-braked hub bearings than is 'normal' with a rim braked wheel.

In addition to the torque loading there is (because of weight transfer) an increased thrust loading on the wheel when the brakes are on whether rim or disc braked. To a first approximation this can (depending on rim stiffness and a few other things) cause a ~25 to 50% reduction in tension in those spokes below the hub at any one time, eg in a rim braked wheel (with a ~100kg rider braking as hard as possible) ~four spokes might each lose 25kg tension and this creates a net upthrust of ~100kg.

If you take a 'worst case' for disc brakes (which is in fact by no means uncommon) then you might have a 28 spoke front wheel with medium-sized flanges and poor torque sharing between them. This can leave just seven spokes on the left side of the wheel that see a tension increase because of the brake torque loads, of which at least one will be running almost slack anyway because of the thrust load (which isn't shared with other spokes in the same way as for a rim-braked wheel). This leaves just six leading spokes doing all the work, basically. These spokes could see tension differentials of ~170kg (or more) vs the trailing ones on the same flange, i.e. if those spokes started at 120kg tension, then trailing and leading spokes could see tensions of ~35kg and ~205kg respectively. The slack spokes will be in danger of simply having the nipples unscrew and the tighter ones are about 2/3 of the way towards yielding in tension; they will undoubtedly accrue significant fatigue damage whenever the brakes are used in this way.

Under the same braking loads there can be a thrust load at the disc caliper of about 500kg. The fork needs to be considerably stiff and stronger in order to safely withstand this load. This load is also reacted through the hub bearings in (vector) addition to the thrust loads that would be present anyway. The net result of this is that a LH disc hub bearing will see a thrust load of at least 500kg under heavy braking, in contrast to a rim brake hub which sees a load increase of ~100kg in each front hub bearing under the same braking loads. 500kg is at or beyond the relevant (static) load rating of many front hub bearings.

In a nutshell the loading in (all parts of) a disc braked wheel and fork is considerably more aggressive than in a rim-braked wheel at the same braking loads. That this doesn't more often cause big trouble is, I suspect, largely because heavy braking does not occupy a very large proportion of the duty cycle of the wheel.

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foxyrider
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Re: Spoke tension underbraking..

Postby foxyrider » 12 Feb 2018, 11:36am

Or more likely Brucey, sheer luck!
Convention? what's that then?
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[XAP]Bob
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Re: Spoke tension underbraking..

Postby [XAP]Bob » 12 Feb 2018, 4:17pm

Airsporter1st wrote:What I have difficulty with is understanding why there should be any difference between discs and rim brakes.

In the fact that rim brakes effectively stop the part in contact with the road, i.e. the tyre, still puts tension on the spokes because the braking force is transmitted through the spokes to the frame. In the case of discs, they are effectively fixed to the frame and thus braking force is transmitted through the spokes to the rim/tyre.

To me its all the same, but I am not daft enough to believe that's the case. I would therefore love to see a clear and concise explanation of the differences.


Because the spoke tension to move the frame about isn't signficant.
The tension required to stop the wheel spiralling into itself is rather high.

A rim braked wheel can be radially spoked - there is no force trying to twist the hub relative to the rim.

A disc/hub braked wheel however has a torque between the rim and the hub - this is what must be countered, and thus requires a cross spoking pattern
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.

fastpedaller
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Re: Spoke tension underbraking..

Postby fastpedaller » 12 Feb 2018, 5:14pm

They probably don't give problems (due to spoke tension) because we don't use the brakes much - Well I don't use them any more than I absolutely have to :lol: which I replicate in the car as well - 50 mpg in mixed driving during the summer (petrol) and I have to stamp on them sometimes to make sure everything doesn't sieze up :roll:

matata
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Re: Spoke tension underbraking..

Postby matata » 12 Feb 2018, 5:19pm

Bob, I take your point about cross spoking/radial but that is because disc brakes can exert a greater breaking force than rim brakes and have to redesigned to cope with the greater breaking. But what if the braking force is the same on both cyclists, say on a gentle braking?. Anybody able to do a convincing diagram to help explain??

Brucey
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Re: Spoke tension underbraking..

Postby Brucey » 12 Feb 2018, 6:10pm

matata wrote:..... but that is because disc brakes can exert a greater breaking force than rim brakes....


no, that is not the case. The braking power is limited on a typical bicycle by the fact that the rear wheel lifts and then you go over the handlebars. Good rim brakes will do this.

You would need at least a basic understanding of how forces act on the bike in order to understand a diagram; I don't mean this unkindly, but from your comments I'm not sure you have that.

cheers
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531colin
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Re: Spoke tension underbraking..

Postby 531colin » 12 Feb 2018, 6:49pm

Brandt has some stuff about spoke tension changes under braking from rim brakes (only)

BigG
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Re: Spoke tension underbraking..

Postby BigG » 12 Feb 2018, 8:19pm

Airsporter1st wrote:What I have difficulty with is understanding why there should be any difference between discs and rim brakes.

In the fact that rim brakes effectively stop the part in contact with the road, i.e. the tyre, still puts tension on the spokes because the braking force is transmitted through the spokes to the frame. In the case of discs, they are effectively fixed to the frame and thus braking force is transmitted through the spokes to the rim/tyre.

To me its all the same, but I am not daft enough to believe that's the case. I would therefore love to see a clear and concise explanation of the differences.

With a rim brake, most of the force is transmitted from the rim to the frame directly via the brake arms and brake mounting point, not through the spokes and hub. Because the braking point is about the same distance from the hub as the tyre/road contact, both apply a similar torque to the wheel and only a small balancing torque is transmitted through the spokes. It may help to think of the situation in static terms with the wheel simplified to a beam running from the hub to the tyre edge. The moment (torque) generated about the hub by the tyre is opposed to that generated by the brake block. These moments are almost balanced because they are at almost the same radius. Only a small residual torque is needed to be supplied by the hub through the spokes. The situation with hub brakes is reversed. Here the friction point is close to the hub axis and a very much larger braking force is required to transmit the required torque to the wheel rim. This is of course transmitted through the spokes.

Brucey
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Re: Spoke tension underbraking..

Postby Brucey » 12 Feb 2018, 9:05pm

with a rim brake there is no 'balancing torque transmitted through the spokes' ; for there to be any such thing there would have to be something resisting hub rotation, and (asides from a little bearing friction I suppose) there is no such thing.
Statically, the moments in a rim brake do sum to zero about the hub, but because the loads are not equal and opposite in magnitude and/or direction, there are net thrusts generated on the hub bearings, and resultant changes in spoke tension.

For several reasons these spoke tension variations are both smaller and better shared in a rim braked wheel than in a disc braked wheel.

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[XAP]Bob
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Re: Spoke tension underbraking..

Postby [XAP]Bob » 12 Feb 2018, 9:41pm

matata wrote:Bob, I take your point about cross spoking/radial but that is because disc brakes can exert a greater breaking force than rim brakes and have to redesigned to cope with the greater breaking. But what if the braking force is the same on both cyclists, say on a gentle braking?. Anybody able to do a convincing diagram to help explain??

Wrong and irrelevant.

Consider the hub of the wheel rather than the rim. Is there anything in a rim brake which tries to rotate the hub? Nothing, it’s free rotating.

On a hub braked bike (disc/roller/hub/???) the hub sees a torque relative to the rim.
Yes, that today grows as braking force increases, but the rim brakes have zero force at any braking force.
As said above - the limit will inevitably be either tyre traction or balance, the forces are proportional up to that point (with the hub torque coefficient for a rim brake being zero)
A shortcut has to be a challenge, otherwise it would just be the way. No situation is so dire that panic cannot make it worse.
There are two kinds of people in this world: those can extrapolate from incomplete data.