crossed spoking vs radial, rear wheel for rim brake

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Bulder
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Joined: 9 Feb 2019, 11:43am

crossed spoking vs radial, rear wheel for rim brake

Postby Bulder » 9 Feb 2019, 2:48pm

I am looking for some new road bike, rim brake wheels. During my research I've seen brands with crossed spoking in both sides of the rear wheel. But also some with crossed in one side and radial in the other. Most common among the latter (I think) is crossed spoking in the drive side and radial in the left. Ex Vision Trimax 30, Fulcrum R3 (and Zonda). But one - Mavic Ksyrium elite - is opposite. Radial in the drive side and crossed in the left. Why has Mavic done this? Are they stronger/weaker or doesn't it matter?

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531colin
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Joined: 4 Dec 2009, 6:56pm
Location: North Yorkshire

Re: crossed spoking vs radial, rear wheel for rim brake

Postby 531colin » 10 Feb 2019, 6:11pm

Making one set of spokes radial is a great way of making it look like somebody thought about it, and it costs the manufacturer nothing.
However, this falls over when a customer notices that 2 people "thought about it" and came up with different answers.
A rim brake rear wheel has no brake torque to deal with, only drive torque. So the driveside spokes crossed "looks right" doesn't it?
Look at a rim brake front wheel. You can see an isosceles triangle made up of the hub and the spokes as they join the rim. The "bracing angle" is the angle between the hub and the spoke, and this is all that supports the rim laterally.
Now look at your rear wheel, and the driveside spokes are almost vertical, so the bracing angle is poor and the rim has little lateral support from one side. You can build wheels like this that work, but its so finely balanced that its very much more difficult to build properly durable 11 speed wheels than 10 speed. The marketing guys didn't tell you that, did they? That "just one more gear" dumps a whole load of trouble in the wheelbuilder's lap.
Unless there are twice as many spokes on the driveside, the tension on the driveside had to be much more than the other side. So much more that you have some basic choices....run the driveside spokes so tight you risk the rim cracking, or run the other side so slack you have to threadlock the nipples, or use a rim with the spoke holes offset, or work out some other way of improving the driveside bracing angle.
That's what the guys with the radial driveside spokes are trying to do: the spoke comes right off the top of the flange, not from a third of the way down as crossed spokes do, and that gives a bit of help for the bracing angle, but more help for the bracing angle comes from running all the radial spokes on the outside of the flange.....its a couple of millimetres, but we are in a place where a couple of millimetres differentiates success from failure.
So my summary would be....go with the radial driveside spokes, just as long as they are on the outside of the flange, and just as long as the hub has a big stiff barrel to transmit the drive torque from the cassette on the right to the crossed spokes on the left....or an offset rim.

Now you are going to think me impossibly old-fashioned, but I have ridden with a bloke who bust a spoke in a factory set of reduced spoke count wheels just as we were climbing out of the top of Swaledale. I'm a wheelbuilder, and I got him going again, and he rode it to Kirkby Stephen and back to the cars in Reeth. If I had not been there, it would have been a bit of a walk just to get a mobile signal. You don't have to be in Outer Mongolia for a simple failure to be a royal pain. A properly-built 36 spoke wheel with a big stiff rim is much more durable, so that the spokes should outlast a few rims as the brake blocks wear the metal away; and even if you bust a spoke the wheel stays in better shape with more spokes.

Only you can judge if the "benefits" of reduced spoke count outweigh the drawbacks, in your use.

Brucey
Posts: 32274
Joined: 4 Jan 2012, 6:25pm

Re: crossed spoking vs radial, rear wheel for rim brake

Postby Brucey » 10 Feb 2019, 6:55pm

FWIW I agree with Colin re the bracing angles.

I'd also point out that you have been able to buy 'better wheels' (for OTP bikes) pretty much for ever; factory built wheels are often not very good. The weird thing is that today, folk often go out and buy another set of 'factory wheels' which just look a bit different, and imagine that they will get some kind of performance benefit. In some cases they might, too, but in many instances you are just trading one set of unwelcome compromises for another.

Currently it is popular to have wheels without very many spokes in. I have no idea why this is, exactly; every kind of engineering assessment tells you that such wheels are inferior to wheels with more spokes, in every respect bar one; aero drag. But this is tiny; so small you have difficulty measuring it in a fully instrumented wind tunnel. In reality when riding the bike you may be losing far more than you would ever gain because wheels without very many spokes in are really very flexy and this takes some of your effort all the time, even if the brakes are not rubbing (which is more common than you might imagine). There are plenty of minimal spoke wheelsets where I basically can't ride out of the saddle without getting severe brake rub.

Anyway if you don't have very many spokes they are (all things being equal) more likely to break and when they break the wheel is more likely to become unrideable. When the rims wear out usually the wheelset is finished; it is very often uneconomic to replace the 'special' wheel rims (two spare wheel rims of the correct type suddenly costs more than the wheelset did originally..... :roll: ) . Also it isn't unusual for the spokes and nipples to be 'special' in some way... like the nipples seize up 'especially' fast and to replace the spokes is 'especially' expensive. In some wheelsets the spokes cost the thick end of £10 each and need special tools to do anything with them. Another unwelcome feature is that (assuming you can that is) should you ride home on a wheel with a broken spoke, often you find that the rim has taken a 'set' and the wheel will never be quite the same again, i.e. it will never be straight and have even tension in, not at the same time anyway.

So maybe a lot of this doesn't matter too much if you are wanting a set of raceday-only wheels or you just want to keep up the with joneses or something. However if you are wanting to ride lots of miles in all conditions probably you could do better than a lot of factory wheels. Depending on how strong and how heavy you are it is possible to recommend a wheelset that will take a lot of wear and a lot of treatment, and can be sensibly repaired should you wear/damage a rim rather than just be thrown away. If correctly built and correctly stress-relieved, you won't break spokes, and if you do they can easily be replaced. Probably if you are running 11s/130mm OLN wheels then an offset rear rim is a good idea as Colin suggests.

BTW you may be using weight as a gauge of how good a wheelset is. Do bear in mind that you may be faster on a stiff wheelset rather than a lighter, flexier one. Also be aware that manufacturers are 'economic with the truth' when it comes to weighing wheels, and they often provide meaningless weights (i.e. without QR and without rim tape etc) anyway. Between all these things it is easy to find that your wheels actually weigh ~250g more than you thought they might....

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

Bulder
Posts: 2
Joined: 9 Feb 2019, 11:43am

Re: crossed spoking vs radial, rear wheel for rim brake

Postby Bulder » 10 Feb 2019, 10:31pm

Thanks for very interesting answers.
So thats why e.g. the Fulcrum R3 has double as many spokes on the crossed driveside, and Mavic an equal number of spokes on each side and an asymmetric rear rim? Is by the way an asymmetric rim the same as an offset rim?
And another question: Are wheels with equal number of spokes on each side easier to true - if that should become nescessary?

Brucey
Posts: 32274
Joined: 4 Jan 2012, 6:25pm

Re: crossed spoking vs radial, rear wheel for rim brake

Postby Brucey » 11 Feb 2019, 1:13am

Bulder wrote:Thanks for very interesting answers.
So thats why e.g. the Fulcrum R3 has double as many spokes on the crossed driveside,


'triplet' spoking more or less equalises the spoke tension each side of a heavily dished wheel (good) but makes the wheel go further out of true if you break a NDS spoke (it is like losing two) and also makes the rim even more 'special' (bad when it comes to replacement). Note that it only works (in most spoking patterns) if the rim is radially extremely stiff, because the radial loads are not evenly distributed.

and Mavic an equal number of spokes on each side and an asymmetric rear rim?


yes but that only improves the tension balance a few percent (for a typical rim with ~3mm offset). It can make the difference between a wheel that works and one that doesn't though.

Is by the way an asymmetric rim the same as an offset rim?
yes

And another question: Are wheels with equal number of spokes on each side easier to true - if that should become nescessary?


it depends on why the wheel is not running true. Rims that are deeper and wider tend to be stiff (and in fact when the wheel flexes there is an element of the whole rim moving around) and if the rim is bascially straight you should be able to get the wheel running true and have the spoke tensions fairly uniform. However if the rim isn't straight any more, you might not get the wheel straight at all and if you manage it, it will be a struggle and the spoke tensions will be all over the place. In general wheels with more spokes and rims which are less stiff are easier to deal with.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~