Spokes on Maddux 3.0 Disc

[alexnharvey]

I follow Shimano's asymmetric method when I build rear disc wheels. Nonetheless, I have seen convincing arguments (quoting Jobst Brandt and others) that the disc brake forces are not significant compared to the overall forces on the wheel and therefore symmetric is fine. I say in that case why not do it asymmetric but some people take the other view.

[Brucey]

IIRC Brandt's analysis assumed a coaster brake hub, which is different from a disc brake hub.

Disc brake hubs for derailleur gears on modern bikes build into quite heavily dished wheels with lowish tensions in the NSD spokes to start with. Part of the issue is that it may not take that much brake torque before the NDS trailing spokes are likely to run slack momentarily. Remember also that in the real world the service loads on the wheel all add together and may produce rather different loadings in the spokes than when they are each considered individually. So just using the brakes is liable to be very different from (say) using the brakes whilst traversing rough ground, with the bike leant over during some transient manoeuvre. On a heavily loaded touring bike the forces in the rear wheel can be greatly increased under braking.

I'd like to see a more up to date analysis of the loadings involved. A while ago I did look at the loadings in a disc braked front wheel and (being larger than the brake loads in a unladen rear wheel) I came to the conclusion that the fatigue loadings in the spokes were significant in some applications. MTB use being one of them; braking can be both frequent and severe. On road going bikes what saves the wheels is that braking is usually not so frequent or severe. Nonetheless some manufacturers take the view that you are chancing your arm if you try and build a disc brake wheel without enough spokes in, so don't bother making disc brake versions of some rim models with fewer than 28 holes in the rim, even if they are prepared to go much less than this in the rim-braked equivalent.

cheers

[LinusR]

IIRC Brandt's analysis assumed a coaster brake hub, which is different from a disc brake hub.

Brandt posted here about this on rec.bicycles.tech

Jose Rizal writes:
> This is in Magura's disc brake manual:
# Use spokes with a diameter of 2mm/1,8mm which you cross three
# times... Head-inside-spokes (arc-outside-spokes) have to be pulled,
# i.e. these spokes point forward on the front wheel; on the back
# wheel these spokes point forward on the rotor side and backwards on
# the drive side.

> Is there any merit to this recommendation?
No, and this is another example of people in charge not being aware of
the technology with which they are working. Had they done an analysis
of forces involved, or read it in TBW ("the Bicycle Wheel"), they
would have known that braking torque doesn't present a significant
spoke load in comparison to the radial load of statically sitting on a
bicycle. They night also have noticed that the so called pushing
spokes in fact push, half the torque load being taken up by a light
tension increase and the other half by a tension decrease, so it
doesn't matter which way the spokes are oriented in the flange. That
is, all spokes of the wheel are involved in transmitting brake torque
from hub to rim (aka pedaling torque to the rear rim).

> The method described in TBW results in the "head-inside" spokes on
> both sides of the wheel pointing in the same direction (towards
> wheel forward rotation for the rear wheel).
That is also explained why one does that and mentions that the
orientation makes no real functional difference.

Jobst Brandt
jobst...@stanfordalumni.org

https://groups.google.com/d/msg/rec.bicycles.tech/rduQB6i9UOg/XsttQJhmHXQJ

I've just checked my MTB and contrary to what I said earlier, the rear wheel is built symmetric but the opposite way to the Brandt method - ie, the inbound spokes face forward not backward.

[alexnharvey]

Ric hjertberg also addressed it here https://www.wheelfanatyk.com/blog/issue ... ke-wheels/

[Brucey]

well neither assessment is entirely correct; if JB was 100% correct you wouldn't see wheels with nothing but broken inside spokes. If RH was entirely correct you would be able to build durable wheels with disc brakes using half as many spokes.

cheers

[LinusR]

well neither assessment is entirely correct; if JB was 100% correct you wouldn't see wheels with nothing but broken inside spokes. If RH was entirely correct you would be able to build durable wheels with disc brakes using half as many spokes.

cheers

If I understand this correctly (doubtful ) to build using the Shimano method for rear disc wheel I would insert "the second set" outbound (instead of inbound as described by JB on p88 above). In other words, instead of turning the wheel over I would drop the spokes in so that the spoke heads are on the inside. And for "the fourth set" so that the heads are on the outside. That would be the only difference when following JB's instructions.

This should give me the Shimano pattern (above). The front disc hub can be laced as usual following JB. Is that correct?

[Brucey]

yes, more or less. However there is more than one way of skinning a cat...

There are many different ways of lacing wheels up but IME lacing a wheel 'skew' (asymmetric) is the easiest of all, provided you are sure which holes to put the second set of spokes in. This is the most common mistake that people make.

The reason I say 'easiest' is that with the JB method you need to rotate the hub before you can fit the third set of spokes. IME if the spokes are a snug fit in the hub (which they should be IMHO) this isn't always easy to do. Indeed, whilst fitting JB's third set, it is often difficult to tell the difference between a wheel with snug fitting spokes and one where you have screwed up and you are trying to build it with spokes that are too short. With the JB method if you have made a mistake with the spoke lengths, it is only apparent when you are 3/4 through the assembling the wheel, which is quite late on.

If you build 'skew' the second set of spokes can be inside spokes too, but they need to be trailing if the first set are leading. This sets the hub twist correctly at an early stage so there is no fighting it later on. If you are not an experienced wheelbuilder, it is less easy to be sure which holes these spokes should go in; best to look at another wheel to be sure. Another way is to look at the holes which you would normally use for the second set, and then (by a process of elimination if you like) use the ones that are 'not those'.

I have even seen folk building symmetric wheels using a similar approach, even though it involves a bit more lacing later on. In this they fit the first set as usual (inside leading, say), and then fit the second set on the other flange outside trailing. This again sets the hub twist at an early stage, and confirms that the spoke lengths are correct at an earlier stage too. In this build process it is best if the third set fitted is the set which JB fits second, i.e. it is the second set of inside spokes. This process requires only a little extra lacing; when you fit the third set, the spokes clear the first set easily, but need to be taken through more spokes before they are in the right place on the rim, which is easier with a small flange hub than a large flange one. The wheel only needs to be turned once with this process; all the spokes in the first and second sets can be inserted from one side. A skilled builder will, I think, usually be fastest with this method.

A compromise between the JB method and others is to fit the first set as per JB, then immediately fit three (in a 36 build) or four (in a 32 build) of the third set, evenly spaced. This sets the hub twist. It also makes fitting all the subsequent spokes slightly more difficult but

a) you will be using a nipple setting tool with most modern rims anyway; having the wheel full slack when fitting the second set is of less advantage than when working with shallower rims (as would have been the case when JB wrote his book)

b) what you lose in lacing you gain in not fighting the hub twist and
c) it tells you very early on indeed if the spokes are about the right length or not.

On point c) if you are in any doubt about spoke lengths at all, you can fit just six spokes (36 build) or eight spokes (32 build) and this will tell you enough. The spokes fitted should be from the first and third sets (as per JB), and evenly spaced.

So more than one way of skinning a cat....

cheers

[LinusR]

yes, more or less. However there is more than one way of skinning a cat...

Don't try to confuse me