Light and strong XCM wheels - the spoke question

[SCD]

So, after much back and forth I settled into all but one component of my new wheel build.

 

RIMS: CSIXX XCM 29er, 28h. 

Why: Wide but not too wide (26mm ID). Reasonable weight (430gr) for the width and thick and strong 3.5mm side walls. Knowing where they are from they will be well designed. And the maker is living nearby should something go wrong. With 30x30mm a stiff yet balanced cross-section. 3mm spokebed asymmetry for evening out spoke stress ratio.

 

HUBS: AM Classic 28h.

Why: Lightest hubs for a reasonable price (225gr + 130gr). Reliable, I put 7000km on a previous pair without a hitch. Strong free hub mechanics that are also easy to service. Off the shelf bearings, easy to service as well. Before it is commented on I am not bothered by the engagement. My second choice DT240 would have been the same without the expensive ratchet upgrade. At first I was a bit put off by what seemed to be a waste of hub flange width by the rather large gap between the NDS flange and the rotor lugs. The total flange to flange width that defines the ultimate wheel stiffness is 50mm. But looking at it more carefully I found that the loss of width is only between 2mm and 4mm compared to major other brands.  Hope Pro4 has 52mm. DT has 53.9mm (3.8% less or 7.2% less). But the center-to-right and center-to-left offset is more balanced than with the other brands so the spoke tension is more equal. Adding the 3mm spokebed asymmetry of the rim the spoke tension ratio of the front wheel will be 93% and for the rear wheel 105% (greater spoke angle on the drive side, surprise!). That is pretty good and should ease the wheel build. And for both wheels the spokes are the same length for the DS and NDS. 

 

SPOKES: That is the remaining question. I already made the call to go for 28 instead of 32 spokes because of the stiff rim needing less support. Now the choice is between standard DB1.8/2.0 or bladed spokes rolled from DB1.5/2.0. I know the answer of every conservative wheelbuilder will be DB1.8/2.0 (after frowning dis-approvingly at the 28h). However CSIXX is building their own wheelsets with 28x DB1.5/2.0 spokes. 1.8/2.0 available on request. The wheelsets are not marketed around their weight. So in some way CSIXX must feel confident that this 28xDB1.5/2.0 is delivering a wheel build up to their standard. Contrary to that, as mentioned above, many wheelsbuilders strongly recommend against anything other 32x 1.8/2.0. Reliability issues are mentioned first, overall strength and stiffness second. Yesterday however I had the pleasure to 'touch' a 28x CXRay (effectively 1.5/2.0) ENVE/CKing wheel on a Scalpel. That wheel was stiffer than my already fantastic 32x 1.8/2.0 AC Wide lighting. So I would happily settle for that. 

So my spoke choices are right now: Standard DB1.8/2.0 or the Alpina DB1.5 offered by Rapide (similar to CXRay but cheaper). Obviously Brass nipples.

 

Purpose of wheel: Cape Town style trail riding, mainly Tygerberg/Jonkers/TM. Local XCM races. Upcoming EPIC (so yes, reliability is important). I am 78kg. Bike is a full-sus RM Element.

 

So I am interested not just in the standard recommendation to go for the bigger spoke for reliability, but some insight into the mechanical reasons behind and the actual real-life issues that might come up with the light bladed spoke in context with the stiff rims and the balanced spoke tension of the rim/hub setup. Does somebody has a DB1.5/2.0 MTB wheel and is happy/unhappy with it?

 

Fire away

 

[Goodbadugly]

28h? Strong? I would not build a 29" wheel with less than 32 spokes.

The weight saving on the extra 8 spokes is just not worth it imo.

 

2-1.5-2mm spokes are fine on marathon/epic type mtb. Again with 32 spokes.

[V12man]

You can build with 1.5/2.0 happily - spokes work in tension, not compression - and DB revolutions are MORE than strong enough for anything you can throw at them.

[David Marshall]

You can build with 1.5/2.0 happily - spokes work in tension, not compression - and DB revolutions are MORE than strong enough for anything you can throw at them.

Agree 100%.  Your spoke tension will be roughly the same no matter which spokes you use.  There is also the in between 2.0/1.7/2.0 double butted.  Your choice is the common 1415 gauge, 1416 (some Pillar and Wheelsmith) and the 1417 gauge (DT Revolution, Sapim Lazer and Pillar).

 

I am not sure how the Alpina flat spokes compare and slot in with the above.  The ones I have are 2mm (1mm by 1.8mm) 2mm.  They build nicely but I personally would use a round spoke as a flat or bladed one that is not positioned in the correct plane is worse than a round one.  Spokes do move around with use.

 

Your more controversial choice is the number of spokes.  People will tell you how strong/stiff/bomb proof their wheels are but I look at it like this.  If you have a bridge and you rebuild it with the same supports but further apart by how much do you weaken the structure?  Discuss it with an engineer.  I don't know the formula but I do know that the decrease in strength is not linear.  Your bridge with the wider supports will also flex more when it is used and therefore will fatigue faster.

 

How many spokes is enough? Boils down to what you want to get out of your wheels and how often you are prepared to re build them.  If they are just for racing you want them light and fast, if you want to train/race you will want them light but more durable.  If they are just for training you want them more durable still.

[SCD]

Agree 100%.  Your spoke tension will be roughly the same no matter which spokes you use.  There is also the in between 2.0/1.7/2.0 double butted.  Your choice is the common 1415 gauge, 1416 (some Pillar and Wheelsmith) and the 1417 gauge (DT Revolution, Sapim Lazer and Pillar).

 

I am not sure how the Alpina flat spokes compare and slot in with the above.  The ones I have are 2mm (1mm by 1.8mm) 2mm.  They build nicely but I personally would use a round spoke as a flat or bladed one that is not positioned in the correct plane is worse than a round one.  Spokes do move around with use.

 

Your more controversial choice is the number of spokes.  People will tell you how strong/stiff/bomb proof their wheels are but I look at it like this.  If you have a bridge and you rebuild it with the same supports but further apart by how much do you weaken the structure?  Discuss it with an engineer.  I don't know the formula but I do know that the decrease in strength is not linear.  Your bridge with the wider supports will also flex more when it is used and therefore will fatigue faster.

 

How many spokes is enough? Boils down to what you want to get out of your wheels and how often you are prepared to re build them.  If they are just for racing you want them light and fast, if you want to train/race you will want them light but more durable.  If they are just for training you want them more durable still.

 

Hi David, thanks for the detailed response.

I keep busy with structural engineering during the day therefore my thoughts on the spoke number in response to the rim stiffness. If you compare the cross-sectional area and the corresponding 2nd moment of inertia (which is the geometric property defining stiffness of a cross-section) of this carbon rim compared to the AC Wide Lightning you will find that you have a much stronger and stiffer bridge, if they were the same material. A well constructed carbon composite however is a stiffer material than aluminium. So the 'bridge' is even stronger.

This does two things: Firstly it needs less supports to support the load of the tyre. Secondly being a prestressed ring beam balanced between tension rods it has greater ability to spread a local impact to more supports. So more spokes will be activated to support such load. That's why I settled on the lesser spoke count.

 

On the smaller spoke diameter: it would make the suspension of the rim ultimately more elastic. The fatigue sensitive points - the J-bent and the threads at the nibble - still have the same diameter so the dynamic stress amplitude would not be different compared to a DB1.8/2.0 spoke. The stress amplitude is certainly different in the thin section. 

 

The Sapim CX Ray as well as the Alpina Flat DB1.5 are both rolled from a doubled butted 1.5/2.0 spoke. The flat section is 1.6x1.1mm. The steel has a strength of 1230-1765Mpa for both, which is very high for any kind of stainless steel. That would allow a spoke tension of 217-312kg before yield or rupture. 

[greatwhite]

Hi David, thanks for the detailed response.

I keep busy with structural engineering during the day therefore my thoughts on the spoke number in response to the rim stiffness. If you compare the cross-sectional area and the corresponding 2nd moment of inertia (which is the geometric property defining stiffness of a cross-section) of this carbon rim compared to the AC Wide Lightning you will find that you have a much stronger and stiffer bridge, if they were the same material. A well constructed carbon composite however is a stiffer material than aluminium. So the 'bridge' is even stronger.

This does two things: Firstly it needs less supports to support the load of the tyre. Secondly being a prestressed ring beam balanced between tension rods it has greater ability to spread a local impact to more supports. So more spokes will be activated to support such load. That's why I settled on the lesser spoke count.

The rim doesn't actually spread the load to local supports (spokes), it does the opposite and unloads them. the load will increase marginally in the spokes away from this area as the rim deflects.

 

I agree that with a more rigid rim a lower spoke count may be employed.

 

On the smaller spoke diameter: it would make the suspension of the rim ultimately more elastic. The fatigue sensitive points - the J-bent and the threads at the nibble - still have the same diameter so the dynamic stress amplitude would not be different compared to a DB1.8/2.0 spoke. The stress amplitude is certainly different in the thin section.

The reason for employing butted spokes (apart from weight saving) is to allow greater elasticity of the narrower mid section. This in turn reduces the stress variation in the fatigue sensitive zones of the spoke. In short, the dynamic stress amplitude is different. Employing this logic only, thinner spoke midsections would by the way to go, to maximise wheel longevity.

 

However, the greater the rigidity of the rim the less this is an issue, since the rim deflects less each time I'd goes round or takes a knock and therefore the amplitude of the fatigue inducing load variation on the spoke is reduced.

 

The Sapim CX Ray as well as the Alpina Flat DB1.5 are both rolled from a doubled butted 1.5/2.0 spoke. The flat section is 1.6x1.1mm. The steel has a strength of 1230-1765Mpa for both, which is very high for any kind of stainless steel. That would allow a spoke tension of 217-312kg before yield or rupture.

 

The only time a spoke should go past yield is when the wheel builder is 'stress relieving' the j-bends. Obviously, you don't want to go to UTS. You should be more interested in the cyclic fatigue characteristics of the material, but I don't ever recall a spoke manufacturer publishing that kind of info.

 

Above said, you have clearly considered your choices logically, but some thoughts you might want to add to your equation:

1. Have you considered straight pull spokes/hubs. This largely eliminates the fatigue problem as the j-bend is eliminated (most spokes fail at the bend). Your DT240 is available in a 28 hold straight pull.

2. Have you considered Lateral rigidity and directness of control? Especially on a 28 spoke build, there is going to be alot of flex between hub and rim of you use 1.5mm spokes.

 

To be fair, in case I am biased, I should also mention that I am using 28 hole syncros hubs (rebadged DT 350 hubs) with 2/1.8/2 DT competition straight pull hub and carbon rims with 25mm inside width - as an XCM wheelset, I'm very happy. With the original DT xr331 alloy rims and aerocomp spokes the wheel were squirelly.

[greatwhite]

Just to add, I would agree with David wrt round spokes. The CXray is a waste of money on an mtb wheel. Not only for the reasons he mentioned, but also an mtb tyre and rim are an aerodynamic disaster area, then just for good measure, you are probably only doing half the speed you would on a road bike, so any potential aero saving would a quarter of what is left.

[energiaMTB]

I am not an expert here, but was considering the same question wrt number of spokes and carbon rims, with the rims being stiffer. I found this link very interesting: http://www.noxcomposites.com/wheel_building

 

Specifically the discussion below:

Number of Spokes

As we said above, number of spokes is the 2nd most important factor when building a set of stiff wheels. Therefore, we use 32 spokes by default. It may not be "trendy," or "look cool" but it's the right thing to do from an engineering standpoint. Many people assume that with super-stiff carbon rims you can "get away" with very few spokes. Perhaps you can depending on what level of stiffness you are happy with, but there is more to the story, and that's opposite load deflection. Read on..

You may have assumed that when wheels rub against the chainstay, fork or cantilever brakes, it is a symptom of flexy wheels. In reality, the situation is more complicated than that. This is because lateral stiffness is affected by two major parts of the wheel; the rim stiffness and the spoke system stiffness. Our carbon rims provide a very high level of rim stiffness, so much so that they can literally overpower low spoke stiffness and this can actually amplify lateral deflection at parts of the rim away from the lateral loading point (the ground). In other words, with all other things equal, carbon rims can increase the rubbing against your chainstays, fork or cantilever brakes! It's a bit counter-intuitive, but this effect is due to our carbon wheels not conforming or bending when subjected to lateral loads, so the effects of low spoke stiffness are actually amplified by the stiffness of the rims. In contrast, a lightweight aluminum rim conforms as lateral loads are applied, and this deformation masks the chainstay rubbing effect (but it is also making your steering feel sloppy and robbing your power!). 

 

[SCD]

Thanks for all the valuable contribution. The wheels are currently being built in the proposed configuration. I will keep this thread alive with my experience as I take the wheels through the next coming months

 

 

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[Captain Fastbastard Mayhem]

Thanks for all the valuable contribution. The wheels are currently being built in the proposed configuration. I will keep this thread alive with my experience as I take the wheels through the next coming months

 

 

Sent from my iPhone using Tapatalk

Which spokes did you go for? In terms of real world difference, there's hardly any between bladed and normal DB. But bladed looks lank kieff on a wheel. 

 

I went with the Alpina bladed spokes for my build on Hope / WTB Asym i35 rims. LOVE them. 

 

And they're not difficult to orient properly. All it takes is a bit of care and attention. 

[Spoke101]

I'm currently riding a set of carbon Reynolds: 29er, 24H, 1.5/2.0 straight pull front and rear.

At 60kg they plenty stiff, mainly used for XCM but have done the odd enduro/trail ride on them with no issues.

Have broken the odd spoke over the last 3 years but all but one have broken at the 2.0 section where the thread starts.

 

PS: always carry a few spare spokes with you because murphy's law when you break one, race support won't have what you need.

[SCD]

Which spokes did you go for? In terms of real world difference, there's hardly any between bladed and normal DB. But bladed looks lank kieff on a wheel. 

 

I went with the Alpina bladed spokes for my build on Hope / WTB Asym i35 rims. LOVE them. 

 

And they're not difficult to orient properly. All it takes is a bit of care and attention. 

 

We are using the Alpina bladed DB1.5. They are well priced. I would probably not spend the money on CX Rays.

The one advantage that I see in bladed spokes is that one can see when the spoke is twisted, and correct it.

[SCD]

I'm currently riding a set of carbon Reynolds: 29er, 24H, 1.5/2.0 straight pull front and rear.

At 60kg they plenty stiff, mainly used for XCM but have done the odd enduro/trail ride on them with no issues.

Have broken the odd spoke over the last 3 years but all but one have broken at the 2.0 section where the thread starts.

 

PS: always carry a few spare spokes with you because murphy's law when you break one, race support won't have what you need.

 

Are you riding a hardtail or a softtail? 

That also makes a big difference in the load a spoke receives from impacts or harsh landings...

[Spoke101]

Are you riding a hardtail or a softtail? 

That also makes a big difference in the load a spoke receives from impacts or harsh landings...

 

Have ridden then on both, Trek Superfly hardtail and now on a SC Tallboy.

I normally race at the pointy end of the field so they have seen a fair amount of abuse.

 

Edit: they have also done some 4/5m gap jumps and drops of 1.5/1.8m on both bikes.

[droo]

I'm not usually one to say this, but with those CSixx rims you'll be fine with 28. And the 1.5 spokes will be plenty, Mark's tested those wheelsets pretty extensively.

 

Just resist the temptation to go with the shiny aluminium nipples.

[SCD]

I'm not usually one to say this, but with those CSixx rims you'll be fine with 28. And the 1.5 spokes will be plenty, Mark's tested those wheelsets pretty extensively.

 

Just resist the temptation to go with the shiny aluminium nipples.

Thanks for your feedback. That is encouraging.

Nope, there will be no alu nipples anywhere near those wheels that is for sure

 

 

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