Carbon disc wheels - 28 or 24 spokes?

My recent build using carbon gigantex disc rims is a good example to open a discussion regarding number of spokes in disc wheels. Is the new trend 24/24h safe for most of the riders? Well, let's try to find out.

Two sets with excellent price to capabilities ratio. Wheels made using Gigantex rims, 40mm front and 50mm rear. Wide and tubeless compatible rims with one of the best quality&finish available in the market. Taiwanese Gigantex is the first factory worldwide who started production of carbon rims and today they are number one leader in technology of carbon products manufacturing. 

Rims were paired with Bitex hubs 106F and 106R. These guarantee long usage due to solid bearings and correct hub's geometry. Worth to mention that those hubs come with all axle standards and CL disc rotor mount so they are the most common choice for road and gravel builds. Price effective with low weight - front 142g and rear 243g. In addition, rear hub has "Anti bite" system so cassette won't damage the free hub during usage.

In terms of spokes, here I used latest generation of Pillar PSR TB spokes with reinforced elbow (2.2mm) and 3D force system. 

For front I gave TB2016 (1.6mm) spokes and for rear I used TB2017 (1.7mm) spokes.

1.6mm and 1.7mm spokes come with weight similar to Sapim D-light spokes, however in order to decrease the weight further I used Kowa spoke cutter to cut spokes and leave min. length of the barrel so effectively I shaved few additional grams over 56 spokes / set.

J-bend spokes were nicely worked out to lay and support the hub's flanges. 

Wheels will be used in all weather conditions, therefore brass nipples were given. About 30g penalty per set but in return they will trouble free for the whole lifecycle of the wheels.

Total weight of the 40/50mm set with 28/28h spokes is 1680g. With alloy nipples those would weight 1650g. Given the solid build and high profile rims I think it's pretty good result.

Price tag: 989 CHF. Set includes wheels, spare spokes and nipples, decals of free colour choice and two layers of tubeless tape.

Wheels can be configured differently, ex. 24/24 spokes and they are available in my shop.

Now, talking about 24/24h vs 28/28h spokes for disc wheels...

The latest trend of 24/24h spokes for road disc wheels is becoming very popular. The argumentation is that the carbon rims are very strong, less spokes is more aero and in addition 24/24 wheels are much lighter. It's true, but only partially. Let me try to put a bit of more light into that.

1. Weight savings. Assuming that each spoke has 5g and we use alloy nipples, the savings will be no more than 45g. For some it's significant, for others not that much. Given that spokes are not entirely a rotation mass and 28/28h wheels are MUCH stiffer, the weight saving is not a plus at all (at least for me).

Exploring that path further, if you look closer to 24/24 disc wheels, most of manufacturers and builders will use thicker spokes (1.8mm) where weight is 6g or more. If we compare that with 28/28h disc wheels with thinner spokes (5g) then the real saving is about 37g only.

2. Stiffness. Surprisingly 24/24h disc wheels are lacking of lateral stiffness. This is caused by hub's geometry where flanges are narrower in order to accommodate disc rotor mount. If you combine that with only 24 spokes, you get a very average wheel's parameters. Because in disc wheels there are no brake pads, wheel won't rub against them and therefore most of the riders don't realise that the flex is there.

3. Aerodynamics. Four less spokes per wheel won't give you any noticeable aero gains. This is just pure marketing.

4. Safety. This is the most important argument which sales guys won't talk about but you as a rider should be aware of.

In every wheel, during every wheel rotation some spokes will get tensioned (top ones) and some others will get looser (bottom ones). If the spoke gets too much loose then for a very short time there is no tension in such spoke, which then creates a significant risk of spoke snap. This is especially important in disc wheels where the torque caused by disc braking is huge and causing leading spokes to further loose tension.

More spokes in a wheel spread the forces (load in usage) across more spokes, therefore they keep the tension better comparing to wheel with less spokes.

Have a look on below calculator, which takes into account rim type, hub type, spoke type, weight of the rider and of course number of spokes. With those parameters it shows the load in usage and therefore I can see if the wheel will be safe or not:

28 spoke rear wheel, 90kg rider+bike

In above simulation, 3 trailing and leading spokes will take 300N of load. On non drive side the tension will be about 645N so taking into account the load, during each rotation bottom spokes will be tensioned to about 350N on non drive side and 900N on drive side.

24 spoke rear wheel, 90kg rider+bike

Same wheel, but 24 spokes. Only two leading and trailing spokes so the load is increased to 441N. During each rotation spokes will be tensioned to about 200N on non drive side, so 150N less than with 28 spoke wheel. 

If you'll apply a torque caused by disc braking, it's almost certain that spokes will be getting completely slack and thus wheels are simple not safe for 90kg rider.

24 spoke rear wheel, 70kg rider+bike

Same 24 spoke wheel but for 70kg rider+bike. As you can see much better parameters. Not that quite as with 28 spoke but much better.

Taking all that into account I can say that for most of the riders, 28 spoke disc wheels will be stiffer, safer and only marginally heavier than 24 spoke wheel. Unless you're not a pro rider where during the race milliseconds are important, think twice (or consult a good wheel builder) before buying a new hoops.

Of course I'm not saying that 24h disc wheels are bad. I'm building such as well, but combination of components (rim+hub+spokes type) and running above simulations taking into account rider's weight is always a first thing I make before I confirm to the customer that I will do the build.

I hope I was able to clarify this at least a little bit. Wheel building is not a rocked science, however few things must be always taken into consideration in order to build safe and reliable wheels.

Thanks for reading

Tomasz

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