Spoke counts

GavH
GavH Posts: 933
edited December 2009 in Workshop
My current setup is running a pair of Shimano WH-R550 wheels with 16 spokes at the front and 20 back. I know this makes them 'lighter' and more 'aero' but I'm also guessing that the added weight and drag of a say, 32h wheel would be negligible. The added strength however would be quite noticeable to the heavier riders i'd imagine.
Can anyone tell me at what weight limits certain spoke counts become ineffective at taking the riders weight and why is it the the front wheels are usually 4 spokes less. Is it simply because the rear wheel needs more due to supporting more of the overall weight?

Finally, what are the relative merits of having a 32f and 32r setup? I notice that the ubiquitous Mavic Open Pro seems to only come in a 32h option, although I believe they used to do 28h not that long ago.

:?

Comments

  • Smokin Joe
    Smokin Joe Posts: 2,706
    If you want to know why front wheels can have fewer spokes than rears, look at the rate of wear on each of the tyres.

    The old standard used to be 32/40, but then 36/36 became universal for decades. I never understood why, probably to make production more economical.

    Weight limits? Dunno.
  • andrew_s
    andrew_s Posts: 2,511
    It's the length of unsupported rim between the spokes that matters. If you take the same rim, and load it more, you need the spokes closer together to prevent excessive rim flex. The rear wheel has to support about double the load of the front one.
    Instead of having more spokes, the alternative is to make the rim stiffer in a vertical plane. This normally means a higher, more aero profile. Given the same rim construction, a high profile rim will weigh more than a flat box section, so there's a certain amount of swings and roundabouts on the weight front.

    The other difference that reducing the number of spokes makes is in how far out of true the rim goes if a spoke breaks. In a 36h rim, breaking one spoke is trivial - you might not even have to open the brake release. If a 16 spoke wheel loses a spoke, the wheel will be so far out of true that it won't go between the fork blades, and you'll either have to phone for rescue, or walk.
    Broken spokes aren't common in good wheels, but it does happen.
  • pianoman
    pianoman Posts: 706
    Given the same rim construction, a high profile rim will weigh more than a flat box section, so there's a certain amount of swings and roundabouts on the weight front.

    Which is why the description for the American Classic 420 is that "deep rims do not need as many spokes"?

    As for tyre wear, I've had to replace the rears on both my sets of wheels after one "season" (winter/summer) but the front ones are still working well. Could you not just swap the tyres round after about 1000km so as to balance the wear rates?
  • Smokin Joe
    Smokin Joe Posts: 2,706
    PianoMan wrote:
    Given the same rim construction, a high profile rim will weigh more than a flat box section, so there's a certain amount of swings and roundabouts on the weight front.

    As for tyre wear, I've had to replace the rears on both my sets of wheels after one "season" (winter/summer) but the front ones are still working well. Could you not just swap the tyres round after about 1000km so as to balance the wear rates?
    Pointless because the overall rate of wear will be exactly the same, and not good practice because the rear squares off much more than the front and will upset the handling if you swap it over.
  • GavH
    GavH Posts: 933
    Andrew thanks for the detailed response. Can I also take it then that a low spoke count wheel will be more likely to flex when out of the saddle than a high spoke count, or is the rim type itself the more important factor? My current wheels feel very 'mushy' when I'm out of the saddle and I can't help but think the low spoke count is to blame. FWIW, I'm 80kg.
  • andrew_s
    andrew_s Posts: 2,511
    Deep profile rims won't be much stiffer sideways than normal rims. What would make a difference would be more spokes, or higher spoke tension. Many of the low spoked rims have special spokes and are built to a higher tension, ending up stiffer than normal. However I dare say some use normal spokes at normal tensions and are less stiff.


    On tyre replacement, best practice is to put a new tyre on the front, and move the old front to the back. That way the front tyre remains fairly new. Front tyres are more important than rear as it's likely to hurt more if something goes wrong.

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