Optium quantity of spokes

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Optium quantity of spokes

Postby mikesbytes » Fri Sep 25, 2009 3:12 pm

This topic doesn't fit particularly well under any particular forum, so I stuck it here

I've heard second hand that there was a study done that determined that the optimum number of spokes in a wheel for aerodynamics was 28. Does anyone know where the paper on this is? I'd like to check it out.
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by BNA » Fri Sep 25, 2009 3:44 pm

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Re: Optium quantity of spokes

Postby sogood » Fri Sep 25, 2009 3:44 pm

If you aim for 1000 spokes, you'd have a disc wheel with dimple effect. :mrgreen:
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Re: Optium quantity of spokes

Postby mikesbytes » Sat Oct 03, 2009 10:03 pm

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Re: Optium quantity of spokes

Postby twizzle » Thu Nov 05, 2009 10:58 am

mikesbytes wrote:This topic doesn't fit particularly well under any particular forum, so I stuck it here

I've heard second hand that there was a study done that determined that the optimum number of spokes in a wheel for aerodynamics was 28. Does anyone know where the paper on this is? I'd like to check it out.


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Re: Optium quantity of spokes

Postby mikesbytes » Fri Nov 06, 2009 7:45 am

Thanks

What does this mean?

Although we cannot test every possible permutation, it has been shown that
for the generic wheel tested here, there is a power minimum in the 24 to 36 spoke range.
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Re: Optium quantity of spokes

Postby sogood » Fri Nov 06, 2009 8:39 am

mikesbytes wrote:What does this mean?
Although we cannot test every possible permutation, it has been shown that
for the generic wheel tested here, there is a power minimum in the 24 to 36 spoke range.

See Figure 6. There's a dip in power (P) around 24 spokes (Ns) and the effect is particularly prominent beyond 30km/h (UB - Velocity of bicycle).

But one should note that the result is only applicable to the specific setup used in the study, which is more of theoretical half size wheel using round spokes rather than any specific factory/DIY wheel. The spokes were superglued to the rim of the experimental model and I don't think you'd want to ride it. ;)
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Re: Optium quantity of spokes

Postby Parrott » Fri Nov 06, 2009 8:52 am

rear :P

front

Rim depth and shape has far more to do with aerodynamics than spoke count, spoke shape apparently matters also here is a bit to sift through
and here
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Re: Optium quantity of spokes

Postby colafreak » Wed Nov 11, 2009 11:51 am

Parrott,

I contend that spoke count has a LOT to do with aerodynamics.

The reason deep section rims are more aerodynamic is because the spokes are shorter. To put it another way, there is less "spoke" being spun through the air.

Reducing the number of spokes has the same effect as increasing the rim section.

As for the optimum amount... very interesting. It would appear there is sort of a "resonant frequency" at which the airflow over the other spokes helps reduce drag on the following spokes. The inference is however, that this will change with different setups, so there is no "magic" number.

Put things in perspective, going from box section to full deep dish carbon rims will add at maximum 0.5km/h at 35km/h. So how big a difference do you think it makes going from 30 spokes to 28? or my pet hate from a traditional carbon rim to a zipp wheel of the same depth with "magical dimples"
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Re: Optium quantity of spokes

Postby sogood » Wed Nov 11, 2009 12:09 pm

I contend that there should be as many spokes as one can fit in the space b/n the hub and rim to get optimum aero response ie. Take it to infinity and we have a fully covered disc wheel.
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Re: Optium quantity of spokes

Postby colafreak » Wed Nov 11, 2009 12:12 pm

So you said in September.
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Re: Optium quantity of spokes

Postby sogood » Wed Nov 11, 2009 12:23 pm

colafreak wrote:So you said in September.

Good. My thoughts are consistent over time. :mrgreen:
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Re: Optium quantity of spokes

Postby Parrott » Wed Nov 11, 2009 12:56 pm

colafreak wrote:Parrott,

I contend that spoke count has a LOT to do with aerodynamics.


Put things in perspective, going from box section to full deep dish carbon rims will add at maximum 0.5km/h at 35km/h. So how big a difference do you think it makes going from 30 spokes to 28? or my pet hate from a traditional carbon rim to a zipp wheel of the same depth with "magical dimples"



I'm time trialing with a full aero set up in the mid 40's kmh region and I can tell you my aero rims are making well over .5 kmh diff, wind resistance is an exponential curve.Even if it was only .5 kmh that is still a good time gap over 40k. Good point about the spokes being shorter, the deeper the rim though, another point in favour of deeper rims :P
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Re: Optium quantity of spokes

Postby colafreak » Wed Nov 11, 2009 1:34 pm

Yeah, look I wouldn't contend that 0.5km/h isn't a big increase. It is, particularly in time trials.

My point is more along the lines of how much faster one set of 50mm tubulars are compared to another set. I think dimples aren't worth paying double for.
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Re: Optium quantity of spokes

Postby twizzle » Wed Nov 11, 2009 3:03 pm

colafreak wrote:Yeah, look I wouldn't contend that 0.5km/h isn't a big increase. It is, particularly in time trials.

My point is more along the lines of how much faster one set of 50mm tubulars are compared to another set. I think dimples aren't worth paying double for.


Marketing over substance....

One of the mags in europe did a race tyre review and was quite surprised that the variation in watts consumed in rolling resistance in the tyres exceeded the difference between best/worst aero rims they had tested in a previous article. In my thinking, the price difference between good/bad aero rims could pay for a lot of good tyres.
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Re: Optium quantity of spokes

Postby Parrott » Wed Nov 11, 2009 3:42 pm

colafreak wrote: I think dimples aren't worth paying double for.


I agree, fair bit of marketing there. Their shape is probably a bit better than some of the cheaper deep rims though and they are stiff and light in a 404 tubular. I choked when I saw the price also though, too much for me, the performance gap wouldn't be anywhere near the price gap.
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Re: Optium quantity of spokes

Postby mikesbytes » Wed Nov 11, 2009 9:48 pm

sogood wrote:I contend that there should be as many spokes as one can fit in the space b/n the hub and rim to get optimum aero response ie. Take it to infinity and we have a fully covered disc wheel.


While your post is in humour, I think this brushes on the discussion, if there are more spokes do the spokes break the wind for the following spoke and hence more spokes might actually reduce wind resistance?
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Re: Optium quantity of spokes

Postby colafreak » Thu Nov 12, 2009 9:29 am

After re-reading the paper, I realised that it actually states that the relationship between spokes and rotational resistance is constant.

The reason there was a dip in the resistance was due to the resistance of the wheel as a whole through the air. Basically what was happening was that the extra spokes were churning up the air and so the airflow over the wheel from front to back was smoothed out. This is the same concept as dimples on a golf ball*

So basically the concept proved is that more spokes can increase the turbulence around the wheel and the airflow over the wheel as a whole can be reduced.


There are a few problems I picked up on though, first there was no fork influencing the airflow. Since the big discovery of this paper was the airflow over the wheel, it should have had a fork as this would significantly affect the characteristics of the wheel. Also, there were no tires on the wheels by the looks of things, also likely to have a big effect on the airflow over the wheels.

Secondly, the wheels were half size. The way air reacts and flows and stalls is not the same at different scales. Therefore, while concept may be proved, it can't be extrapolated that the magic figure for 700c wheels lies in the same area as for these half-size ones. It may be that you can only experience this effect between 38 and 46 spokes, or between 4 and 8 spokes or something impractical like that.



The giveaway is how poorly the disc and tri spoke performed. It should have been a red flag that there was a problem with the model. The effect as a whole is proved, but not where it sits in relation to real world wheels.


My advice would be (if you can) to get out on the track with your powermeter and have a nice bike shop mechanic build up a couple of practical solutions and see if you can notice any similar effect with different spoke numbers (don't forget to tape the spoke holes closed).




*SHOCK!! That's the concept on which Zipp's idea is based on of course, but remember that the effect of the spokes would have to be MASSIVELY greater than the effect of dimples on the rim surface. The wire spokes are like a giant whisk, whereas the variance in the surface of the rim due to the dimples is very very small.
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Re: Optium quantity of spokes

Postby bracko » Fri Nov 20, 2009 12:23 pm

Mike, regarding your last post the answer is it depends.... the size and shape of the spoke dictate how well the cutting effect will be and how much dirty air is left behing for the oncoming spoke to cut through.

I'm not entirely sure that the tests can't be extrapolated for a full size wheel... without going into the article too much, as long as the airflow is fast enough, which i believe it is, you should be okay before you get more secondary forces coming into play (a'la the first Audi TT's high speed issue where it turned into an aerofoil).

the main advantage with deep dish wheels is that the exterier of the wheel, i.e. at the rim is where you will have the most drag force, remove that with a solid rim, remove the largest force. the distribution of the drag force is not linear either.
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Re: Optium quantity of spokes

Postby colafreak » Fri Nov 20, 2009 3:12 pm

bracko wrote:Mike, regarding your last post the answer is it depends.... the size and shape of the spoke dictate how well the cutting effect will be and how much dirty air is left behing for the oncoming spoke to cut through.

The article does not state that the dirty air off the spoke decreases drag on the following spoke. It states that this turbulence reduces teh drag of oncoming air around the whole wheel. Very different concept.

I'm not entirely sure that the tests can't be extrapolated for a full size wheel... without going into the article too much, as long as the airflow is fast enough, which i believe it is, you should be okay before you get more secondary forces coming into play (a'la the first Audi TT's high speed issue where it turned into an aerofoil).

the main advantage with deep dish wheels is that the exterier of the wheel, i.e. at the rim is where you will have the most drag force, remove that with a solid rim, remove the largest force. the distribution of the drag force is not linear either.


No, the main advantage is reducing the total length of all the spokes spinning through the air. It reduces rotational drag the same way using less spokes reduces rotational drag.
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Re: Optium quantity of spokes

Postby bracko » Fri Nov 20, 2009 3:26 pm

colafreak wrote:No, the main advantage is reducing the total length of all the spokes spinning through the air. It reduces rotational drag the same way using less spokes reduces rotational drag.

is that in reply to my last statement? we're fundamentally agreeing, you just have a 'no' infront of your sentence. however, the drag is still higest at the outer rim.
Last edited by bracko on Fri Nov 27, 2009 4:37 pm, edited 1 time in total.
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Re: Optium quantity of spokes

Postby colafreak » Mon Nov 23, 2009 12:04 pm

I see what you're saying. The key is in the previous part of the post where I quoted you and then commented in bold.

In your last paragraph, it appeared to me that you're talking about the longitudinal drag when taking into account the rest of the post. I have now re-read and can understand where you're coming from.

The other comment though still stands. The idea of one spoke decreasing the drag of the following one is not supported by the experiment. What it is saying is that at some points, as rotational drag increases (more spokes) longitudinal drag can decrease.

Rotational drag continues to increase as expected (exponentially) as more spokes are added, no exceptions.

Apologies for telling you you're wrong and then agreeing with you! lol, my bad.
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