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Aerodynamics - front / rear difference

by bechdan
Thu, Jun 14, 2018 19:36
16

with regard to wheels, does it make any difference if the rear wheel is not as aero as the front? I cant see how it would make any difference to have a deep / aero profile on the back. What do you think?

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16 comments

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Mungecrundle | 6 years ago
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Being honest with myself, my number 1 priority, after them actually being the right size, brake compatibility, attachment system, a preference for tubular tyres and not being absolute stupid money, would be "Do the wheels look good on my bike?" Because the marginal gains in any category between different wheelsets would be lost on me.

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CXR94Di2 | 6 years ago
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I have two pairs of 60mm aero rims with the same tyres on both.  One pair cost £650 come with 'U' shaped profile sapin spokes, all very neatly laced so there is hardly any spokes exposed until near the hub(DT Swiss straight pull )  The other pair £279 ebay 'V' shaped.  There is no descernible difference in speed.

The cheaper 'V' shaped aero wheels are more susceptible to crosswinds steering and make riding a bit more twitchy

 

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HowardR | 6 years ago
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Deep section wheels arguably have the advantage of being quite 'sexy' <I admit it ~ I'm a velo-pervert> - but I was wondering, if in extensive wind tunnel testing, it was proven beyond a doubt, that an equivalent aero benefit [to the most expensive of deep section wheels] could be attained for just a couple of quid by attaching some pink tassels to your handle bars & a small doll sat on a little plastic seat mounted behind ones saddle - how many of us would go for it?

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Canyon48 replied to HowardR | 6 years ago
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HowardR wrote:

Deep section wheels arguably have the advantage of being quite 'sexy' <I admit it ~ I'm a velo-pervert> - but I was wondering, if in extensive wind tunnel testing, it was proven beyond a doubt, that an equivalent aero benefit [to the most expensive of deep section wheels] could be attained for just a couple of quid by attaching some pink tassels to your handle bars & a small doll sat on a little plastic seat mounted behind ones saddle - how many of us would go for it?

 

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HowardR | 6 years ago
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Am I right in remembering that handlebars with a 'aerofoil' top section provide a similar drop in drag to most deep section wheels (& the flattening doesn't have to be massively pronounced)?

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Canyon48 replied to HowardR | 6 years ago
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CXR94Di2 wrote:

The difference between £2k deep aero and £200 aero wheels is minimal.  If you want to pay silly money, do so, but it wont make you much faster than you are already.  Getting more aero on the bike has the most significant difference plus getting fitter/stronger.

I definitely agree that body position and fitness (power) give you greater gains than aero wheels... however, a £2k set of deep aero rims will save a VERY significant amount over a £200 wheelset.

The aerodynamic difference between £1k aero wheelset and a £2k aero wheelset will be reasonably small (in terms of the bike and rider overall). The real difference comes in the weight and materials used, switching from aluminium to carbon, reducing the weight.

https://www.swissside.com/blogs/news/cfd-development-for-the-hadron-aero...

To compare rim profiles, given two rims of the same depth; one generic flat-sided, V-section rim (like you can get cheap from PlanetX etc etc) and the other a CFD-designed U-section rim (DT Swiss, Zipp, etc). At 0 degree yaw, there will be little difference between the two, at yaw angles between 5 to 10 degrees (typical for normal riding) the U-section wheels will perform vastly better and handle better in crosswinds.

HowardR wrote:

Am I right in remembering that handlebars with a 'aerofoil' top section provide a similar drop in drag to most deep section wheels (& the flattening doesn't have to be massively pronounced)?

Aero handlebars do provide a small advantage and they work on similar principles. Aero bars give no way near the same drag reduction as aero wheels (4 watts compared with 23 for this data)

https://www.shopforwatts.co.uk/blogs/news/watts-it-worth

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fukawitribe replied to HowardR | 6 years ago
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HowardR wrote:

Am I right in remembering that handlebars with a 'aerofoil' top section provide a similar drop in drag to most deep section wheels (& the flattening doesn't have to be massively pronounced)?

Nope

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CXR94Di2 | 6 years ago
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The difference between £2k deep aero and £200 aero wheels is minimal.  If you want to pay silly money, do so, but it wont make you much faster than you are already.  Getting more aero on the bike has the most significant difference plus getting fitter/stronger.

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HowardR | 6 years ago
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There's more to areodynamics than rim depth.

This test, admitedly of some years back lists the shallow section Shimano C24's only a notch or so below the C50's and well ahead of some much deeper sectioned rims:

http://www.rouesartisanales.com/wp-content/uploads/2008/01/aero_english.jpg

From: http://www.rouesartisanales.com/article-15505311.html

Well worth a read.

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Canyon48 replied to HowardR | 6 years ago
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HowardR wrote:

There's more to areodynamics than rim depth.

This test, admitedly of some years back lists the shallow section Shimano C24's only a notch or so below the C50's and well ahead of some much deeper sectioned rims:

http://www.rouesartisanales.com/wp-content/uploads/2008/01/aero_english.jpg

From: http://www.rouesartisanales.com/article-15505311.html

Well worth a read.

It's worth noting that this wind tunnel test is fundamentally flawed as it can't be extrapolated to real world conditions.

The vortices that develop either side of the wheel are not present here as the entire wheel is presented to freestream air.

This picture illustrates the vortices https://www.google.co.uk/search?tbm=isch&q=Bicycle%20wheel%20cfd&oq=Bicy...

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madcarew | 6 years ago
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In short, yes, there is a point in the rear wheel being aero, in a TT, but it is just about the least important thing on the bike. In that common yardstick, a normal box section wheel (25mm rim, 24 spoke) to a 808 deep dish back wheel the alleged savings are about 8 - 12 sec in a 40k TT. This is less difference than between types of tyre on the front wheel.  The time saving is realistically the same regardless of if you're doing 30 kph or 45 kph. Disc wheels are a slightly bigger saving (20 sec over a standard 25mm 24 spoke rim) and are most effective from about 5 degrees to 18 degrees of yaw. 

By comparison moving from on the hoods to full TT position is about 2.5 minutes.

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check12 | 6 years ago
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Deep rims are better when the wind isn’t head on so the rear wheel gets that benefit. 

Rear disc, think the speed you benefit is >25mph so have to be a pretty decent tt-er

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Welsh boy replied to check12 | 6 years ago
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check12 wrote:

Deep rims are better when the wind isn’t head on so the rear wheel gets that benefit. 

That is not 100% true, different profiles behave differently at different yaw angles, it is not just the depth which affects the drag.  There was an article on this site a few months ago about a manufacturer designing a new wheel which touched on yaw angle, speed and rim depth, it is worth digging around and reading it if you can

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check12 replied to Welsh boy | 6 years ago
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Welsh boy wrote:

check12 wrote:

Deep rims are better when the wind isn’t head on so the rear wheel gets that benefit. 

That is not 100% true, different profiles behave differently at different yaw angles, it is not just the depth which affects the drag.  There was an article on this site a few months ago about a manufacturer designing a new wheel which touched on yaw angle, speed and rim depth, it is worth digging around and reading it if you can

 

didnt really have time to go in to detail, just made a generally true comment, yes crap wheel is going to be crap whatever depth and a well designed rim profile will be better at a lower depth than another wheel at a bigger depth, lean towards swissside hed flo rather than Planet X 

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Canyon48 | 6 years ago
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A rear aero wheel definitely makes a difference, however, it isn't as important as the front wheel (SwissSide website).

Wheel profiles are optimised only for the front wheel as it makes CFD much simpler due to less turbulence (again, ref SwissSide).

There is data available online about the drag reduction benefits of rear aero wheels.

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Welsh boy | 6 years ago
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It depends on hoow fast you are riding and how much difference an aerodynamic wheel is really going to make to your performance.  Ask yourself why a lot of track riders use a disc wheel in the back and a deep section spoked wheel in the front.  I know that a spoked wheel in the front handles better than a solid disc but if there was no aero benefit in having a disc rear then surely they would ride a deep section spoked wheel in the back too.

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