Aerodynamic rear wheels?

[graymee]

I'm watching the highlights of the TdF team time trial and was wondering why all the riders had filled in rear wheels. Is it a cover over the spokes? Someone will probably say it's for streamlining but why not use one all the time for ordinary road stages for example?

[[XAP]Bob]

It is an aero aid, but they also catch side winds (hence not used up front outside)

I imagine that in a peleton the shifting winds as you go to the lee of others with such wheels could cause serious issues...

[uphillbothways]

UCI rules say that no part of a bicycle may exist for purely aerodynamic reasons, but must also be a structural part of the bike. The disc wheels you see in time trials have no spokes and are one piece of carbon fibre, so the skin of the wheel is a structural part rather than a fairing. Wheels of this type are forbidden in massed-start races for safety reasons, as they act like a sail in sidewinds.

On a normal stage, you'll see that the wheel rims are very deep. These deep carbon rims provide substantial aerodynamic benefit, but are a compromise between speed and stability in sidewinds. You will occasionally see traditional shallow rims being used on very windy stages, or on races like the Paris-Roubaix or the Eroica where rough roads could damage fragile carbon rims.

[theopr]

Deep section rims provide a noticable aerodynamic advantage at higher speeds but as the speed drops off so does the aerodynamic effect. Below a certain speed lighter shallow section rims will have the advantage. I expect the main contenders will all be on shallow rims in the mountains.

[[XAP]Bob]

UCI rules say that no part of a bicycle may exist for purely aerodynamic reasons, but must also be a structural part of the bike. The disc wheels you see in time trials have no spokes and are one piece of carbon fibre, so the skin of the wheel is a structural part rather than a fairing. Wheels of this type are forbidden in massed-start races for safety reasons, as they act like a sail in sidewinds.

So if I ditch the conventional frame and make an exoframe of fine CF...
It's clearly a faring, but it would still be an upright, and it would be structural...

[Neil Mc Ivor]

For any New Scientist readers almost the same question is asked as "This week's question" in "The Last Word" page of the magazine 2 July 2011.

[SilverBadge]

So if I ditch the conventional frame and make an exoframe of fine CF...
It's clearly a faring, but it would still be an upright, and it would be structural...

The dimensions of frame members are strictly defined in the UCI regs because they have anticpated people like you There is no such thing as purely structural or purely aerodynamic so the rules are framed such that shape choice doesn't get too gratuitous for aero purposes. There's always HPVA if you feel too constrained.

[[XAP]Bob]

So if I ditch the conventional frame and make an exoframe of fine CF...
It's clearly a faring, but it would still be an upright, and it would be structural...

The dimensions of frame members are strictly defined in the UCI regs because they have anticpated people like you There is no such thing as purely structural or purely aerodynamic so the rules are framed such that shape choice doesn't get too gratuitous for aero purposes. There's always HPVA if you feel too constrained.

I don't feel constrained - I just don't race

The UCI just don't like progress - when were the numbers last changed?

[uphillbothways]

The UCI just don't like progress - when were the numbers last changed?

They've been changed repeatedly, getting progressively stricter as technology advances. The most recent was the rule mandating a maximum aspect ratio of 3:1 - no part of the frame or handlebars may be greater than three times as deep as it is wide. It nearly bankrupted some handlebar manufacturers, who were left with a huge investment in tooling for aerobars that became illegal overnight.

Astonishingly, there are still several rules in the book that were written specifically as a result of Graeme Obree, some of which cause real problems for bike designers and fitters. Almost all modern time-trial bikes come with a dual-position seatpost to alter the effective seattube angle - one position for triathletes, another to meet a UCI rule introduced to ban Old Faithful.

I think the main gripe is the inconsistency and uncertainty around the enforcement and interpretation of UCI regulations. A blind eye was turned to all sorts of blatantly illegal stuff, while at the same time people were being stung on technicalities. A lot of people in the industry feel that UCI regs exist mainly to benefit the marketing departments of big manufacturers.

If you want to get your head around the current regs, there's a condensed guide available from the UCI, although even that isn't particularly clear.

[Steve Kish]

I used an early disc wheel in the late-1980s for time trials. I didn't really notice any benefit over my 28 spoke rear CX18 and as this was a solid lump of carbon, it was very twitchy on the corners. Used it for a year before I sold it on.

In all probability, they've now improved quite a lot.

[gilesjuk]

The UCI hold back the development of bicycles. If F1 had such a body they would still be driving wingless cars made of steel that kill people when they crash (carbon fibre breaks and absorbs energy).

[SilverBadge]

They've been changed repeatedly, getting progressively stricter as technology advances. The most recent was the rule mandating a maximum aspect ratio of 3:1 - no part of the frame or handlebars may be greater than three times as deep as it is wide. It nearly bankrupted some handlebar manufacturers, who were left with a huge investment in tooling for aerobars that became illegal overnight.

The rules underwent a major revision for 2000, the "changes" since then have largely been clarifications to eliminate some wishful thinking on behalf of equipment manufacturers. All any manufacturer has to do is make one design that is UCI-legal and they can make plenty of other gear for triathlons etc.

Astonishingly, there are still several rules in the book that were written specifically as a result of Graeme Obree, some of which cause real problems for bike designers and fitters. Almost all modern time-trial bikes come with a dual-position seatpost to alter the effective seattube angle - one position for triathletes, another to meet a UCI rule introduced to ban Old Faithful.

The main thing about Old Faithful was the handlebars, though the seat angle was aggressive. Unsurprisingly, once this genie was let out of the bottle, the rule to prevent it has to exist for ever. People riding in extreme positions with reduced levels of machine control was deemed to be not the route the sport should pursue. It's entirely up to manufacturers how they accomodate this and other branches of the sport.

I think the main gripe is the inconsistency and uncertainty around the enforcement and interpretation of UCI regulations. A blind eye was turned to all sorts of blatantly illegal stuff, while at the same time people were being stung on technicalities. A lot of people in the industry feel that UCI regs exist mainly to benefit the marketing departments of big manufacturers.

Whilst simultaneously bankrupting handlebar manufacturers? Perhaps you could list some of the "all sorts of stuff".

The UCI hold back the development of bicycles. If F1 had such a body they would still be driving wingless cars made of steel that kill people when they crash (carbon fibre breaks and absorbs energy).

Though on here carbon is regarded as the stuff that fails catastrophically and unpredictably F1 has plenty of material restrictions - some for safety (magnesium less than 3mm thick, chrome plated steel), some for performance restriction for safety and improved racing (e.g brake caliper material) and some for environmental reasons (e.g beryllium alloys). UCI has a minimum bike weight and safety tesing of wheel designs (though no specification of required materials. F1 has far more dimensional restrictions too.

[uphillbothways]

Whilst simultaneously bankrupting handlebar manufacturers? Perhaps you could list some of the "all sorts of stuff".

Niche, high-end manufacturers can be absolutely crippled by a rules change, as it can undermine their entire business. For the likes of Specialized, Giant and Trek, racing is only a small part of their business and retooling costs are easier to finance and amortize. It was the likes of 3T and Oval that suffered, not the big boys that hold sway in Switzerland. The same argument is being made about the UCI frame approval - for volume manufacturers, £8000 per design is hardly noticable, but for little guys and custom manufacturers it's a serious expense.

As regards all sorts of illegal stuff, you just need to look over the pro bike photos at Velonews or Pez with a copy of the rulebook in hand. The most obvious and egregious example is article 1.3.007, which mandates that bicycles or accessories must be "of a type that is sold for use by anyone practising cycling as a sport". The regulation has been enforced to prohibit custom, prototype or modified parts, but there seems to be no consistency in enforcement and non-conforming parts are commonly permitted to race. Likewise with wheels not complying with 1.3.018 - it's quite common to see wheels not on the approved list, albeit usually with a different manufacturer's stickers on the rim.

[mr riff raff]

On a normal stage, you'll see that the wheel rims are very deep. These deep carbon rims provide substantial aerodynamic benefit, but are a compromise between speed and stability in sidewinds. You will occasionally see traditional shallow rims being used on very windy stages, or on races like the Paris-Roubaix or the Eroica where rough roads could damage fragile carbon rims.

Err, apart from this year when Johan van Summeren won on Mavic Cosmics.

[uphillbothways]

On a normal stage, you'll see that the wheel rims are very deep. These deep carbon rims provide substantial aerodynamic benefit, but are a compromise between speed and stability in sidewinds. You will occasionally see traditional shallow rims being used on very windy stages, or on races like the Paris-Roubaix or the Eroica where rough roads could damage fragile carbon rims.

Err, apart from this year when Johan van Summeren won on Mavic Cosmics.

I didn't say that nobody used carbon rims on pave, just that shallow rims are often preferred in difficult conditions. By my estimation about half the field at this year's Roubaix started on box rims. The front-runners were mainly on carbon, but that's a much easier gamble when you've got a domestique ready to give up his wheel.