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Why you should ride a disc wheel in a time trial

One of the things that puzzles me the most when I rock up to a time trial, especially a significant one, is the lack of disc wheels on bikes.

At club level you could be forgiven, especially in Canberra where time trials are few and far between, as the disc is not a cheap bit of equipment.  On average you would be lucky to get to use it three of four times a year.  At over a thousand (and sometimes over two) dollars a pop, that equates to $250+ per ride for the year!  Even over a few years, maybe say 10-15 races, that’s still pretty expensive – more than the race entry fees themselves.

But costs aside there are a range of reasons that are often cited to justify not riding a disc, even when the rider has one available.  It’s too windy.  The course is too hilly.  It’s heavy. I am too small and not strong enough to push it.  These are the most common but in reality you should really be using a disc in 99% of cases. 

Here is why.

Debunking the myths about using disc wheels.

It’s too windy.

This is one of the most common reasons given for not riding a disc.  Claims that it will affect the handling of the bike, make it unstable or just be plain slower than a spoked wheel.  Let’s take a look at these a little closer.

Handling and stability

The issue here is that people perceive the disc for what it appears to be, a solid piece of carbon that will catch a lot of cross wind.  However, when you consider the situation more closely its actually one of the last things to worry about.

A rear disc wheel is situated behind the steering axis of the bike and therefore any sideways pressure on the disc has little affect on the handling.  Sure there is a little pressure but once you're used to how it feels then it shouldn't pose any problems and most riders hardly notice it.

The location of the rear wheel also means a disc has a lot of weight (from the rider) over it, essentially pushing it in to the ground and stabilising it.  The front wheel however doesn’t support as much of a riders body weight and it’s more susceptible to movement from sideways forces. 

The third piece of the puzzle is actually the rider.  The human body is the single largest component of the ‘cycling system’ and as any good time trialist will tell you, having the smallest, most aerodynamic frontal profile is really important when trying to go fast.  The same rules apply for cross winds but its a lot harder to hide the shape of the body and it’s actually the body that catches most of the cross winds.  As the torso is primarily above the centre of gravity point of the bike it therefore has a greater, negative influence on the stability of the bike.

Finally, the aerodynamic properties of a disc wheel are quite unique in cycling and the solid design actually accelerates the flow of air across it from the windward side and releases the low pressure on the downwind side, which also helps stabilise entire bike.  As an added advantage, this also places more pressure over the back of the bike and reduces the load on the front wheel making is less susceptible to steering issues.

If you still need convincing, grab a disc wheel and put it on the back but leave your normal, high spoke box rim on the front and take it out in a cross wind.  It wont take long before you realise its the front wheel that’s giving you all the grief, not the disc at the back.  Then up the anti and put your 50mm or 60mm deep carbon wheel on the front and note the difference in handling.

What this highlights is that while the disc is the logical choice in the wind, the front wheel needs to be chosen with great care and you should experiment with different options and above all, practice riding your preferred combo in training.

 

Aerodynamics and Speed. 

It’s a pretty well known fact that a disc wheel is faster as it creates a ‘sail effect’.  If you look at the wind tunnel test data for disc wheels some even go into negative drag at certain yaw angles (more on yaw angles in a moment).  That means its actually pushing you forward, hence the term ‘sail effect’.

But what about in different crosswinds?   Well that’s where yaw angle comes in to play.  When absolutely stationary the effective wind angle (or yaw angle) on an object is exactly the same as the direction of the wind.  For example a rider facing due west on a start line for a time trial with a wind coming from due north will have an effective wind or yaw angle of 90 degrees.   At this angle there is lot more sideways pressure on the rider.

But what happens when the rider starts moving in that westward direction?   You might be surprised to know that the effective wind angle actually reduces, and even more interestingly the faster you go the further the angle reduces.   To provide an actual example (real numbers), lets take our westward bound time trialist who has a 15kmh crosswind directly from the north.  At the start line the yaw angle is 90 degrees as they are stationary.  But once they start and hit 20kmh (quite slow really) the wind angle changes considerable, reducing to ~37 degrees - that’s a reduction in the yaw angle of a whipping 53 degrees!  Add another 10kmh to the velocity taking it up to 30kmh and the wind angle drops to ~27 degrees.  At 40kmh the wind angle is further reduced to ~20 degrees and then to ~16 degrees at 50kmh.

So you can see that once the rider gets up to speed (even a modest 30kmh) the wind is suddenly acting more like a headwind (or mild cross headwind) and this in turn further reduces the sideways forces on the rider, AND the disc!

But what about stronger winds?   Lets look at a 30kmh northerly wind acting on our westbound rider.   At a ground speed of 30kmh the rider will experience an effective wind angle of ~45 degrees.  So a solid cross headwind, but still a 50% reduction in the effective wind angle from a stationary position.   Up the riders speed to 40kmh and the angle comes down to ~36 degrees and then ~30 degrees at 50kmh.  So a stronger cross wind but still nowhere near as severe as most riders ‘think’ it is.

Why is all this important, well for a disc wheel it means that the sideways forces on the disc are greatly reduced and this in turn further reduces any instability.  The front wheel on the other hand is a whole different story as it’s the foremost part of the bike and how it catches the wind has a large part to play in the overall aerodynamics and stability of the bike. This is why wheel choices in cross winds mainly need to be concerned with front wheels rather than the rear disc.

As for the aerodynamics of a front wheel.  Well that’s a little beyond the scope of this article, however the more the effective wind angle becomes like a head wind, the faster a deeper section front wheel will be.  But at a certain angle, called the ‘stall point’ the wheel will actually increase drag, become slower and more susceptible to ‘buffeting’ from the cross wind and therefore feel unstable.   Various wheel/rim profiles and designs have different characteristics at different yaw angles and one of the reasons a lot of the pro tour men are using the uber deep front wheels is that they travel at very fast speeds (50-60kmh) and at this velocity the effective cross wind can be as low as 10-25 degrees, even with strong cross wind (30kmh).   The deeper wheels provide a larger aerodynamic advantage in these conditions but if your average, everyday punter used the same wheels, on the same course and with the same wind conditions they would find their bike felt less stable as their lower speed increased the effective wind angle operating against the front wheel.

Rider Size

This one has two aspects that need to be considered, physical size and power output.

Firstly, if you go back and ready the section above on handling you will recall that body size has a lot to do with the stability in cross winds.  A smaller rider presents a much smaller sideways cross section that a larger rider and as such they are less likely to experience stability issues when riding any combination of wheels, disc or otherwise.  A larger rider on the other hand will catch more wind, but they are also often capable of much larger power output and this leads to the second compensatory affect – forward velocity and effective wind angles. 

Larger riders, or any rider for that matter, that is capable of achieving higher power outputs will be able to sustain a higher forward velocity and therefore reduce the effective wind angle on therefore increase their stability. Smaller riders with higher power outputs also present a minimal frontal area and reduced aerodynamic drag.  So small, pocket rocket types, get multiple benefits from riding the disc in the cross winds.  This is most notable for professional female cyclists, who are often only 50-60kg but can travel at speeds greater than their larger male rivals.

At the other extreme is the rider with a lower power output, and therefore lower relative speed.   If the rider is has a larger body surface area then they are likely to have higher frontal drag as well as a large side-on profile.  The lower speed will therefore increase the effective wind angle (remember the example above 37 degrees at 20kmh) but the extra weight will also help stabilise the disc so there is some offsetting of the disadvantage.  This then leaves the smaller rider with lower power as the one (of two) general exceptions to the ‘just use the disc’ rule and in the end it will very much depend on the course and prevailing conditions as to whether the disc is the right choice for this type of rider.  If its a calm day then by all means use the disc, but if you’re in this category you should make sure you practice with a disc in various wind conditions and pay extra note of the wind speed and direction on race day to ensure you make the right choice.

It’s too hilly

When most people are faced with a hilly time trial their first thought is to ditch the disc in favour of light weight climbing wheels, which usually have a much lower profile, anywhere from 30-60mm deep.   The issue here is more of a lack of understanding about the effects of the different elements of the rider and bike on velocity. Everyone knows (or thinks they do) that lighter climbs faster.  Right!   Nup – wrong.    At moderate gradients a rider can still maintain a sufficiently high velocity that the wind resistance still plays a significant role in determining the overall speed of the bicycle.  As such, the aerodynamic properties of the rider and their equipment is also important.  

But at what point do aerodynamics gains and the extra weight cancel each other out.  Various studies have shown that the aerodynamic benefits are usually still the winner on slopes of up to 5-6%.  That’s actually quite a solid climbing gradient and with lots of long hills averaging somewhere between 4-7% it means you need to seriously consider that the disc will still be fastest option.  Even if your course does have a sharp climb in it, say 10% for 1km, its not automatic rejection of the disc as you then need to factor in the remainder of the course.  The disc may slow you down a little on the climb, but there may still be plenty of flatter kilometres to ride where that small time loss can be wiped out and still more time saved.

What this means is that unless you have a predominantly uphill time trial, or have a course with a prolonged climb of 5-6+% then the disc is still going to be your best option.  Of course the actual weight and aerodynamic properties of your specific disc wheel will play a part in exactly what gradients benefit the disc or climbing wheels.

A final point worth noting about the disc on a hill.  One of the primary benefits of a light weight climbing wheel is it’s low rotating inertia.  This makes it easier to accelerate and this can be quite important when climbing in road races.  However, as a time trial is best paced more evenly, the heavier disc doesn’t limit the rider as much as it would in a road race scenario.  In fact the disc tends to carry more momentum and this can actually help maintain speed on a climb, especially over small rises on an otherwise flat course!

A note on front wheel stability

While this blog article is primarily about the benefits of riding a disc wheel it is worth spending a moment to consider the real issue of front wheel stability in windy conditions.

If you have read this far it should be pretty clear that the design of the front wheel plays a big role in the stability of the bike.  Now, when doing a time trial there is one thing that will slow you down more than anything - sitting up out of your aero position.  But this is exactly what people do when they feel unstable.  They get up off the aero bars, grab the hoods or bull horns and try and stabilise the bike.  If that’s you, then you seriously need to consider changing your front wheel – or just spend more time getting use to the movement.  Sometimes it helps to think of the bike as a yacht, tacking it’s way along the road, however in the end only experience will tell you whether the bike is truly unstable or not and whether you need to change your front wheel.

For what its worth, a tri-spoke is usually the best option when you feel there is a stronger cross wind and if you can get one of these you should notice that you feel more stable when its windy, and more stable means more time getting good power to the pedals.

Summary

The humble, but otherwise expensive, rear disc wheel has put up with a lot of flack over the decades.  It gets left at home wondering why it’s been ignored, knowing full well it would make its owner faster on the day.   It can’t understand why you don’t want to use it but hopefully now, after having read this article you will be able to start to see things from your disc wheel’s point of view.  It’s your friend.  It wants to make you faster and more stable and all it asks in return is that you take it out on race day (and some practise runs as well) and show it some love.

 

Jason Mahoney Thursday 26 September 2013 at 8:17 pm | | Tips




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