Wheel Selection   The three most common questions I get asked concerning race wheels are: Which wheels should I use? How much time will they save me? How fast do I have to go for them to work?   I’ll deal with the easy questions first…   How much time will they save and how fast do I need to go to get a benefit?  Time savings with aero wheels will be around the 1 minute mark at Olympic distance, 3 minutes for a Half and around 6-8 minutes for the full 180 kilometers.  If it’s a windy day with lots of cross winds these time savings can increase even more.  You can also expect some time gains on the run from saving some watts while on the bike.  Anyone that has ridden a really nice set of aero wheels knows they are faster but they may not know why or by how much.  It doesn’t matter what speed you travel at (from 25kph to 45kph) you’ll get a similar time saving advantage.  There is a common misconception (particularly about disc wheels) that you need to travel at a certain speed for them to work (speeds of above 35kph are often listed as the ‘minimum’ speed required to obtain an aero advantage).  In short this is absolute rubbish.  Anyone that has gone for a ride in baggy clothing will quickly realise that the drag associated with what you are wearing can be felt at speeds at low as 10-15kph – aero wheels work along similar lines.  As long as you can go faster than about 20kph then your aero wheels are working for you.  Interestingly in gross percentage terms of time saved, a slower rider will save a greater percentage of time with aero wheels than a faster rider so don’t worry about how fast you need to travel to justify a new wheel set, you already go fast enough.   Now for the really difficult question – what wheels will work best?   As a general rule you want to run the deepest set of aero wheels that allow you to remain on the aerobars.  As soon as you need to lift of the aerobars and revert to the base bar to control the bike (when hit by a side gust of wind) you blow all your aero advantage out the window.  You are roughly 30% less aero on the base bar than you are on the aero bars and at this point your expensive deep sided wheels have become part of the problem, not the solution.  It’s important to realise that what we’re really talking about here is the depth or surface area of the front wheel only, not the total wheel package.  It’s the front wheel that is predominantly responsible for moving a bike around on the road and making it unstable.  On an aero tubed tri bike, (and to a lesser extend on any bike) the rear wheel is shielded by the frame, the rider and the flow of air down the side of the bike.  Side winds have only a limited effect on the rear wheel, so it’s really only the front wheel that is a major concern from a bike handling perspective.   In summarising which wheels to use I’ll refer to Zipps various depths (404, 808 etc).  I’ll do this for two reasons, firstly, most people know what sort of depths I’m referring too when I list these wheels as examples (Zipps have become the defacto tri standard in rim depth description) and secondly because it’s a concept that Zipp pioneered so I’m giving them the credit for the wheels that we now all take for granted while on course.   For the larger, heavier rider with above average handling skills the fastest wheel will be either a deep bladed wheel (Hed’s famous H3 is the best known of these) or a very deep section spoked wheel (Zipp’s 1080 is perhaps the most recognised in this category).  BUT, only if the rider can keep the bike straight and stay down on the aerobars in windy conditions.  Remember, as soon as you have to leap on the base bar to control the bike you’ve shot your aero advantage to pieces.  For an averaged size triathlete (lets say around the 5ft10’ mark) with average bike handling skills then a wheel of 808 depth or similar on the front will normally be the fastest and most controllable in a wide range of conditions.  A disc will nearly always be the fastest option on the rear wheel but a disc has a few issues specific to this type of wheel (see notes below on the pros and cons of choosing a disc).  So the average person with average skills will nearly always get a fastest result on a set of 808 depth wheels.  For the shorter, lighter and less confident in the bike handling department then move to a lesser depth of rim such as a 404 (add a slightly deeper rear wheel for virtually no loss of handling and a slight edge in speed).  If conditions become so severe (unpredictable side gusts or extreme weather) that using any deep rim wheel presents a problem then a standard set of road wheels is the only option.  Its worth noting that not all deep rimmed wheels are created equal.  Rim shapes with a toroidal bulge shapped rim (pioneered by Zipp and Hed) are vastly less affected by side loading and easier to control on windy days.     A simple guide to wheel tech terms:   Yaw– In simple terms let’s look at yaw as being the apparent angle that the wind hits a wheel.  Imagine travelling at 30kph on a completely still day.  The wheel would be hit by a 30kph head wind (apparent) at this speed.  Now imagine you are travelling at 30kph and you are riding with a 30kph wind that is hitting you directly on your side.  Your wheel would feel an apparent wind angle of around 45 degrees from the front and the wind strength would feel closer to 45kph.  The most common apparent wind angles are usually in the realm of 15-20 degrees from your front.  Again, in general terms, a deeper wheel will yield a proportionally greater advantage at greater yaw angles.  In simple terms deep wheels provide an even greater advantage in cross winds and cross winds are more common than no wind or an exact head/tail wind.  To further complicate matters, a bladed wheel (back to our Hed 3 spoke) will be much faster than any other wheel option but only through a very small (say 10 degree) wind angle at around 20 degrees.  The best results in a wide range of wind angles are obtained by the mid depth of wheel when taking bike handling issues into account.  Yaw angles are complex to calculate exactly and on any given course they change all the time so picking a wheel to be best in ‘most’ conditions is the aim.   Toroidal shapes.  Invented by Hed and Zipp, perfected by Zipp.  Rim shapes now resemble aerofoil shapes more than wheels to provide less drag across a greater range of yaw angles.  Another spin-off of this shape is that it is less wind affected in cross winds.  The latest Zipp ‘Firecrest’ shape is almost completely unaffected by side loading and gusts.  All the marketing hype aside, it actually works – get some, if you can afford them.   Blades vs Spokes.  At around 20 degrees of yaw or more (apparent wind) the bladed spoke wheel will usually test fastest of all.  At these greater yaw angles the wheel is working like a wing and developing its own lift, effectively driving the bike forward.  While this sounds great there are some drawacks.  The wheels are massively affected by side loading/gusts (you can end up in on the side of the road wearing your bike) and angles of between 10-20 degrees yaw are more common.   Dimples– in a straight line with no change in wind volume or direction, dimples offer no speed advantage that can be measured.  So why have them?  Other than the obvious marketing bling advantage, dimples offer a solution to the problem of changes in wind angle and strength.  Rapid changes of wind direction across a wheel cause the wheel to stall and adds drag.  Dimples assist the air to re-attach faster in a smooth laminar flow so they reduce the incidents of the wheel being ‘stalled’.  Don’t worry to much about dimples – while very clever, the effects are minor.   Discs – Generally faster than any other wheel option but there are some trade offs.  The rigid nature of a disc makes it prone to skipping out on rough corners and the ride comfort is no-where near as nice as a spoked wheel.  This may not matter for a 20km TT but on a rough road a 180k effort on a bone jarring disc can hurt you on the run.  Also, the gyroscope effect of the disc is more pronounced which is great in a straight line but again will hamper your cornering speed.  Discs are best used on flat and smooth courses.  Interestingly disc wheel covers offer similar time advantages as the disc and have the benefit of spokes for comfort.  Zipp’s Sub9 (the disc with a toroidal bulge near the rim) offers some compliance and generates lift (forward movement) at greater yaw angles, probably the best rear wheel for triathlon but does cost more than most bikes.