Talk to any non-cyclist, and it won’t be too long before they might ask you: "Why don't cyclists use cycle lanes?" Or alternatively (and preferably): "What’s the fastest you’ve ever gone on a bike?" Obviously, the answer that we all want to be able to give to the second question is that three-figure number of over 100kph; however, it’s certainly easier said than done to hit this elusive number. Here's my experience of trying to break the barrier.
A few years ago I did actually proudly join the 100kph club, which is just over 62 miles per hour for those who prefer old money. This was on a very long and very straight descent in the alps, but there's always a chance it could have been a one-off or a GPS error. It would be nice to do it again with more proof.
Usually, when working on a feature like this I'm pretty sure that I am actually capable of achieving the feat before starting. With this one, I was less confident. What that does mean is that you can benefit from my first-hand trial and error approach and/or watch me walk and ride back up a hill MANY times!
Descending at high speeds: the disclaimers
Just to be clear, the point of this article and video is certainly not a call to encourage you to head out and try to hit an arbitrary (very fast) speed on the first steep road you come across on your next ride. My attempts took place on very well maintained, deserted roads with lots of visibility, and I'm an experienced descender.
The most important takeaway from this should be to ride safe when hitting fast speeds. Keep your wits about you, make sure your bike is mechanically sound and remember that this isn't something to be attempted on a whim. Just take a look at how rarely even the pros hit these speeds (on fully closed roads) and that should give you some indication of how difficult it is for even the most experienced riders.
For most riders with some road cycling experience, hitting speeds over 50kph is going to feel very fast, and it's important to work on bike handling skills and confidence to improve descending speed. Check out our top tips for better descending as a very good starting point to get down descents safely and quickly.
> Best aero road bikes 2022 — wind-cheating bikes with an extra turn of speed
To re-emphasise those disclaimers again, this is not something I would ever want to attempt in the UK, or most places for that matter. Not only are many roads poorly maintained where I live, but they can also be slippery and busy with traffic. To do anything like 100kph, you need a clear view of a LONG way in front of you. Luckily for me, a recent training camp was the perfect opportunity, with long, fast and straight traffic-free descents.
So, for my initial attempt, I found a nice straight road for obvious reasons. I GoPro’d myself up and waited for a dry day, as it’s a really bad idea to fall off doing anything close to 100kph... I lost enough teeth and skin at 50kph! Rain won't slow you down exactly, but it will drastically reduce your visibility and grip, and increase your stopping distance if you need to brake.
Most of the best cycling computers have the functionality to tell you both your current speed and your max speed of that ride. Please oh please, if you're trying to descend quickly, don't glue yourself to the screen! On a descent you need to focus on the road ahead of you and not a tiny display, seeing the number won't make you any quicker. There is only one exception to this and it's if you're looking at a map. It can be useful in some cases, and it's been known for professional riders to bring up the route on unknown descents to visualise corners in front of them.
Hitting higher speeds
To gain those extra 20 or so kilometres per hour, it’s clear that some changes were going to have to be made. More weight can increase descending speed, and while I'm committed to the cause I wasn't going to put weight on specifically for this exercise, so I was stuck at around 72kg.
> Watch: Lorry-drafting Brazilian cyclist hits 168 kilometres an hour
The single largest difference that you can make to your top speed (other than drafting something of course) is the where and the when. Even the strongest riders can't hit 100kph on a twisty, shallow descent with a headwind. You'll also need a serious amount of stopping distance, with good visibility of traffic and your surroundings with no junctions, crosswind gusts or anything else unpredictable.
In these situations apps like Strava and komoot are your friend, because they allow you to quickly and easily find steep and straight descents that are going in the same direction as the wind. I quickly pinpointed my target hill and the section that I would be aiming to hit my top speed on. All I had to do then was wait for right wind conditions, a 20kph tailwind ideally.
Next, I considered what’s actually slowing me down when barrelling down a descent; and there are only really two things. This is a vast simplification, but I’m ignoring drivetrain losses as I probably won’t be pedalling when hitting my top speed. Vibrational losses are still a fairly grey area, and there's not a whole heap I can do to improve them.
That leaves two things. Firstly my rolling resistance, which is caused by the contact patch between my tyres and the road. Your contact patch on a road bike is only about the size of a two pence piece, but to minimise resistance as much as possible I switched to a set of racey summer tyres. I opted for Challenge’s new handmade Criterium RS TLR tyres (full review coming soon). I opted for a 27mm size to balance aero and rolling resistance.
I also set the tyres up tubeless as we’re told that this offers the lowest rolling resistance because there is no energy wasted on deforming a tube.
The second thing that slows descents down - and this is the big one - is aerodynamic drag. This comes in two main forms: skin friction drag, basically the air sticking to clothes and the body on its way past; and form drag, which causes a big turbulent wake behind the rider, and is influenced mainly by frontal area.
As a cyclist's speed goes above around 20kph, it's drag which becomes the main force impeding you. At 100kph any other forces are all but negligible, and small differences can begin to have a large effect.
From the equation above we know that drag equals 0.5 x Rho (air density) x Velocity2 x Coefficient of Drag (the shape I am) x My frontal area.
So, what did I do to make myself more aerodynamic? Well first up is a wheels swap to try and reduce that Coefficient of drag. I ditched the shallow sections for a set of Parcours Chrono hoops (review coming soon) that are 68mm and 75mm deep front and rear respectively. I also sized down my clothing to make sure it sat as close to my body as possible and didn’t flap about, which should be of benefit to both my coefficient of drag and frontal area. I also opted for a semi-aero helmet, aero socks and shaved my legs (in February, this is serious!)
Having failed another few times, I set about doing a bit more research on the weather. I waited for a day with a lovely tailwind and fitted a full-size 53t chainring instead of the 52t one to help me continue pedalling up to a higher speed. I also do finally spin out on the steepest part of the descent, going into the top tube 'super tuck' position to try and really reduce my frontal area (just nobody tell the UCI).
> Should you buy an aero road helmet? How much faster could you be?
A study by Bert Blocken (linked above) shows that the super tuck, where you sit on your frame's top tube, is significantly faster than other positions. Whereas in my previous attempt my position was 'Back down 2', the super tuck or 'Top tube 4' position is around 3% faster at the 72kph test speed, and likely even more at my 100kph target.
How challenging is it?
Even with all these changes it still took me most of a day to reach my target speed, which shows you that 100kph is pretty unobtainable in most situations. When travelling at high speeds on a descent it's important to try and stay relaxed, because if you clutch at the bars then you will just exacerbate any speed wobble. This could be a recipe for disaster.
To finally hit the elusive 100kph (it's elusive for good reason) I had to enlist the help of a mate for a lead out. Whilst drafting someone you'll be expending around 20-40% less energy, and in this case when speeds are high it would likely be near the top end of that.
Was it worth it? No, probably not... I rode up and down one hill FAR too many times! Even with an entire day at my disposal, perfect conditions and the perfect road, this was certainly a challenge.
Once again, the most important factor when riding your bike is safety. If you do feel that you have the necessary experience and skill to see how fast you can go on a descent, and find yourself in the required location where this is both safe and legal, then make sure your bike has had a good check over first, preferably with a second opinion from a professional mechanic.
What's the highest speed you've hit on a bike? And where did you do it? Let us know in the comments section below...
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