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Why riders like you need to get more aero and wheel weight doesn't matter

Wind tunnel evidence that it really is aerodynamics that makes the difference

I went to a wind tunnel and learnt a few things. The first thing I learned is that a wind tunnel isn't necessarily a tunnel.

I had an image in my mind of a fully enclosed and pristine tube like you see in the car ads. Octagonal, probably. Maybe with smoke trails, and definitely with a big fan at one end. The reality was a bit different. 

Swiss Side Wind Tunnel 25.jpg

The GST wind tunnel at the Airbus Defence and Space facility on the outskirts of Friedrichshafen scores on the fan front. But basically it's just a room. There's a table in it, with a bunch of tools on, and a stepladder propped up in the corner. There's some feather flags filling some space, and a projector hanging from the ceiling. Pristine it ain't.

Swiss Side Wind Tunnel 22.jpg

It's an old facility; it used to be the Dornier wind tunnel when Dornier were making planes, and there's still a goodly number of switches and filament bulbs in the control room. Now it mostly splits its time between bike parts and scale models of drones. But it's a good tunnel, says Jean-Paul Ballard of Swiss Side wheels, who's invited me to spend an hour or two looking at the process. He's got a bit of experience with aerodynamics, having worked for 14 years in Formula 1 as the lead engineer on a number of teams. Between them, the team behind Swiss Side have over 50 years of F1 experience, which obviously includes a whole stack of wind tunnel time. 

"It doesn't look like much but it's incredibly consistent", he says of the GST tunnel. "One of the problems with an enclosed tunnel is the blockage effects. This is important when testing with a rider as results can be significantly influenced. Here the flow isn't constrained". And, for the record, the fan sucks. "We laugh when we see pictures of wind tunnels in adverts with the bike pointing the wrong way". It's obvious once you think about it: if you want a clean flow of air you don't want a massive fan chopping it up a few feet from your test rig.

So the fan pulls the air through, and the intake at the other end is as smooth and uniform as possible to create the best conditions for consistent results. And the extra space in the room means the natural flow expansion and turbulence created has somewhere to go without affecting the readings on the rig. And all that makes for lovely, repeatable numbers. Stepladder or no.

We spent a bit of time running a standard series of tests. There's plenty to take out of the few tests that we did (and some we didn't), so let's pull out some of the most interesting bits of information we garnered. Starting with this...

1. The slower you are, the more important aero is.

Well, up to a point.

That's right: all you choppers out there – and I count myself firmly among your number – need aero most of all. That's madness, right?

Wrong. This seemingly counter-intuitive bit of information is explained by a single graph from the basic tests we ran during our hour at the tunnel. Here it is:

Swiss Side Wind Tunnel 29.jpg
Drag v yaw angle (click image to embiggen)

So what's this showing? It's showing the drag of a range wheels at various angles of yaw. The blue line at the bottom is an Argon 18 time trial bike; ignore that slippery beast for now and concentrate on the four lines at the top. That's four different wheels aboard the same frameset, a Cervelo R5 (Not a P5 like the legend says). They're all Swiss Side wheels: The box section Heidi (light blue) and three depths of their toroidal Hadron rim. The 80mm Hadron 800+ is in red, The 62.5mm Hadron 625 in orange and the 48.5mm Hadron 485 in green. Got that?

What's yaw?

Yaw is the angle of the wind relative to the bike. So if you're cycling directly into a head wind the yaw angle is zero, and if you're at a standstill with a direct crosswind, it's 90°. When you're moving, the yaw is a combination of the wind speed, the wind direction and your speed.

The first take-home point here is to look at the drag from the bike at a yaw of zero. Whatever the wheel fitted, the drag of the bike as it faces directly into the wind is more or less the same. We (well, I, anyway) mostly have a notion that those big, chunky rims make a nice shape that slices through the air straight on. In reality, that's not where the gains come from.

Once the yaw angle starts to creep up then you start to see a marked difference in the aero-optimised wheels compared to the box section. The Heidi wheel keeps increasing in drag over about 6° while the Hadron wheels begin to drop back down again until at 20° of yaw their lines are well underneath the standard rim.

Why? Because the air flow stays attached to the rim, rather than breaking off and causing turbulence that drags against the bike. It's at these higher yaw angles that the gains are most apparent. That's where you're making your time up.

More speed, less yaw

Swiss Side Wind tunnel 10.jpg

So, imagine a rider travelling at the speed of light. The wind will always be straight on, a yaw angle of zero: however hard it's blowing, the wind speed and direction will have no discernible effect. As you slow up, the fact that you're moving more slowly means that a greater range of yaw angles are possible for any given wind speed. And if you slow right down until you stop, anything goes: the yaw angle is whatever the wind direction is, relative to the way you're facing.

Followed all that? The practical upshot is that the faster you go, the lower the range of relative wind angles you'll experience. Professional riders in normal conditions won't see anything over about 10°, whereas us sportive bashers and lower-category chuffers will see much higher yaw for the same wind, because we can't go as fast through it. And it's those higher yaw angles that see the biggest gains, up until about the 20° mark when the airflow detaches from the rim and you lose the aero advantage.

Swiss Side Wind tunnel 5.jpg

"Faster riders generate more drag", Jean-Paul adds, "because drag is proportional to the square of velocity. But faster riders are also on the course for less time, and experience a narrower range of yaw angles. Through our simulations, we see that slower riders actually save more absolute time. They're out on the road for longer and can therefore benefit from the bigger aero gains for longer."

So go out and buy some new aero wheels, ordinary rider. Tell them I said it was okay. Make sure you fit the right tyres, though. Because...

2. Fit the wrong tyres and your wheels won't be aero

This was perhaps the most surprising bit of information from the whole session. It's not something that we tested on the day, because unfortunately there wasn't time to swap masses of tyres out and in, but Jean-Paul from Swiss Side told me they'd been testing plenty, and the results are interesting. More than interesting. It's all about what kind of airflow is passing over the wheel.

You'll often see trips on aerodynamic profiles; small raised or rough sections. Some bike wheels even have them. These trips cause turbulence, and the turbulent layer of air helps the flow to stay attached to the surface. The tread pattern of a tyre can act as a trip and help the wheel to remain aerodynamically efficient at higher yaw angles. It can do that. It doesn't always do that.

Swiss Side Wind Tunnel 28.jpg

"The best tyre we've found is the Continental GP4000S", Jean-Paul told me, and that's what our test bikes were shod with. Is the tread pattern designed for that job? Probably not, he conceded, it's more than likely just a coincidence. But there are other tyre manufacturers that are definitely designing their sidewall profiles with aerodynamics in mind, whether they're talking about it or not.

And how much difference does it make?

"If you fit a GP4000S to one of our Hadron wheels the flow will stay attached to 18 or 20° of yaw," Jean-Paul said. "With another tyre, one with a completely smooth profile, the figure drops to 8-10°".

One look at our yaw graph above shows that you're losing almost all of the aero advantage of a deep section if that's the case. That's a major issue, and one that most people won't have considered at all in the whole aero equation.

So how will you know whether your tyre is a good one, or a bad one? Well, you won't. Except if you're running GP4000S tyres, in which case you're in luck. Or if you're running tyres that don't have any tread profile at all, which is probably bad. The Swiss Side guys have been running tests on a wide range of tyres: there's boxes of them knocking about at the wind tunnel. I'm not sure they have plans to release that information to the general public, at least for now.

Swiss Side Wind Tunnel 27.jpg

So, you've got your deep section wheels and they look pretty pro, and your tyres with the (hopefully) effective trip profile. But those big old rims are heavy: maybe a couple of hundred grams over a climber's wheelset. Isn't that going to negate all that precious aero advantage?

Well, no. because...

3. Weight is a secondary issue almost all the time

What's more important, light weight or aero? That's simple, according to Jean-Paul. Aero trumps light almost every time.

Swiss Side's aero range has - up until now - used a non-structural carbon aero profile with an alloy rim and braking surface. It's simple enough to get the right profile with a non-structural fairing, and the alloy braking surface means more consistent performance from your brakes. The alloy bead section is easy to extrude, too, which means the wheels are cheaper. The penalty in terms of weight is about 100g over going to a full carbon construction, and you're adding that on to the extant weight penalty of just having a bigger rim made of more stuff. What kind of time penalty is that going to add up to?

Swiss Side feed aero and weight data into a model that crunches the numbers for different types of ride profile and length, and then spits out the likely speed and timing penalties based on a reference ride. One of their programs is a 120km rolling ride with 1200m of height gain. Their 'average' rider completes this parcours at exactly 30km/h (211.4W average power), which left me wishing I was average. But enough about my failings. What difference would one hundred grams (from an 8kg bike to an 8.1kg bike, with a 75kg rider) do to the ride time?

Swiss Side Wind Tunnel 31.jpg

Well, it would increase it.

By three seconds.

Adding weight to a rider going that fast, over that terrain, makes precious little difference, really. 100g is 1.25% of the wheel weight; even at four times that, the penalty is just 17 seconds.

Changing that 1.25% weight penalty to an aero penalty - upping the overall drag of the bike by the same percentage – gives a 22-second penalty, and quadrupling the drag penalty pretty much does the same to the time lost: 87 seconds at 5%.

Now these still aren't big numbers: just under a minute and a half in four hours of riding. But the difference is certainly significant: aero gains are worth six times what weight gains are, and a fair conclusion from Swiss Side's stats would be that on rolling terrain it's worth going heavier and more aero.

But surely there's a tipping point? Of course, but it's a long way north of where you might expect. 

This graph shows what a average rider might do on a 20km climb with an average gradient of 4%, knocking out 211W for not much under an hour. That's certainly achievable for many of us.

Swiss Side Wind Tunnel 30.jpg

This is the tipping point, more or less, for this imaginary rider. If your ride has an average gradient of 4% or more then the weight gains will mean bigger time savings than the aero ones. So you're still permitted to take a drill to your levers for the local hillclimb. But unless it is a hillclimb – or a really, REALLY hilly ride – then you're better off optimising your bike to cut through the wind than you are shaving off the grams. If that's a surprise to you, well, it was to me as well. Obviously it doesn't mean that a 10kg beater will be as quick as a 6kg superbike. But any time you're spending stressing over the odd hundred grams here or there is basically wasted.

A big thanks to Swiss Side for their time at the wind tunnel.

Dave is a founding father of road.cc, having previously worked on Cycling Plus and What Mountain Bike magazines back in the day. He also writes about e-bikes for our sister publication ebiketips. He's won three mountain bike bog snorkelling World Championships, and races at the back of the third cats.

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55 comments

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barbarus | 4 years ago
1 like

Savage to find out that my best bike is in fact a 10kg beater!   

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JEMVisser replied to barbarus | 2 years ago
0 likes

Yeah interesting! That explains why I always rode so fast on my old steel road bike with frame shifters... It had good tires, semi aero wheels rven for that period, hey are deeper than those box standard wheels, and it put me in this aero position that was really good. It wasn't the most comfortable position, but I got used to it now and the vibration dampening was awesome! The only thing was the braking, awfull.

My new carbon cx bike rides smooth, fast and good on all roads. Would need to upgrade the tires if I want to ride on roads but that's fine, I've got gravel roads out my front foor so...

If I were to be a professional I'd want to go full damn aero, always. Deepest wheels available (if no crosswinds eh), dropper post (aero gains downhill), and generally as light as possible if aero isn't compromised, say no lightweight frame instead of an aero one, yes lighter pedals vs heavy (pedals make no difference in terms of aero gains? Nope, must be minimal at most). Also, this bike is faster on really mountainous stages than an outright climbing bike because of the weight limit. Today, even outright aero bikes are near 6.8 kg!

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Cugel | 4 years ago
5 likes

As then (when this article was written) so now - the effects on the time taken for a ride of any meaningful length is barely affected by these teeny changes in weight or "aerodynamics" supposedly provided by these products and their endless tweaking. What is affected is the ability of the manufacturers to flog their latest gew-gaw to the gullible and fashion victims.

Why does a ride have to be a minute or so shorter than it was yesterday, unless you're doing a time trial? Are all these pseudo TT riders with the latest £1000 wheels needing to get home in time to watch an important advert on the tele for something else they don't need?  

It seems to be a vicious circle of adverts to promote products based on "professional" demands of bikes driving the notion that the professionals' demands of bikes are an appropriate benchmark by which normal cyclists should choose products.

Does every bike ride consist of a race with yourself or those you're with? If so, you're missing an enormous amount of cycling pleasure. Sad, really, how cycling has become dominated by consumerist hysteria for the new! improved! and latest silly fashion,  every day that mass media like this gets another manufacturer's PR blurb to churn out as exciting "news".

Sad that hordes of lads (and quite a few lasses) go about as if they're in a race when they'd likely fall off in the first half mile of any real race. The power of the glamorous image, eh!? 

One day I will open a therapy centre for burnt-out MAMILs and similar fashion victims. It will be free (they've spent their last penny on a go-faster plastic hat) and consist of treatments involving picnics on a Pashley.

Cugel

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McVittees replied to Cugel | 4 years ago
2 likes

I feel reassured by the fact that by not having an aero setup I lose such a minuscule amount of time that when the time comes for a new bike it can be based purely on aesthetics, ride characteristics and price. Aero be damned!😊

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Simon E replied to Cugel | 4 years ago
0 likes

Cugel wrote:

It seems to be a vicious circle of adverts to promote products based on "professional" demands of bikes driving the notion that the professionals' demands of bikes are an appropriate benchmark by which normal cyclists should choose products.

Professionals ride what they're given, they don't demand deep wheels, 12-speed derailleurs or aero frames be developed to enable them to turn a pedal.

They are simply part of the marketing dept for each brand they use, and for the image of pro cycling as a sport. We are supposed to look up to them, admire them for their prowess and want to emulate them, which is where the problems begin.

I also think it reflects the current orgy of consumerism, our love of shopping, where people think that by owning x sets of Nike trainers, the latest smartwatch and smartphone and this model car is part of the image of yourself you want to portray (and so often simply a copy of some insta-celeb or shallow social media 'influencer').

My impression is these are all indicators of deep insecurity and a need to 'have the right gear', 'look the part' - to belong, to be accepted into whatever tribe they think they must join. But they shouldn't need to do that to be accepted, they should be able to ride for fun or utility or even to race - or even to not ride when they don't want to - whatever brings them genuine satisfaction or serves a useful purpose. I love seeing people (especially kids) out on bikes, regardless of what they wear. My only gripe is that many of them really ought to lube that bloody chain!

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fukawitribe replied to Simon E | 4 years ago
3 likes

You clearly don't believe that people should be able to do "whatever brings them genuine satisfaction" - and if they do buy something you deem as image or market based flim-flam, it's an indicator of deep insecurity - an overwhelming need to join some "tribe". Bit presumptive don't you think ?

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Simon E replied to fukawitribe | 4 years ago
2 likes

fukawitribe wrote:

You clearly don't believe that people should be able to do "whatever brings them genuine satisfaction" - and if they do buy something you deem as image or market based flim-flam, it's an indicator of deep insecurity - an overwhelming need to join some "tribe". Bit presumptive don't you think ?

I think it's you presuming. But that's OK, I don't care what you think.

I don't mind if you buy £1,500 60mm wheels for your £7,000 custom bike, I will think no more or less of you than someone riding a step-thru with a wicker basket. I'll say 'hi' to each of you if we pass and would be happy to stop to assist if you had a mechanical.

If you want to have a level-headed discussion about things like consumerism, tribalism and social norms then that's fine too.

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fukawitribe replied to Simon E | 4 years ago
1 like

If it's presumption, then i'm not understanding your generalised point. I get the consumerism angle, and agree with that, but your own words seem to conflate buying these things with insecurity and a need to join a tribe - and conform to some state to belong to it. I don't agree with that in general - sure it happens, and may well happen a fair amount for all you and I know, but to seemingly attribute those properties on someone because of what they buy seems over-reaching to me. 
If i've mis-understood, can you rephrase to make it more specific. Cheers.

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Rich_cb replied to Cugel | 4 years ago
3 likes

The way I like to ride my bike is better than the way you like to ride your bike.

Rich_CB

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hawkinspeter replied to Cugel | 4 years ago
4 likes

...but I like shiny things!

Bike rides are always a race if there's another cyclist around - unless they're quicker than you in which case you're just taking a leisurely recovery ride.

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CyclingInGawler | 4 years ago
0 likes

In the absence of any statement to the contrary I assume that the drag (W) vs yaw angle chart was developed from tests at a constant wind speed. But aerodynamic profile drag rises and falls with the square of the change in speed, so the slower rider might have a greater yaw angle and that might create a small increase in drag, but I'd suggest that the yaw effect is completely overshadowed by the fundamental reduction in drag from the lower wind speed.

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Nick T | 4 years ago
1 like

"Why riders like you need to get more aero and wheel weight doesn't matter"

(Narrator: it's because this company that makes heavy, deep profile wheels wants to sell you them)

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Griff500 | 4 years ago
1 like

"100g is 1.25% of the wheel weight"

Those are some heavy old wheels!

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TrueFalcon | 4 years ago
0 likes

It's more than a bit annoying to read an article dated today and after reading a dozen or so comments, you noticte they are TWO YEARS OLD. When Conti 5000 tyres weren't being mentioned, I wondered why. I'm riding TLs and wondered how they fared in this tunnel.

How can 2 year old comments be relevant to an article written today? They can't be commenting on something written after their comments?

Why does road.cc redate old articles and publish them as new??? I've noticed this happens a lot and it's a disservice to the readers.

It looks like it's time to dump this tab from my batch of open-all-the-time tabs.

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check12 replied to TrueFalcon | 4 years ago
0 likes

5000 TL are 25mm min not sure what actual size but conti 4000 23mm were/are 26mm measured, so if tl 25mm are same or less than 26mm, great, if more than 26mm then will be worse aero wise

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fukawitribe replied to check12 | 4 years ago
0 likes

Maybe, maybe not. Wheel-system drag is solely related to frontal area only in the most (over) simplified of models.

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RobD replied to check12 | 4 years ago
0 likes

The overall width of the tyre isn't the only issue with regards to how aero or not it is, it depends on the interface between the tyre and rim, as well as the tyre surface. Otherwise modern super wide rims would be less aero because of the width, which is the opposite of reality.

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davewyman | 5 years ago
0 likes

I like riding uphill. I like the feel of acceleration on the flats and reaching a high speed. As my last official race was decades ago, I don't care about setting any speed records on descents. 

That why I've got a climber's bike that, with pedals, weighed less than 15 pounds when I bought it. Now my bike has a set of aero wheels in place of the wheels that came with the bike, that make it even more lightweight. 

So I think I have the ideal bike for what I most like to do (not, somewhat regrettably, that this is my only bike.) 

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Hobbanero | 5 years ago
0 likes

the guys at Deceunink Quickstep apparently didn't get the memo about disc brakes being slower.

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Simon E replied to Hobbanero | 5 years ago
0 likes

rws wrote:

This article seems to have been written for time triallists - for a road racer it’s not the overall course gradient that matters, it’s where you plan to attack, and 4% or higher gradient is not unusual. Furthermore, when it comes to attacking, it’s not all about weight - inertia matters too.

No, it wasn't written for time triallists, but they benefit most and care most about the subject. Does nobody use aero bikes, wheels & clothing in your races?

You might want to launch your attack on a 6% gradient but unless it's really steep and you're going very slowly then the aero benefit is still there, and it will be even more effective on any subsequent descent, when you definitely won't want to be using your weight-weenie climbing wheels!

Hobbanero wrote:

the guys at Deceunink Quickstep apparently didn't get the memo about disc brakes being slower.

Things have improved on this front since the article was published, as madcarew's post might suggest. And if your rivals also run disc brakes...

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rws | 5 years ago
0 likes

This article seems to have been written for time triallists - for a road racer it’s not the overall course gradient that matters, it’s where you plan to attack, and 4% or higher gradient is not unusual. Furthermore, when it comes to attacking, it’s not all about weight - inertia matters too.

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MarkiMark | 5 years ago
1 like

I rode the Tour de Yorkshire Sportive on Sunday. The 'race' (against my mate) culminated with a sprint finish for (our) 1st and 2nd place. The crowd were roaring. Long short, I came second by a wheel, and now I learn that I could have gained a valuable 3 seconds over the course if I'd had more aero tyres!!!!! Marginal gains indeed.

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madcarew | 5 years ago
1 like

Dave, just wondering if you could look at the more recent iterations of discs and aero testing and include that in the last part of your article. Re-cycling (sorry) old articles is fine, but it'd be of more interest to many of us if they were updated along the way.

Cheers

Paul

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mrchrispy | 5 years ago
2 likes

you guys are totally missing the biggest take away from this article. 

we now have empirical evidence to justify buying some 40-50mm section carbon wheels to overcome the drag effects of my disc brakes.  re-fecking-sult! 

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fukawitribe replied to mrchrispy | 5 years ago
2 likes

mrchrispy wrote:

you guys are totally missing the biggest take away from this article. 

we now have empirical evidence to justify buying some 40-50mm section carbon wheels to overcome the drag effects of my disc brakes.  re-fecking-sult! 

You'll also need to set aside some cash for beer for the ultimate performance set-up

 

https://www.youtube.com/watch?v=Z4Te-wrCQsM

(GCN - Why A Beer Belly Actually Makes You Faster)

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xerxes | 5 years ago
2 likes

"And, for the record, the fan sucks"

It looks OK to me.

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Spock Icus | 6 years ago
1 like

Thanks for the article it has helped me make up my mind - why i don't need deep section aero and weight does matter. You see you provide two points of reference for the tests concluding that weight does not matter:

1. 1200m elevation gain over 120km completed at 30 km/h at 211 average watts

2. a 4% gradeint

In the case of point 1 you conceded a 3 second difference with 100 grams. The problem is that this is an insignificant elevation gain for a 120 km ride constituting the kind of elevation i'd expect from a hilly ride. Far from it, it would appear to resemble a loop i do for flat crit type training - it has one moderate hilll. Try upwards of 2500m elevation gain for that distance and tell me the difference will be 3 seconds...

 

With point 2 i have to say that if you consider 4 % to be the tipping point then you can expect some rude surprises up a proper incline which in my books starts at around 9% and upwards. So if 4% is your tipping point then i'm going for weight over aero.

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Paul J | 7 years ago
2 likes

alotronic: The strong older guys aren't riding sportives, they're still racing. There's guys in their 60s and 70s around here still doing TTs who can still leave most younger cyclists for dead.

Also, bear in mind that the cyclists you catch up with on the road /must/ be a good bit slower than you. Physics means it's rare that you ever catch up with cyclists around your own level, unless you happen to join the same road at the same time.

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alotronic replied to Paul J | 7 years ago
0 likes

Paul J wrote:

alotronic: The strong older guys aren't riding sportives, they're still racing. There's guys in their 60s and 70s around here still doing TTs who can still leave most younger cyclists for dead.

Indeed! I didn't mean to dis the old roadies. I don't ride sportives either, I have slid into the warm haven of Audax. I imagine that if I ever got the racing bug again (I am only 50!) I would be trying to get every watt advantage I could as I would certainly have less of them to muster  3

A

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Beecho | 7 years ago
3 likes

No mention of cake drag.

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