Lightweight or deep rimmed?

greasedscotsman

ugo.santalucia wrote:

And let's not forget the fastest Paris-Roubaix ever in 1964!
http://paolocoppo.drupalgardens.com/con ... ics-really

And let's not forget that the route has changed significantly since then.

Also your blog is out of date, the 2nd fastest was set by Fabian Cancellara this year.

awsm

I have Dura-Ace C24's & C50's and I mostly ride hilly routes. I prefer the C50's and run them pretty all the time.

edit: I skipped most of the posts just noticed the discussion about aero and the benefit to slow vs fast riders and remembered Cervelo just published an article on this:

http://www.cervelo.com/en/engineering/t ... iders.html

ugo.santalucia

greasedscotsman wrote:

ugo.santalucia wrote:

And let's not forget the fastest Paris-Roubaix ever in 1964!
http://paolocoppo.drupalgardens.com/con ... ics-really

And let's not forget that the route has changed significantly since then.

Also your blog is out of date, the 2nd fastest was set by Fabian Cancellara this year.

I know... I should update it...

boblo

Aaah so, everyone's telling the truth from their perspective and the OP would be better off spending his £500 on a training camp for maximum return (assuming standard wage earning part time amateur rider). That about it?

Alex_Simmons/RST

ugo.santalucia wrote:

Alex_Simmons/RST wrote:

ugo.santalucia wrote:

Just better aerodynamics works better at low speed than high is simply not true. You can plot your car fuel consumption vs speed and you will see that the curve is not linear... there is point where your car will use a lot of fuel to go just a tad faster... that's where you would like your car to have the shape of a rocket... while at 40 mph it won't make pretty much any difference in the fuel consumption.
My car is shaped like a brick and it is incredibly fuel efficient up to 60 mph, pass that and it becomes among the worst in its range of engines

You are confusing the difference in the power demand of riding at different speeds with the relative speed improvement from the same aerodynamic change (i.e. a fixed percentage reduction in CdA) a rider attains (be they riding at 30 or 50 km/h).

Why my car, which is very aerodynamically inefficient has the same fuel consumption than a different model with the same engine at 60 mph, but a radically different one at 70 mph and ourageously different at 80 mph?
I have owned both cars: they both did about 60 mpg at 60 mph, the current does about 48 at 70mph (former did 54) and about 35 at 80 mph (former did 48). Being a stingy persona, I do tend to look at these numbers.
Am I wrong or this shows that aerodynamics is more relevant at speed? Where am I confused?

EDIT: the understanding I have of speed and power is that to move from saying 10 to 20 mph you have invest X power, but to move from 20 to 30 mph you have to invest a lot more than X power and to move from 30 to 40 mph (on a bicycle) you have to invest a hell of a lot more than X.
Typically sprinters use 1000-1400 Watt to go at 40-42 mph
Time triallists use 400-450 Watt to go at 30 mph
Amateurs need about 200-250 Watt to go at 20-22 mph
In which way this relationship is linear? It seems to me rather exponential and it's all down to aerodynamic drag

You are clearly confused as to what I am saying so I suggest actually reading what I wrote.

It's clearly obvious that the relationship between speed and power is a cubic equation. I am not disputing that.

Let me make it clear:

If you have two riders, one capable of sustaining 30km/h and another say capable of sustaining 45km/h.

Then apply the same reduction in CdA to both riders (e.g. by providing the same equipment change option).
What is the relative speed change and reduction in time over a fixed distance for each rider?

You will find that both riders go faster, and they both go faster by very nearly the same percentage. And the consequence of that is the absolute time saving over a fixed distance course (which is what TTs are) for the slower rider is greater than it is for the faster rider.

Is that really so hard to understand?

Alex_Simmons/RST

OnYourRight wrote:

Alex, on your blog you write:

“Whether a slower rider should be putting time/energy/effort/resources into gaining or buying an aero improvement when they might perhaps be better focussed on losing weight and training more (or harder or smarter) is a moot point. Really, though, such an argument is a false dichotomy. Why not do both?”

It is a moot point, but for the true meaning of ‘moot’ rather than your usage here. The argument that a slower rider actually gains more from aerodynamic improvements than a faster rider because he or she gains a bigger absolute time advantage is classic sophistry, i.e. hogwash. It’s like suggesting it’s more important for the US government to save $1000 than for a wage-earner to save $500, because 1000 is a bigger number than 500.

Slow cyclists and Team Sky alike have limited resources, and in the case of slow riders the limits are typically severe. In these circumstances it makes little sense to try to save sungod’s suggested 6 watts at 19 MPH by buying Zipp 303 Firecrest wheels – and that 6 watts is based on Zipp’s claims, which are undoubtedly based on extremely felicitous circumstances. It wouldn’t surprise me at all if the difference is more realistically 3 watts or less.

There are easier ways than buying new wheels to save 6 watts from the typical professional cyclist, never mind slow amateurs.

Is anything I have said incorrect, meaningless or insincere? The physics and maths are sound and I am just correcting false information/myths such as aerodynamics doesn't have an impact for slower riders, when it quite clearly does.

People constantly present false dichotomies, as if someone can't choose to improve both fitness and reduce the resistance forces as means to improve their performance.

Pithy Power Proverb: "Maximise the power you can produce, minimise the power you must produce" - Jim Martin

As to specific individual circumstances, I have not made any comment pertaining to those, so attributing such to me is, in your words, hogwash (i.e. misleading and insincere).

ugo.santalucia

Alex_Simmons/RST wrote:

You will find that both riders go faster, and they both go faster by very nearly the same percentage. And the consequence of that is the absolute time saving over a fixed distance course (which is what TTs are) for the slower rider is greater than it is for the faster rider.

Is that really so hard to understand?

I guess I am a bit thick...
I think I am confusing power output/fuel consumption with time saving. So basically what you say implies that time saving is also a non linear curve and whilst at low speed the time saving can be great for a small reduction in CX, at greater speed the time saving is smaller... or in other words saving 4 minutes whilst going slow is pretty easy, whilst saving 4 minutes at great speed nears impossible.
Or in other words, the "slow" rider who improves his PB by 4 minutes fools himself into believing that this is a massive improvement, as it might equate to a fast rider trimming 10 seconds off his PB... which goes on well with real life observations and of course it is my spin/bias on the all subject.

Somehow the Zeno paradox is buzzing in my head though...

BTW: apologies for my very first post... I was distracted and didn't realise we were talking about a member of the forum... I receive quite a lot of that treatment myself, so I know it's not nice and I don't use that tone when discussing with another forum member.

Bar Shaker

The biggest part of a wheel's aero drag comes from the spokes. Replacing spokes with a smooth surface that can generate laminate flow is way better than throwing spokes forward at twice the speed of the bike. The fastest part of the spoke is the outer section and this is travelling forward fastest at the top of the wheel. Replace this with rim and see a big reduction in drag.

Hence why discs are so damn fast.

ugo.santalucia

Bar Shaker wrote:

The biggest part of a wheel's aero drag comes from the spokes. Replacing spokes with a smooth surface that can generate laminate flow is way better than throwing spokes forward at twice the speed of the bike. The fastest part of the spoke is the outer section and this is travelling forward fastest at the top of the wheel. Replace this with rim and see a big reduction in drag.

Hence why discs are so damn fast.

Indeed... which is the reason why it is difficult to work out the aero advantage of these wheels at a given speed

http://paolocoppo.drupalgardens.com/con ... -they-work

cycleclinic

We have all learnt something I think from this which is the whole point of the forum. Still my first question is what is the actual change in CDa going from 32 spoke count shallow rims to 50mm deep rims roughly that is. Without any specific data I find it hard to know if it is worth or not. Alex do you have such data or are those CDa numbers you gave typical of these two wheels in which the difference is very big.

Bar Shaker

ugo.santalucia wrote:

Bar Shaker wrote:

The biggest part of a wheel's aero drag comes from the spokes. Replacing spokes with a smooth surface that can generate laminate flow is way better than throwing spokes forward at twice the speed of the bike. The fastest part of the spoke is the outer section and this is travelling forward fastest at the top of the wheel. Replace this with rim and see a big reduction in drag.

Hence why discs are so damn fast.

Indeed... which is the reason why it is difficult to work out the aero advantage of these wheels at a given speed

http://paolocoppo.drupalgardens.com/con ... -they-work

I have been going on about this for ages. At last someone else gets it.

Alex_Simmons/RST

ugo.santalucia wrote:

Alex_Simmons/RST wrote:

You will find that both riders go faster, and they both go faster by very nearly the same percentage. And the consequence of that is the absolute time saving over a fixed distance course (which is what TTs are) for the slower rider is greater than it is for the faster rider.

Is that really so hard to understand?

I guess I am a bit thick...
I think I am confusing power output/fuel consumption with time saving. So basically what you say implies that time saving is also a non linear curve and whilst at low speed the time saving can be great for a small reduction in CX, at greater speed the time saving is smaller... or in other words saving 4 minutes whilst going slow is pretty easy, whilst saving 4 minutes at great speed nears impossible.
Or in other words, the "slow" rider who improves his PB by 4 minutes fools himself into believing that this is a massive improvement, as it might equate to a fast rider trimming 10 seconds off his PB... which goes on well with real life observations and of course it is my spin/bias on the all subject.

Somehow the Zeno paradox is buzzing in my head though...

BTW: apologies for my very first post... I was distracted and didn't realise we were talking about a member of the forum... I receive quite a lot of that treatment myself, so I know it's not nice and I don't use that tone when discussing with another forum member.

I know it can seem a little counter intuitive at first, but once you think about it, it makes sense.

I don't rank improvements, just the fact that somebody is making positive progression is a good thing.

Now I'm certainly not the only or first person to make this observation, it's come up on forums many times over the years and others have written about it as well. Cervelo did this item which perhaps gives another explanation that might help:
http://www.cervelo.com/en/engineering/t ... iders.html

Alex_Simmons/RST

thecycleclinic wrote:

We have all learnt something I think from this which is the whole point of the forum. Still my first question is what is the actual change in CDa going from 32 spoke count shallow rims to 50mm deep rims roughly that is. Without any specific data I find it hard to know if it is worth or not. Alex do you have such data or are those CDa numbers you gave typical of these two wheels in which the difference is very big.

Jack Mott has recently begun to collate a range of resources related to aerodynamics, so perhaps that's a good place to start.
http://www.aeroweenie.com/data.html

There are numerous sites with such comparison data.

In an example of testing the aerodynamic difference between rear wheels, I tested:
- a 32 spoke Velocity Aerohead rim with
- a rear Zipp Disk,
in low yaw conditions (0-10%).

The CdA improvement from changing the rear wheel was ~ 0.023m^2, which equates to more than 2 seconds per kilometre faster*. e.g. over a 40km TT, I expect that such a change would reduce time to complete the course by at least 80 seconds.

I've tested various wheels and combinations over the years.

And in one instance I coached an older master female cyclist who by young guys standards is not fast, but she was fast enough to win a masters world championship. Even at her very modest speed, good aero choices made enough of a difference to help her win.


* you can use the rule of thumb that at normal flatter ground cycling speeds:
5W ~= 0.005m^2 CdA ~= 0.5 seconds per km

i.e. drop CdA by 0.005m^2 and you gain 0.5 seconds per km in speed, and that's about the same speed gain from increasing power output by 5 watts.

Crimmey

Are those figure for a riders cdA or the wheel only cdA and at what speed?

Alex_Simmons/RST

Crimmey wrote:

Are those figure for a riders cdA or the wheel only cdA and at what speed?

Which figures?

The ones where I show a drop in CdA by changing rear wheel represents the absolute change in the bike+rider CdA. In the case of swapping the rear 32-spoke Aerohead rim for a rear Zipp disk, it represented a drop in total CdA of nearly 9%.

CdA is independent of speed (over the range of speeds a cyclist will travel at). Indeed good testing protocol usually tests at various speeds. It can however vary with yaw angles, and some wheels perform significantly better in crosswinds than others, even though their low yaw performance may be similar.

Alex_Simmons/RST

ugo.santalucia wrote:

Bar Shaker wrote:

The biggest part of a wheel's aero drag comes from the spokes. Replacing spokes with a smooth surface that can generate laminate flow is way better than throwing spokes forward at twice the speed of the bike. The fastest part of the spoke is the outer section and this is travelling forward fastest at the top of the wheel. Replace this with rim and see a big reduction in drag.

Hence why discs are so damn fast.

Indeed... which is the reason why it is difficult to work out the aero advantage of these wheels at a given speed

http://paolocoppo.drupalgardens.com/con ... -they-work

Assessing the aerodynamics of wheel options (well any change really) is not all that difficult. If you have a good power meter*, a reasonable venue and conditions, use a good test protocol and apply some maths such an assessment is pretty straightforward.

Fortunately there are tools available that can do the maths for you, including some tools that I published to the public domain at some time in the last decade although others have since provided newer tools (although the maths is essentially the same). With good protocol and venue it's possible to have excellent precision at least equivalent to the better wind tunnels.

Field testing in less than ideal conditions provides less precision, or to attain high precision requires more sophisticated technology, which exists but is not a cheap/simple matter.


* Indeed it's also possible to do field aero testing without a power meter, if you have a good speed recording device (wheel based, not GPS) and a suitable venue for coast down testing, one can apply the principles of "virtual elevation modelling" to such data to get reasonable estimate of one's CdA.