# Don't believe what they all say about "weight".



## gavroche (3 May 2013)

Many cyclists go on about body weight ratio to power ratio with the belief that the lighter you are, the faster you go. Well, I weight 13.75 stones for 5'7 and age 62 and my weight tends to remain constant as I don't do diet and eat what I want.
Now, when I am on a ride, many a time, I am passed by people much heavier than me judging by their body frame. These guys seem to have very powerful legs though and loads of stamina as I watch them pedal with a regular rhym, even when going uphill. I was behind such a cylist last week-end. I was just starting up and still warming up when he passed me. I jumped into his wheel and followed him for a couple a miles before we went our separate ways.The road was gently undulating and he never changed gear once, just pushing harder on the pedals when needed. I use my gears all the time, according to the terrain. So bang goes the theory about being slim and super fit I thought! It might be ok for real professionals but don't seem to affect the common mortal so much on cycling runs. I think the muscles in your legs are more important than your body weight, up to a point of course.


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## ianwoodi (3 May 2013)

All depends on how fast you were going up the hill. I am 5ft 7in like you and a year ago around same weight now i am 10st and can reach the top of hills alot faster


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## Cycleops (3 May 2013)

Although muscle is obviously important I thought it was more to do with stamina and your heart's ability to pump the blood around your body. This is why Bradley Wiggins has been so successful as I understand he has a huge capacity heart enabling him to keep going for longer. I know this is where I fail, being the same age as the op I struggle over longer distances even though I am very slim.


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## Sittingduck (3 May 2013)

CV fitness is very important but to discount weight is foolish. You need to output more power to maintain the same speed as a lighter cyclist, if you are heavier. That isn't to say that it cannot be done but that extra has to come from somewhere, right...? I am talking mainly about going uphill and on inclines. Obviously a heavier person can get more speed downhill if simply freewheeling and this can help carry us over rolling terrain. When you're going uphill though - all things being equal between two people (e.g. CV fitness), the the lighter guy will either be faster or be reqired to output less power to climb at the same speed, which in the end - will be telling.


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## T.M.H.N.E.T (3 May 2013)

It's entirely possible to be fat and fit. Being rake thin isn't a sign of "fitness" even though that word,has multiple definitions.


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## 400bhp (3 May 2013)

Bang goes the theory based on some bod that Gavroche sat behind on some incline or other.


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## youngoldbloke (3 May 2013)

Could it be that age and cardio-vascular condition has something to do with it too?


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## Flying Dodo (3 May 2013)

I've lost 6 kg in 6 weeks this spring, and the same hills feel faster. Haven't timed myself up hills though, although having checked the clock, my flat-ish commute into work is definitely faster.


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## GrasB (3 May 2013)

Disclaimer: this makes a whole shed load of assumptions & is very simplistic. I just don't want to write a huge essay on the subject.
All mass on a persons body helps produce power, including fat it's just fat doesn't help produce many watts, even if someone has a low PWR a lot of weight produces a high absolute power figure. When aerodynamic drag is the primary source of resistance then absolute power trumps PWR. On gently undulating terrain aerodynamics is going to be your primary source of drag.

However when you get sustained gradients absolute power isn't as important as PWR. Lighter people typically have a higher PWR than heaver people for the same level of training & ability. But in this case without sustained climbs you'll not see this effect. 

Heaver people also tend to be able to apply more force to pedals as they're constantly doing resistance training compared to a lighter person. This isn't a good thing or bad thing, it's just different. I ride fixed gear because it forces me to be flexible in my riding & gets me used to muscling up hills etc. I race on geared because I can maximise my performance by not muscling up hills but saving that high force bits for accelerating hard when needed etc.

'Endurance' is a funny one. Often bigger people who look like they can go on forever hit simply hit their personal endurance brick wall. Lighter people tend to have more of a ramp to climb up so have some warning before they run out of endurance.


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## Crackle (3 May 2013)

It's true what they say about wait.


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## derrick (3 May 2013)

Crackle said:


> It's true what they say about wait.


What do they sat about wait.


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## Crackle (3 May 2013)

derrick said:


> What do they sat about wait.


All things come to those who weight


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## Rob3rt (3 May 2013)

derrick said:


> What do they sat about wait.


 
That before you say something dumb, you should wait. This decreases the chance of making a fool out of yourself!


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## derrick (3 May 2013)

Rob3rt said:


> That before you say something dumb, you should wait. This decreases the chance of making a fool out of yourself!


You obviously don't.


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## gam001 (3 May 2013)

gavroche said:


> Many cyclists go on about body weight ratio to power ratio with the belief that the lighter you are, the faster you go. Well, I weight 13.75 stones for 5'7 and age 62 and my weight tends to remain constant as I don't do diet and eat what I want.
> Now, when I am on a ride, many a time, I am passed by people much heavier than me judging by their body frame. These guys seem to have very powerful legs though and loads of stamina as I watch them pedal with a regular rhym, even when going uphill. I was behind such a cylist last week-end. I was just starting up and still warming up when he passed me. I jumped into his wheel and followed him for a couple a miles before we went our separate ways.The road was gently undulating and he never changed gear once, just pushing harder on the pedals when needed. I use my gears all the time, according to the terrain. So bang goes the theory about being slim and super fit I thought! It might be ok for real professionals but don't seem to affect the common mortal so much on cycling runs. I think the muscles in your legs are more important than your body weight, up to a point of course.


 
As a larger (being polite!) cyclist myself (5'8" and 15.5 stones), I can quite easily motor along on flat or rolling terrain with some good club cyclists. I can even muscle my way up short, sharp inclines in the big ring and recover OK the other side. I quite often overtake other cyclists on this kind of terrain.

However, anything longer than a couple of hundred yards at 10%+ gradient and I have to slow down and gear down, as I just can't maintain the power needed for the duration. I have a power meter and you won't believe how much extra power is required to maintain speed up a steep hill when you're heavy, eg 400-500 watts, which is a lot for mere amateurs for more than a minute or two! 

So I agree with you to a point, but as soon as it gets steep it's a totally different ball game gav


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## gavroche (3 May 2013)

gam001 said:


> So I agree with you to a point, but as soon as it gets steep it's a totally different ball game gav


 And I couldn't agree more with you and that goes for every one. Only true climbers can substain prolonged effort over very steep gradients.


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## Rob3rt (3 May 2013)

Still not understanding this are you?


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## gam001 (3 May 2013)

gavroche said:


> And I couldn't agree more with you and that goes for every one. Only true climbers can substain prolonged effort over very steep gradients.


When there's lots of hills involved gav, it's all about PWR - it just is pal - it's the laws of physics. EG I'm sure Cancellara could maintain more power on a sustained climb than Wiggo could, but he's a lot heavier and will therefore (and does) go slower on lumpy terrain (although he then has to descend like a demon to try to catch up!).


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## gam001 (3 May 2013)

[quote="(although he then has to descend like a demon to try to catch up!).[/quote]
Watch this for descending...

View: http://www.youtube.com/watch?v=RxXqQqAc2pA


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## Sittingduck (3 May 2013)

Never get sick of seeing that vid.


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## Hip Priest (3 May 2013)

I am a fat cyclist (6ft 2, 16.5st), but I also consider myself relatively fit, as I can do a flattish 20 mile TT in an hour, and I've got a resting HR of 41.

However, where my weight really hinders me is on climbs.

I suffer on anything from 10-15% and if it gets any steeper than that, it's the walk of shame, even with a 34-26t.

I should try to lose weight really, but I love bad food and booze too much!


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## Hacienda71 (3 May 2013)

That has got me in the mood to go up in the hills this weekend.


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## 400bhp (4 May 2013)

Hacienda71 said:


> That has got me in the mood to go up in the hills this weekend.


 
Yes, you fat b'stard


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## Banjo (4 May 2013)

I think its completely pointless making assumptions about other riders you pass or get passed by on the road. The thin racer you overtook could be recovering from illness the fat guy that passes you could be an ex olympic champion . Or just a fat guy sprinting round a very short route.


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## ComedyPilot (4 May 2013)

I was 5' 10" about 20 years ago...and still am.....


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## Bill-H (4 May 2013)

I have lost a stone since Christmas hills are easier i find i can now do some in big ring i used to use granny ring on however i still use granny ring on others but my average speed is usually the same downhills are as slow as usual fear factor


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## ComedyPilot (4 May 2013)

Bill-H said:


> *I have lost a stone since Christmas* hills are easier i find i can now do some in big ring i used to use granny ring on however i still use granny ring on others but my average speed is usually the same downhills are as slow as usual fear factor


Also lost the *,* and *.* buttons on the keyboard?


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## Bill-H (4 May 2013)

,.,.,.,.,.,.,.,.,.,.,.,.,........,,,,,.,.,.,.
Found them lol.


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## Hip Priest (4 May 2013)

Banjo said:


> I think its completely pointless making assumptions about other riders you pass or get passed by on the road. The thin racer you overtook could be recovering from illness the fat guy that passes you could be an ex olympic champion . Or just a fat guy sprinting round a very short route.


 
Talking about assumptions, I was in hospital recently and a nurse taking my blood pressure remarked that I had a low HR. I said "I know" and she asked "Did you used to be really fit or something?" Ha!


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## ayceejay (4 May 2013)

The OP's premise seems to be a logical fallacy. I am fat - I overtook a skinny guy - therefore fat cyclists are faster than skinny cyclists.


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## Peteaud (4 May 2013)

I think i am slower due to my gut acting like a parachute.


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## Lyrical (4 May 2013)

Bigger guys push tyres down harder and creates more down force.

This is like F1 Cars

This is why big guys faster than small guys... right?


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## gam001 (4 May 2013)

400bhp said:


> Yes, you fat b'stard


Edit...
faSt b'stard


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## Roadhump (4 May 2013)

I'm a 5' 9", 15 stone stocky guy, quite powerful but with a bit of a beer belly. My weight has been up and down over the years though. I find age is also a performance inhibitor. I am almost 53 now and just cannot reach the average speeds I used to to reach 15 years ago (I have an old file on which I recorded some from that time - a bit geeky, I know). 20 years ago I used to make it to the top of a hill about 8.5 miles away in 30 minutes on an inferior bike to the one I now use, but now it takes me about 35 minutes.


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## Cycleops (7 May 2013)

Roadhump said:


> 20 years ago I used to make it to the top of a hill about 8.5 miles away in 30 minutes on an inferior bike to the one I now use, but now it takes me about 35 minutes.



I know the feeling, and it takes you all night to do what you used to do all night!


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## Shut Up Legs (7 May 2013)

I'm a faster cyclist and better climber than I was at 20 (I'm now 45). Youth was wasted on me, I'm afraid. I'm sure I could have been a much better cyclist back then if I'd only bothered to work on it.


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## Crankarm (8 May 2013)

T.M.H.N.E.T said:


> It's entirely possible to be fat and fit. Being rake thin isn't a sign of "fitness" even though that word,has multiple definitions.


 
Yeah, but to be a fast cyclist over most terrain you have to be lighter and stronger. So power to weight ratio is important. Even a sprinter like Cavendish is pretty thin compared to most of the Joes riding on here who claim fat and fit is fine and climbers KOM riders are like skellingtons compared to nearly every plodder on here. It's not just visible fat but fat around your vital organs that counts and fat that silts up the arteries. Eat less crap, moderate portion size, ride harder, more frequently and for longer.


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## Crankarm (8 May 2013)

T.M.H.N.E.T said:


> It's entirely possible to be fat and fit. Being rake thin isn't a sign of "fitness" even though that word,has multiple definitions.


 
Depends what you define as "fit". Moving to the kitchen to grab another pack of Doritos whilst watching the TdF might qualify but if you wish to do any intense sustained sporting activity then, no.

Shovelling in the pies just because you ride 1 mile to work and back each day and weighing 15+ stones does not mean you are fit. You are a heart attack waiting to happen, not to mention a candidate for diabetes with all the Coke you'd drink and cakes you'd probably eat as well.


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## Rob3rt (8 May 2013)

There are several things that ought to be recognised.

On the flat, power to weight is largely irrelevant, power to CdA is the dominant factor to how fast you will go. The frontal area of a heavier and a lighter rider is generally not that different (within reason, discounting extremes) so the rider with the highest absolute sustainable power (for the relevant duration) will often be quickest.

The reason that many heavier, more muscular riders are okay on the short steep stuff is because generally the heavier riders are relativelly very powerful and/or explosive (i.e. short duration power output is very high) thus they can put out a lot of power for the shorter duration to get up the short climbs with the bunch. As the hills gets longer they suffer though as a typical sprinter has a distinctly downward sloping power profile (i.e. power drops off sharply the longer the duration) so they will fatigue and slow substantially. It is here that your power to weight ratio (as most seem to refer to it) really starts to dominate.

Of course in the grand scheme of things, being as light as possible while being as powerful as possible is desirable, why carry something you don't need (plus having less fat has benefits other than reduced weight) but the reality is, depending on your discipline you will need to compromise, i.e. if you are a climber, you will need to give up some power to get light, if you are a sprinter, you will need to get heavier to gain explosive power, etc.

It should also be noted being powerful is not the same thing as being strong!


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## wait4me (8 May 2013)

(I have an old file on which I recorded some from that time - a bit geeky, I know).

Old files are like us --they lose their cutting edge over the years. Was going to put something about bastard in here but couldn't think of a link


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## 400bhp (8 May 2013)

Rob3rt said:


> There are several things that ought to be recognised.
> 
> On the flat, power to weight is largely irrelevant, *power to CdA* is the dominant factor to how fast you will go. The frontal area of a heavier and a lighter rider is generally not that different (within reason, discounting extremes) so the rider with the highest absolute sustainable power (for the relevant duration) will often be quickest.
> 
> ...


 
What does that mean?


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## Rob3rt (8 May 2013)

CdA = drag co-efficient which is largely dependant on your position on the bike and equipment choices. It is also not static and varies with yaw angle.

Flat speed would be a compromise between being aero and being powerful, to get more aero, it usually involves getting lower and narrower at the front end. Both things which compromise power (either by closing the hip angle or by constricting your chest). So you need to balance how aero you are with how much power you put out.


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## Rob3rt (8 May 2013)

I also forget to mention in my rather long post, people seem to refer to power to weight ratio as a single figure, this is not the case, power to weight ratio varies with duration (unless you are a Duracell bunny and are either on or off and your power is constant across all durations)!


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## 400bhp (8 May 2013)

Rob3rt said:


> CdA = drag co-efficient which is largely dependant on your position on the bike and equipment choices. It is also not static and varies with yaw angle.
> 
> Flat speed would be a compromise between being aero and being powerful, to get more aero, it usually involves getting lower and narrower at the front end. Both things which compromise power (either by closing the hip angle or by constricting your chest). So you need to balance how aero you are with how much power you put out.


 
Long time since I did physics, but have done a bit of research around motor racing.

So, co-efficient is between zero and 1 is it? What's the formula that incorporates mass/acceleration/speed? (combination). What's "yaw" for a bike?


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## Rob3rt (8 May 2013)

I think CdA can be greater than 1. Formula is given on the Wikipedia page: http://en.wikipedia.org/wiki/Drag_coefficient

Yaw is the effective incident wind angle as far as I understand (function of wind angle and speed?).

The following may be interesting: http://anonymous.coward.free.fr/wattage/cda/indirect-cda.pdf

It is a way to estimate CdA using a power meter, Golden Cheetah has this method built into a nice GUI called Aerolab, it also covers the equations and any assumptions and simplifications, apparently this method is quite good and has good sensitivity, there are a few variations out there too.

GrasB is your man for this stuff, or get yourself over to the Wattage group or TTF.

There is never a day with little to no wind here, so I haven't had the chance to give this a go. I will do at some point though. I think my club mate has done some testing of this type on the velodrome. I think he was testing helmets and I seem to think the KASK Bambino won (not to say it is the best helmet, but the best fit for him and his ride position, in those conditions, he uses a Bambino and another helmet on the road, seems to vary which he uses).


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## 400bhp (8 May 2013)

Rob3rt said:


> I think CdA can be greater than 1. Formula is given on the Wikipedia page: http://en.wikipedia.org/wiki/Drag_coefficient
> 
> Yaw is the effective incident wind angle as far as I understand (function of wind angle and speed?).
> 
> ...


 
I'm not sure he can use a sentence without some form of abbreviation


Would be nice to understand the mechanics behind it at some stage.


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## Roadhump (13 May 2013)

Cycleops said:


> I know the feeling, and it takes you all night to do what you used to do all night!


I can still drink 8 pints of bitter in 2 hours if that's what you mean....perhaps its not just age!!


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## Hip Priest (13 May 2013)

Crankarm said:


> Depends what you define as "fit". Moving to the kitchen to grab anotwher pack of Doritos whilst watching the TdF might qualify but if you wish to do any intense sustained sporting activity then, no.
> 
> Shovelling in the pies just because you ride 1 mile to work and back each day and weighing 15+ stones does not mean you are fit. You are a heart attack waiting to happen, not to mention a candidate for diabetes with all the Coke you'd drink and cakes you'd probably eat as well.



Fitness is relative, but I can do a 20 mile TT in a hour and an undulating century (5-6000ft of elevation) in about seven hours. Compared to a racing cyclist I'm unfit, but compared to the general population, I think I'm doing alright.

And I love Coke, pies & Doritos!


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## GrasB (13 May 2013)

400bhp said:


> So, co-efficient is between zero and 1 is it? What's the formula that incorporates mass/acceleration/speed? (combination). What's "yaw" for a bike?


CdA = co-efficent of drag multiplied by area where:
- Cd is a co-efficent of the aerodynamic drag of a perfect sphere flat sheet (Cd = 1). Cd can be well over 1.
- A is the area in m^2

Thus for Cd = 0.5 & A = 0.5
CdA = 0.5 * 0.5 = 0.25

CdA is normally used as separating Cd & A is very difficult for a cyclist.

Nominal values of CdA for a bike + rider start at a little under 0.18 for a UCI track legal track bike (not UCI hour record legal) up to around 0.65 for sit up & beg butchers bike.

Some sample numbers for you (0-yaw):
Semi-aero road bike tops - ~0.320
Aero road bike hoods - ~0.270
Aero road bike drops - ~0.245
TT bike - ~0.205
Best track bike I know of - 0.162
My low racer - 0.158
My low racer no brakes or gears - 0.149
Aurélien Bonneteau Mid-racer (Unfared recumbent hour record) - ~0.135


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## VamP (13 May 2013)

GrasB said:


> CdA = co-efficent of drag multiplied by area where:
> - Cd is a co-efficent of the aerodynamic drag of a perfect sphere flat sheet (Cd = 1). Cd can be well over 1.
> - A is the area in m^2
> 
> ...


 
Ha you thought that you gave us enough numbers, but no; we want more.

Do you know what a non-aero road bike in the drops would produce? With non aero clinchers, and with 50mm carbon tubs (say 303 firecrests). I know the size and position will have a huge effect, but assume a biggish rider (6'1'', 78kg), slammed bars, and good posture.

Roughly


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## GrasB (14 May 2013)

VamP said:


> Ha you thought that you gave us enough numbers, but no; we want more.
> 
> Do you know what a non-aero road bike in the drops would produce? With non aero clinchers, and with 50mm carbon tubs (say 303 firecrests). I know the size and position will have a huge effect, but assume a biggish rider (6'1'', 78kg), slammed bars, and good posture.
> 
> Roughly


At a guess 0.275-0.28. That said it depends how 'clean' the bike is. With the same fit a round-tube bike with well routed cables, really nicely wrapped bar tape & tyres which sit nicely on the rims could end up with a lower CdA than a top of the line aero bike with bad cable routing, scruffy tape over/under sized tyres for the rims.

Put another way with exactly the same bike & a lot of thought put into cable routing my road bike's CdA went from 0.266 to 0.259.


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## Ningishzidda (14 May 2013)

GrasB said:


> CdA = co-efficent of drag multiplied by area where:
> - Cd is a co-efficent of the aerodynamic drag of a perfect sphere flat sheet (Cd = 1). Cd can be well over 1.
> - A is the area in m^2
> 
> ...


 
You'll find a 'Flat lamina, face on' is Cd 1.3. Even Wikipedia publishes a figure close to this, 1.28.
CD 1.0 applies to a bloke stood up with tight fitting smooth clothing. This can also be applied to a cyclist in tight lycra clothing sitting on a bicycle in the upright position. Cd 1.2 can be applied to a bloke stood in the street wearing a floppy rain mac and flappy trousers.
The CTC accepts a figure of 1.0 for the typical touring cyclist.

Getting a figure for your own Cd is a simple matter of freewheeling down a known gradient and recording the maximum speed attained. Cd is a 'by the way' as the more important result of the test is a graph of kW vs kmh.

Me on my road race bike 0.88. Me on my tourer 0.96. Me on my TT bike 0.72.
Cd values of 0.75, when multiplied by an area of 0.5 m^2 gets a CdA of 0.375, which is typical of a race boy in the crouch.

Read this
http://www.fredericgrappe.com/cv/bibliographie/d31.pdf

Incidentally, Area does not need to be known to ascertain Cd or kW vs kmh. Mass does.


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## GrasB (14 May 2013)

Ningishzidda said:


> You'll find a 'Flat lamina, face on' is Cd 1.3. Even Wikipedia publishes a figure close to this, 1.28.


Assuming the plate has edges...



> CD 1.0 applies to a bloke stood up with tight fitting smooth clothing. This can also be applied to a cyclist in tight lycra clothing sitting on a bicycle in the upright position. Cd 1.2 can be applied to a bloke stood in the street wearing a floppy rain mac and flappy trousers.


And?...



> The CTC accepts a figure of 1.0 for the typical touring cyclist.


Assuming a frontal area of 0.5m^2 which is almost certainly incorrect.



> Getting a figure for your own Cd is a simple matter of *freewheeling* down a known gradient and recording the maximum speed attained. Cd is a 'by the way' as the more important result of the test is a graph of kW vs kmh.


freewheeling does not deliver your correct CdA try again while pedalling & get a different result.



> Me on my road race bike 0.88. Me on my tourer 0.96. Me on my TT bike 0.72.
> Cd values of 0.75, when multiplied by an area of 0.5 m^2 gets a CdA of 0.375, which is typical of a race boy in the crouch.


I very much doubt those numbers are correct -
Measured CdA of me on a my race day bike...
Tops - 0.309
Hoods - 0.259
Drops - 0.238

... on my fixed/SS road bike..
Tops - 0.314
Hoods - 0.282
Drops - 0.255

... a friend's road bike.
Tops - 0.326
Hoods - 0.276
Drops - 0.248
Those figures used when modelling power v's speed match up very accurately for mesured required power.



> Incidentally, Area does not need to be known to ascertain Cd or kW vs kmh. Mass does.


Area DOES need to be obtained to work out the Cd when dealing with measured drag as the amount of drag in grams is proportional to the frontal area. You do not need to know area when dealing with CdA


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## VamP (14 May 2013)

GrasB said:


> At a guess 0.275-0.28. That said it depends how 'clean' the bike is. With the same fit a round-tube bike with well routed cables, really nicely wrapped bar tape & tyres which sit nicely on the rims could end up with a lower CdA than a top of the line aero bike with bad cable routing, scruffy tape over/under sized tyres for the rims.
> 
> Put another way with exactly the same bike & a lot of thought put into cable routing my road bike's CdA went from 0.266 to 0.259.


 
Thanks.

And how much difference between aero/non aero wheels?


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## Ciar (14 May 2013)

Going down hills my michelin man styled body (6ft 19st) helps me go faster, just need stripes! I would just like to mention i hate hills ;-)


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## GrasB (14 May 2013)

VamP said:


> Thanks.
> 
> And how much difference between aero/non aero wheels?


The HED cycling site is a good place to look for approximations of drag loss. Take the Jet 6 FR for example. The actual CdA gains will be different depending on the rest of the bike.


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## PK99 (14 May 2013)

gam001 said:


> As a larger (being polite!) cyclist myself (5'8" and 15.5 stones), I can quite easily motor along on flat or rolling terrain with some good club cyclists. I can even muscle my way up short, sharp inclines in the big ring and recover OK the other side. I quite often overtake other cyclists on this kind of terrain.
> 
> However, anything longer than a couple of hundred yards at 10%+ gradient and I have to slow down and gear down, as I just can't maintain the power needed for the duration. I have a power meter and *you won't believe how much extra power is required to maintain speed up a steep hill when you're heavy, **eg 400-500 watts,* which is a lot for mere amateurs for more than a minute or two!
> 
> So I agree with you to a point, but as soon as it gets steep it's a totally different ball game gav


 

useful calculator here>>>> http://bikecalculator.com/wattsUS.html

at 15.5 stones, level ground
15mph = 110 watts

15.5 stones, 1.5% grade (the same as a well laid garden patio)
15mph = 220 watts
3% = 335 watts
5% = 485 watts


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## Rob3rt (14 May 2013)

PK99 said:


> useful calculator here>>>> http://bikecalculator.com/wattsUS.html
> 
> at 15.5 stones, level ground
> 15mph = 110 watts
> ...


 
I would be skeptical of how well it works! But a few brief tests where I know the true power output suggest that it works better than the power estimator on Strava!


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## Ningishzidda (14 May 2013)

GrasB said:


> Assuming the plate has edges...
> 
> 
> And?...
> ...


 
What test, what facilities and what measurement instrumentation did you use to get an accuracy to 3 decimal places ???



For the likes of us, get someone to take a piccie of you on your bike with a brick wall behind you. Count the number of bricks you obscure.
My touring bike AREA is 4.8 Sq ft, I freewheel down a 4% at 26 3/4 mph and the vehicle weighed 220 lbs.
The Tyre coeff was 0.0052. ( Crr )
This equated to 0.7 HP for 26 3/4 mph.
Prr for the tyres was 0.08 HP.

CdA came to 4.78.

For any speed, Pf = ( Crr x (lb/375) x V ) + ( CdA x (V^2/391) * (V/375) ) in Horsepower.

Therefore, Cd was 0.99, which I was happy with.

To you, that'll be 0.990, 0.010 away from 1.000 and TOO inacurate


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## GrasB (14 May 2013)

Ningishzidda said:


> What test, what facilities and what measurement instrumentation did you use to get an accuracy to 3 decimal places ???


Power Tap & very careful measurement can get accuracy of +/-0.0015. At least 2 other people can back work weights on a climb to the error margin introduced by water loss during the climb. That required 3-4 sf accuracy of the CdA & Crr.




> For the likes of us, get someone to take a piccie of you on your bike with a brick wall behind you. Count the number of bricks you obscure.


You've measured your silhouette not you frontal area. The two are different.



> My touring bike AREA is 4.8 Sq ft, I freewheel down a 4% at 26 3/4 mph and the vehicle weighed 220 lbs.
> The Tyre coeff was 0.0052. ( Crr )
> This equated to 0.7 HP for 26 3/4 mph.
> Prr for the tyres was 0.08 HP.
> ...


How did you work out your Crr? 0.0052 is light weight tubular territory!
Tempo II = 0.00582
Ultremo HT = 0.00619
Ultremo ZX = 0.00652
Durano Plus = 0.00719


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## GrasB (14 May 2013)

Rob3rt said:


> I would be skeptical of how well it works! But a few brief tests where I know the true power output suggest that it works better than the power estimator on Strava!


Depends on how close its assumptions are to your actual data.


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## gam001 (14 May 2013)

PK99 said:


> useful calculator here>>>> http://bikecalculator.com/wattsUS.html
> 
> at 15.5 stones, level ground
> 15mph = 110 watts
> ...


As Rob3rt says, the absolute values are rubbish - e.g. I'd be at around 150 watts on the hoods for 15mph on the flat.
The relationship between the numbers looks sensible though, and illustrates the point well


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