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

[400bhp]

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).

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.

 

[Roadhump]

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!!

 

[Hip Priest]

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!

 

[GrasB]

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

 

[VamP]

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

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

 

[GrasB]

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.

 

[Ningishzidda]

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

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.

 

[GrasB]

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

 

[VamP]

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?

 

[Ciar]

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 ;-)

 

[GrasB]

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.

 

[PK99]

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

 

[Rob3rt]

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

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!

 

[Ningishzidda]

Assuming the plate has edges...


And?...


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


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


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.


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

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

 

[GrasB]

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.

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

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