Getting more aero on a touring bike

[mrcharly-YHT]

   
I tried some thought-experiment calculations on surface areas of objects on my bike.

For my body (ex-legs), I came up 1225cm2, riding on hoods. If I get down low onto the bars (as if I were in a tri-bar position), that drops to 800cm2.

My current mudguards, excluding area masked by tyre, are 380cm2. That's just the vertical area.

A whole third of the surface area of my body!

Hey, Gonzo, you're the expert here. How much difference does a change in frontal area make?

[mattc]

My current mudguards, excluding area masked by tyre, are 380cm2. That's just the vertical area.

A whole third of the surface area of my body!

I don't believe that. Mudguards DO have a lot of drag* , but 1/3rd of your body's area just sounds completely wrong.

<maths teacher>
Please show all your working!
</maths teacher>


*(front ones I reckon have a lot due to their scoop shape facing forwards - rears I suspect aren't as bad as they're in a turbulent region, but that's not what we're measuring here ...)

[mrcharly-YHT]

Guards are 7.5cm wide. About 50cm of forward-facing guard. So (7.5-3.7)*100=380. They are humoungus guards mounted nearly 5cm from the tyre.

On the hoods, less than a foot of my body is vertical forward area (guessing). I'm skinny. That's how I end up with 1225 for body.

It might be an under-estimate. Even doubling the body frontal area means the guards are 17% of the body when on the hoods, 23% when I'm tucked down.

It seems a great deal to me.

[Greenbank]

But how much of the area of your guards is shared by the frame, forks and tyres?

[gonzo]

D = Cd * ½ * (rho) * V² * A
Cd =  a shape factor, between 0.8 to 1.2 depending on position
(rho) = density = 1.224
V = velocity in m/s
A = frontal area in m²

CdA should be about 0.3 when TTing.

The rule of thumb is that the following are all roughly the same:
5 W reduction
0.005 m² CdA  reduction
0.5 s/km reduction
0.5 N reduction

[mrcharly-YHT]

Going by Gonzo's figures, changing my mudguards~ 15W reduction in power needed to maintain the same speed.

[mattc]

Guards are 7.5cm wide. About 50cm of forward-facing guard. So (7.5-3.7)*100=380. They are humoungus guards mounted nearly 5cm from the tyre.

On the hoods, less than a foot of my body is vertical forward area (guessing). I'm skinny. That's how I end up with 1225 for body.

So by "body" I guess you mean "all of me except my legs and feet".
Given that, and your huge guards, I can believe your estimate

But then:

But how much of the area of your guards is shared by the frame, forks and tyres?

... so I think you need a wind-tunnel to get any sensible figures!

[ You could probably do some before/after tests at constant heart-rate, but it sounds like you want to avoid that sort of thing! ]

Perhaps google

drag  mudguards -caterham -seven .

[mrcharly-YHT]

Guards are 7.5cm wide. About 50cm of forward-facing guard. So (7.5-3.7)*100=380. They are humoungus guards mounted nearly 5cm from the tyre.

On the hoods, less than a foot of my body is vertical forward area (guessing). I'm skinny. That's how I end up with 1225 for body.

So by "body" I guess you mean "all of me except my legs and feet".
Given that, and your huge guards, I can believe your estimate

But then:

But how much of the area of your guards is shared by the frame, forks and tyres?

... so I think you need a wind-tunnel to get any sensible figures!

[ You could probably do some before/after tests at constant heart-rate, but it sounds like you want to avoid that sort of thing! ]

Perhaps google

drag  mudguards -caterham -seven .

Well I did subtract the area of the mudguards masked by the tyres - at 3.7cm they are wider than the frame.

The difference in mudguard width is still large. Everything else is unchanged, so the 15W probably still applies.

Plus, if I can get new guards, I will be mounting them closer to the tyres, which should smooth airflow a bit.

[mattc]

Perhaps google

drag  mudguards -caterham -seven .

Having just removed my audax bike's fenders guards for the Mersey 24, this is a subject I cannot help thinking about. So I googled. And the best I could find was:

I have a 5 mile descent which with fenders I max out at 42 mph, without the fenders (same bike) I can hit 44 mph.

I'm too fuzzled right now to do the maths/fizziks to convert this to "Matt" speeds, especially as we have no idea of the reliability of this anecdote.

[gonzo]

Take a front on photo with you on a bike in front of a white sheet.

Use a histogram feature of a photo package to work out how many pixels are you then convert between pixels and m² and you've got your answer.

[mattc]

Well I did subtract the area of the mudguards masked by the tyres - at 3.7cm they are wider than the frame.

The difference in mudguard width is still large. Everything else is unchanged, so the 15W probably still applies.

Plus, if I can get new guards, I will be mounting them closer to the tyres, which should smooth airflow a bit.

Intuitively I'd expect narrower guards to reduce drag. BUT i can't help thinking of GB's point: your forks are still wider than your mudguards.

I still think forks are more slippery than scoopy mudguards, so these frontal area measurements may be irrelevant.

I like the fact that you can find data on the web for the drag of every component under the sun - expect mudguards, which presumably aren't owned by any of the people doing these test!

[mrcharly-YHT]

lol

My mudguards are 75mm wide.

My forks are 100mm wide.

There is a substantial gap there, so must be some airflow through it. I don't think it can be simplified down to an outline size.

Guess there is no alternative; I'll simply have to build a windtunnel.

[gonzo]

I still think forks are more slippery than scoopy mudguards, so these frontal area measurements may be irrelevant.

From the equation I gave above, Cd is much higher for guards than forks.

I like the fact that you can find data on the web for the drag of every component under the sun - expect mudguards, which presumably aren't owned by any of the people doing these test!

You can't just say that guards have a drag of X and the bike has a drag of Y thus the total is X+Y.