Al – I have no idea of the point you are trying to make.

It makes no sense to me at all. I suspect you have concepts confused as what you are claiming disagrees with my understaning of teh situation -but you are unable to explain what you mean – so you get offensive again.

Nice.

You tell me I am wrong – that something I have experienced cannot have happened.

You come in with some calculations that make no sense and that you are unable or unwilling to explain.

I don’t understand where you get your numbers from. I don’t think you understand the concepts hence your confusion

I don’t understand your need to put me down all the time.

If it helps at all (probably not, but IIRC TJ has trusted my science before) in general I agree with Al.

To sum up the point he’s making and keeping it as simple as possible:
If your terminal velocity is 45mph without panniers and 35mph with, then using
drag force = A*Cd * v^2
rearrange:
A*Cd = drag force / (v^2)

At terminal velocity drag force must be the same for both since it counteracts the force of gravity (which is constant for a given gradient). Therefore A*Cd is inversely proportional to v^2 – ie at 45mph
A1*Cd1 = k / 2025
at 35mph
A2*Cd2 = k / 1225
Bunging those two equations together:
A2*Cd2 = A1*Cd1 * (2025/1225)
A2*Cd2 = A1*Cd1 * 1.65
(so 65% more drag rather than the 70% Al claimed, but we’re in the same ballpark – I think he just approximated a bit earlier).

{for the physics pedants I should point out I’ve simplified slightly by removing the p/2 term from my first equations, but that’s constant so makes no difference}

I am not saying it didn’t happen, I am expaining that there must have been other factors, as the 30% reduction in speed would have required a 70% increase in drag, therefore your experience can’t be down to the panniers, and is a bit meaningless.

I can’t make the calcs any simpler I don’t think, in any event it would be obvious to anyone that understood.

You’ve gone from me being wrong and you being right to you not understanding but me still being wrong (like many times before…) – can you see that might be frustrating?

EDIT yes AR I rounded the difference to 30% when it is in fact 2/7.

2 people backing me up TJ, am I still wrong?

Oh and for the record, I don’t believe panniers have anywhere near a 70% increase in drag – not even front ones. I think that’s the point TJ is making (so I kind of agree with him too), the trouble is, that means the 35/45mph figures are wrong. It’s worth pointing out that front panniers will also have a hugely bigger influence on drag than back ones, given the back ones do sit in your wake.

Aracer – drag force 65% more but how does that equate into increase in surface area?

Al -at no point did I claim what you say I did. Go back and read it.

I still think you are confusing things.

“and as its exponential (square IIRC ) its not 30%
but more like 70%.”

that bit. no way can panniers add 70% more drag.

Edit – aracer – the panniers were huge – around 80-90 cm wide and 50 high sitting up high on the front of the bike

the panniers were huge – around 80-90 cm wide and 50 high sitting up high on the front of the bike

fairy nuff – in which case you’re proving that atypically large front panniers have a significant influence on drag – not that much more relevant in the context of this thread than skidartists stop/start around town experience (admittedly my kiddy seat/trailer data may not be much better 😉 )

aracer. Could you please give a reference for your
drag force = A*Cd * v^2
formula please I’d like to see where such a simple formula for drag comes from. I obviously know Stokes law as a simple drag relationship in a Stokes flow (which I am of course aware this is not) and have calculated drag using triple deck theory before but have never seen such simple approximation of drag unless I’m forgetting something very simple.

I’m not trying to say it is not valid but I’d like to see the derivation from the Navier Stokes equations that is all for my own interest.

indeed aracer – thats all I said. IMO IME a large set of front panniers made a significant difference

I really tried to make it clear that this was not good data. Why al had to make such a pedantic attack on me I don’t know.

Found something in Prandtl’s Essentials of Fluid Mechanics – Herbert Oertel

Not pleased with that approximation!

I was using the same sim[le drag formula as aracer, I have seen it in bycycle science amongst other places

For the record I didn’t say TJ was right or wrong about the effect of panniers I just said that for 2 bikes of equal mass and the same drag coefficient. Sadly a tandem is a poor example as it will have so much more weight.

However Cynical for the case of to bikes free wheeling at terminal velocity one at 30mph the other 45 mph then the drag forces are equal. They both are in equilibrium. They both have the same force acting down the slope so they both have the same drag. The slower bike is less aerodynamic so travels slower for the same drag force. TJ was clearly understands this point

I think there was some confusion and misunderstanding on both sides. trying to describe physics in imprecise words

90cm wide? really?

the post you’ve quoted above is what aracer has explained.

it seems you’re trying to say you have some relevant date and at the same time that there are so many caveats it’s irrelevant. which is it?

Al – why don’t you read what I wrote. Why do you have to search out points to attack me on all the time?

“cynic-al – Member

it’s an attractive idea that aerodynamics suffer with panniers but I doubt any of us can really state that authoritatively, or that it’s significant. There are too many other factors.”

True but I do have anecdotal evidence – a wide set of front panniers ( big rears as well)on the tandem our top speed down glencoe was 35 mph. On similar roads without panniers we reach 45+easily mph.

Lots of factors for sure but certainly the frontal area was greater and it appeared to me that the aero drag built more quickly that without the panniers

That is clearly anecdotal and clearly full of caveats and I agree with you that it is multifactrial. Just a bit of info to teh debate.

Why you feel it necessary to attack me in such a way – its not as if I siad “I know best this is what happened”

I think there was some confusion and misunderstanding on both sides. trying to describe physics in imprecise words

Did writing down the equations actually help then?

For TheBrick http://en.wikipedia.org/wiki/Drag_equation (as I said, I left off the p/2 constant) Too many years since I did FD properly, so no idea how that relates to Stokes.

ok TJ why did you say I was wrong? you’ve not proved that.

Can I throw something else into the mix. Years ago I went touring in the Alps. My friend and I were riding near identical bikes, we had bar bags and rear panniers and similar tyres. His decending speed was significantly faster than mine. We put it down to his having Campagnolo Super Record hubs and I had some cheap Maillard hubs.

Could friction in the hubs add significantly to drag?

Al- have a think about it. You have got your exponential s muddled

Wind resistance is proportional to frontal area. Its exponential to speed.

So at a steady speed double the frontal area double the #wind resistance. As speed increases wind resistance increases exponentially.

My last post on this.

As I said I think there is confusion on both sides – you think we are talking about the same things but we are not.

Its in the imprecise use of language.

Why can you never accept that you might have got he wrong end of the stick

3 times this week you have told me that my experiences are wrong or that kit I have fitted to my bikes does no exist

In your eagerness to attack me you don’t stop to think that you might have something wrong

mcmoonter. Identical physique and all up weight?

I’ve oftem wondered abou hub friction etc. but its at its least important when your going fast, that is least importnat relative to air resistance

mcmoonter. Identical physique and all up weight?

Almost identical.

Mmmm

Plenty of bickering useful information, thanks folks.
I reckon the on/off public transport issues may kibosh the trailer idea.
I might consider getting a frame bag made up, if I can come up with a decent template.

Could friction in the hubs add significantly to drag?

after extensive totally unscientific testing with hope hubbed bikes v. cup and cone hubbed bikes with all sorts of different tyres, i am utterly convinced you CAN tell the difference, and that the hope hubs were better.

the ‘test’ involved riding along at the same speed down a tarmac slope and seeing who gained on the other.

Pete, no way! sorry.

but if your panniers were a few mm bigger that could have done it

😉 JOKE JOKE JOKE

Pete, no way! sorry.

I was convinced his hubs were frictionless and have lusted after quality hubs ever since. 😕