I will not be using a trailer then.
Surely it should be fun and not a worry over frontal area and castor!!
Bike Forum
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Trailer vs Panniers for a road-based cycle tour
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Posted 1 year ago #
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No your born with the physics. If you haven't got it now you won't
Posted 1 year ago # -
It doesn't matter what the question is, a trailer isn't the answer.
Posted 1 year ago # -
Al - at no point did I say that.
You claimed that - I did not. I siad that with the bulky front panniers on out terminal velocity of the Glencoe road was around 35mph when on similar roads without panniers we reached 45+.
This was an observation that I stated was anecdotal.
Drag is directly proportional to frontal area. I don't understand how you get the 70% figure from - it makes no sense.
Posted 1 year ago # -
Posted 1 year ago # -
TJ of course you didn't say that. but it's not wrong.
your point is that the panniers created enough wind resistance to slow you down by 10mph or 30%.
as wind resistance increases with the square of speed that would require the wind resistance to have increased by the square of 30%. you calculate that by squaring 1.3, which gives you 1.69 which I rounded to 1.7, ie 70%.
if you don't understand this then fair enough but that doesn't make it wrong (and can a numeratd person back me us please).
Posted 1 year ago # -
Come on, dear. Wake up dear.
Mother!
Come on dear.
So, it was all a dream.
No dear, this is the dream, you're still in the cell.Posted 1 year ago # -
nightmare more like!
Posted 1 year ago # -
Cynic-al is correct in that if the drag factor is the same then a 30% decrease in terminal speed means a 70% increase in area. Thats keeping the mass of both bikes the same....
Posted 1 year ago # -
NO! wind resistance, not just area.
Posted 1 year ago # -
Al - it is far more complex than that and you are getting confused anyway.
Wind resistance increases in proportion to frontal area. The squaring only take place a s you bring speed into it. which produces the drag
You are confusing concepts here and its led you into an error. drag coefficient ( wind resistance) and drag ( retarding force)
Posted 1 year ago # -
ok by wind resistance I mean drag (ie force), coefficient of drag is different I know and I thought that would have been obvious, how could anyone confuse that with force?
if I am wrong them what is the explanation?
Posted 1 year ago # -
The explanation is that the panniers increased the drag to the point that the bike reached a lower terminal velocity and that I could feel this.
My post above is somewhat garbled as well. and I have reached beyond my knowledge now.
All I know is I could really feel the increased wind resistance with a bulky set of front panniers. I don't know how much they added to the frontal area or the drag coefficient. Just that it was a lot, it was obvious and it slowed us right down
You brought the maths into it and what you have presented makes no logical sense to me. i think you are confusing concepts.
Posted 1 year ago # -
ah, so you don't know what I am talking about and yet you are arguing I must be wrong.
a new low.
must stop wasting my time.
Posted 1 year ago # -
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.
Posted 1 year ago # -
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.
Posted 1 year ago # -
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}
Posted 1 year ago # -
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?
Posted 1 year ago # -
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.
Posted 1 year ago # -
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
Posted 1 year ago # -
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
)
Posted 1 year ago # -
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 [url=http://en.wikipedia.org/wiki/Stokes'_law]Stokes law[/url] 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.
Posted 1 year ago # -
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.
Posted 1 year ago # -
Found something in Prandtl's Essentials of Fluid Mechanics - Herbert Oertel
Not pleased with that approximation!
Posted 1 year ago # -
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
Posted 1 year ago # -
I think there was some confusion and misunderstanding on both sides. trying to describe physics in imprecise words
Posted 1 year ago # -
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?
Posted 1 year ago # -
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"
Posted 1 year ago # -
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.
Posted 1 year ago # -
ok TJ why did you say I was wrong? you've not proved that.
Posted 1 year ago # -
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?
Posted 1 year ago # -
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
Posted 1 year ago # -
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
Posted 1 year ago # -
mcmoonter. Identical physique and all up weight?
Almost identical.
Posted 1 year ago #
Topic Closed
This topic has been closed to new replies.
