Numpty IM Bike Thread

Bike ponces only welcome if they don't speak in tongues

11,881 to 11,900 of 11,977 messages
09/07/2015 at 15:36

I'll attempt (probably get it wrong too) an answer at this, but I'm sure someone like the Engineer will come along and give a better physics lesson. Apologies if this derails into drivel! 

The trouble with cycling is that you need to contend with both land speed and air speed. Land speed is the speed the bike travels over the ground (hopefully also with you attached). Air speed is the total of land speed + air speed opposite to the direction of travel. 


You're going at land speed of 10mph into a headwind of 10mph. Airspeed is therefore 20mph.

You're going at land speed of 10mph with a tailwind of 10mph. Airspeed is therefore 0mph.

You're going at a land speed of 20mph with no wind. Airspeed is therefore 20mph.

So - why is airspeed important? Mainly because this is the main component of what determines the air resistance stopping you going any quicker (F=MVsquared from memory - meaning a very large force is required to overcome a very small increase in speed).

So lets for the sake of argument now ignore rolling resistance, bearing friction etc. and assume that all things being equal, on a day with no wind, you're capable of travelling at a land speed of 20mph before the resistance due to the wind becomes too much to go faster. This is basically saying that you're capable of overcoming an airspeed up to 20mph.

If we now go back to our examples, but keep the airspeed constant and look at what that does to landspeed:

Travelling with a 10mph headwind means a max landspeed of 10mph (airspeed 20mph)

Travelling with a 10mph tailwind means a max landspeed of 30mph. (airspeed 20mph)

Travelling with a 20mph headwind means a max landspeed of 0mph (will be very hard work! - airspeed 20mph)

Notice how in all the above, body position hasn't been mentioned. Body position will change the amount of resistance for a set airspeed. Tucking in will reduce the resistance (make you go quicker) and sitting up will increase the resistance. Lets say that the 20mph max airspeed was due to an upright riding position, and a tucked position would allow you to reach 30mph airspeed.

Our examples now become:

Travelling with a 10mph headwind means a max landspeed of 20mph (30mph airspeed)

Travelling with a 10mph tailwind means a max landspeed of 40mph (30mph airspeed)

Travelling with a 20mph headwind means a max landspeed of 10mph (30mph airspeed)

So in short - no, sitting up won't have any benefit, because you should still be pushing hard enough that there will still be an effective headwind - you'll just be travelling alot quicker in terms of land speed! If you were to sit up and stop pedalling, you'd get pushed along, yes, but at a much slower speed than you should be capable of.

tl;dr version - It doesn't get easier, you just go faster! 

09/07/2015 at 16:11
DaylightR wrote (see)

I'll attempt (probably get it wrong too) an answer at this .........

tl;dr version - It doesn't get easier, you just go faster! 

Wow!  You could have done another training session in the time it took to write that  

Good post though

Booo    pirate
10/07/2015 at 09:26

Brilliant stuff, thanks DR .. I think I understood it (on about 4th reading )  ... so its aero (the term being used loosely here)  all the way then !

Razor51    pirate
10/07/2015 at 10:34
Squeaky bottom bracket, could it just need cleaning and regreasing if so what grease?
Bobs Your Father    pirate
10/07/2015 at 12:28

I once asked a very good time triallist for advice on getting faster.
His sage words of advice were:-

"Get lower and pedal harder!" That is all....

Edited: 10/07/2015 at 12:28
27/07/2015 at 22:43

ok, whats the likely cause ?

just finishing of rebuilding my travel bike. boardman alloy frame. ultra 6700 parts. all seconds.

fitted new bars and gear/brake cables. everything works ok without being on it.

BUT, when riding and specifically, when pushing the chin jumps of the big chain ring.


not when on the smaller chainring. - I think the rear slips occasionally also.

chain is near new. chainset is second hand - maybe its worn to much? could it be the big ring?

maybe the adjustment is out - chain length is good. needs tweaking a little for the rear maybe. front seems ok. just under load it slips pretty much every time of the big ring.

was able to clim out of the saddle on the smaller chain ring. can't do out with the big ring.

chain seems to be loose of the big ring. lots of side movement. but the same on the small ring



Cheerful Dave    pirate
28/07/2015 at 07:46

I had a similar problem a while ago, and it turned out that the problem was a stiff link in the chain that would cause it to jump at the back, but it only happened when I used the large chainring, probably because when I used the small ring the sharper turns through the rear mech sorted out the stiff link.

Otherwise, if it only happens when you're working hard, it might be that the frame is flexing and the front mech is pushing the chain off. 

28/07/2015 at 11:12

so, will lube and flex the chain. might be the front mech. setting it up was a bit of a ball buster. 

its dropping of from a standing start. might be frame flex ? hope the frame isn't busted- can't see a sign of it.

29/07/2015 at 11:35

another ride. any pushing on the big ring, at any point, on either side and the chain slips off. not even with a great amount of effort.

little ring is fine, all the rear is fine ( minor adjustment on the downpipes to the cable )

no flex in the frame I can sense - solid out of the saddle and over the 'pending repair' cobbled roads of Toddington area.

chain works fine on the little ring through the block - although if on the littlest front and rear the chain seems long ( rubs at the derallier )

potentially, a buggered outer ring - seems these are not cheap to replace either ?  having someone take a look.

29/07/2015 at 11:37

having the chain "large" at the front and "large" at the rear, or the other way round is call cross chaining, And the fastest way to knacker the chain and the block.

Edited: 29/07/2015 at 11:37
29/07/2015 at 13:02

Any damage to the teeth on the cogs or is the large ring slightly bent? either could make the chain slip.

cougie    pirate
29/07/2015 at 14:44
Weird. Does the chain slip off to the outside - so towards your pedal ?

And the front mech isnt pushing it off ?

I've never seen that happen before.

Its not possible that the big ring is on the wrong way ? Outer face inward ?

I'd have thought tooth damage would be easy to spot.
29/07/2015 at 14:47

Easy to spot ?  This is Lard, he has only just learnt to recognise a bike.....

cougie    pirate
29/07/2015 at 15:02
cougie    pirate
29/07/2015 at 15:04
The best numpty thing I ever heard was some dude complaining about his tyres not lasting very long before they went furry.

Turned out he'd managed to install them inside out.
29/07/2015 at 15:30


i was checking the chain for length, not riding it on the extreme. its slightly too long - a link only maybe.

cable has been reconnected tot he front mech on the inside and not on the outside ( which makes more sense in a picture - so picture the cable coming up wrapping on the inside of the nut and then out, rather than on the outside of the nut and wrapping in ) - its easier to change up/down now.  

chain appears to have a twist on it for a couple of links. so obviously removing one of those might help. as would a new chain maybe

has slipped both ways - tends towards inside though. teeth on ring look fine ( worn but ). nothing missing, not bent. and I think it only fits one way - (ultegra 6750 compact 50T )

bike was easy to spot ( wheels, no rubber suit )  its the training I keep missing dave.????

29/07/2015 at 15:35

Wrong chain ? 



cougie    pirate
29/07/2015 at 16:02
Bizarre. You'd think it'd be secure on the big ring - there must be some kind of force acting on it to pull it off the big ring.

BB not in cockeyed is it ? Although I'm not even sure thats possible.
29/07/2015 at 16:06

Bent axle thro the BB ? 

I can see an 8 spd chain jumping off an 11 spd block, but don't see why it would jump off at the front ? 

Is the  chain sitting all the way down on the teeth of the big ring ? 

cougie    pirate
29/07/2015 at 16:17
Any photos of the chain on the chainset and the teeth of the chainset ?
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