Gear ratios?

Numpty question that's probably been asked many times

1 to 20 of 27 messages
29/03/2014 at 21:23

Currently running a standard 53/39 - but having just moved to a TT bike I am currently suffering with loss of power with the new position . . 

I am on the same ratios as my roadie but not climbing with the same power - the question is, should I change to a compact 50/34 or just HTFU?

citizen 146    pirate
30/03/2014 at 00:52

you are right similar questions come up all the time , should be able to find them in the old threads

30/03/2014 at 07:49

I don't have a TT bike, so all I can offer my thought process in that situation.  There is a trade off between an aero tuck Vs producing power.  

If you look at Fabian Cancellara, his body is a little more upright than it could be.  Big Mig was a little this way too.  Some smaller bodied people can get crumple up into a very aero position- Wiggo for example.

It may be that your new bike has forced you into a more streamlined position, which your body cannot hold.  Perhaps your knees press into your chest a little?  This would reduce your leg power.

I would go for a RETUL bike fit.  As I understand it, they allow for your body shape and flexibility when optimising your bike positioning. It will definitely solve the problem.

 

30/03/2014 at 07:51

What is or are your target races for this year? If your doing say the Outlaw then stay with the double as it flatter then a witches tit.....If your doing IM Wales then i would consider a compact. Are you planning on entering any TT's?

30/03/2014 at 09:10

Retul done:

Not up to IM wales just yet . . 

Problem being that I live in Norfolk so not getting any practice in on any "proper" bumpy stuff.  From reading up it looks like a temporary loss of power is quite normal and does come back once the body adapts to the different position on the TT bike and uses the different muscle groups.

I think it's my rubbishness on the hills that is going to show as soon as the road goes up though - and it's always embarrassing to be passed by someone on a mountain bike whilst grinding up a small incline!

Hit the bottom of a small 15% grade on a training ride recently and the guy in front of me just flew up whilst I was breathing out my arse by the time I got to the top - it's only about 400 metres!!!

I was also thinking about keping the legs fresh so higher cadence verses grinding it out . . .

so HTFU it is then unless I get anywhere really hilly!!!

 

Andy

30/03/2014 at 19:48

Are you staying on the aero bars up those hills? Probably not much point at 15%. If you sit up you should have pretty much the same power output as the road bike right? As you've concluded I wouldn't switch to a compact unless doing a really hilly course. 

I wouldn't notice much difference between the road bike and TT bike so I would consider your position. For example if the saddle is too low that limits the power in your quads in a big way.

TheEngineer    pirate
01/04/2014 at 14:14

Also, check your crank length.

01/04/2014 at 17:03

Mr Engineer sir - do elucidate?

01/04/2014 at 17:27

In imprecise terms, longer lever - greater force.  Same principle as the weight you have to put through an allen key to undo a stuck pedal.  The longer the crank arm, the more power you're producing on a revs per minute for comparable revs per minute on a shorter crank.

I was pleased to have received this advice when buying my new bike (didn't know it was an option).  Now, the speed I can produce from my tree trunk legs benefits from the extra few CM on my crank arm.

Awaits more scientific input from Chief E.

01/04/2014 at 18:11

 you'd think a longer lever would mean greater power but in fact studies don't show that is the case.  Obviously for a given cadence you get a higher power but you could say the same about fitting a 55tooth chainring.

A longer crank also means a lower saddle height (as you have further to reach at the bottom of the stroke) so harder to get a good aero position too.  

 

 

01/04/2014 at 20:16
Smaller bike, longer lever, worked a treat for me. But of course, you're only getting a greater power output for comparative cadence.
02/04/2014 at 21:45

May have done but it has nothing to do with a longer lever meaning greater force - google it for the reason why.   

citizen 146    pirate
03/04/2014 at 12:28

liking this technical discussion, surely the engineer would be the man to listen to, this has just sparked a massive discussion at work , not googled it yet ...

03/04/2014 at 14:46

Pops, I look forward to hearing the technical info.  I only studied physics to GCSE and therefore, perhaps naively, assumed the longer crank arm allowed greater leverage from the pivot point.  Perhaps I've just simply my understanding.  Either way, let the (re)education begin...

03/04/2014 at 16:47

You'll def get more leverage with longer crank and the pedal at 3 o'clock position but when you're actually pedalling the larger cranks can make your pedalling less efficient with a bigger dead spot so you've got the possibility of power loss.. The ideal length of crank is going to vary person to person anyway and nobody has defined what 'long' is. What length crank did you switch to and from? I've got 170mm on my TT bike and 175mm on the road bike. Pedalling the road bike feels very clumsy compared to the TT bike. Only for the fact i had a bike fit done recently i'd switch the 175 for a shorter crank. 

TheEngineer    pirate
03/04/2014 at 17:37

Sorry PRW, longer lever doesn't make you any faster at a given cadence. The force required to turn the pedals through one full rotation is still governed by the system resistance (inertia if accelerating + rolling resistance + aerodynamic resistance). 

http://www.slowtwitch.com/Tech/Crank_arm_lengths_for_tri_727.html

http://www.slowtwitch.com/Tech/Crank_Length_and_Gearing_4095.html

That should get you going. If that's too much reading the summary is that shorter cranks are good from a fit perspective, but if making wild changes then one must consider gearing - not because the mechanical advantage has changed but because the speed of your foot has changed (owing to the diametral difference). 

The reason I mention it here is that if Oneof42 is running a longer crank on the TT (which used to be the 'conventional wisdom'), he/she may well be finding that because the range of motion has increased they are not accustomed to the change in biomechanics. It may even be possible that they've closed up their hips so they are inhibiting bloodflow - this doesn't to me sound like a gearing problem.

 

Edited: 03/04/2014 at 17:38
03/04/2014 at 18:28

Robbie, I didn't even take physics to o level so probably not the person to explain why.   I just know studies show no real advantage with long cranks and then the disadvantages for bike fit mean I'd say for anyone unsure err on the side of shorter - but I'm all for people going for what they find works.  

Can anyone explain why greater leverage doesn't give an advantage - is it because gearing effectively allows us to increase leverage or is that different?

04/04/2014 at 22:02

My brain hurts . . .

OrangeCannon kona-5    pirate
08/04/2014 at 23:10

i have a sram wifli climbing gearset on the rear of my TT bike, and a full fat chainset on the front..... looks silly but means i can spin up stuff.   its slightly lower gear than a compact / stnd rear casette.  

maybe a bit cheaper too....

10/04/2014 at 21:17

I'm going with changing the cassette to a 12-30 to see how I get on . . .

Was passed by a colleague on a local hill this morning . . . 

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