How Many Teeth?

[dtp]

Hello all, I've been lurking for a little while picking up little peices of knowledge here and there and now's the time for my first question!

I'm just about to buy a ribble and have the option to specify the number of teeth on the casette and the length of the crank arms on the chainset. My problem is i'm not entirely sure what I should be going for! I've had a look on sheldon brown's site and searched here but couldn't see anything.

My option on the cassette ranges from 12-23 to 16-27 with a bunch of options in between.

The option on the chainset is a little more academic seeing as I want a double not a compact - but does crank length make a huge difference? I can choose from 170 - 175 mm.

I'm just wondering what the most popular arrangement is? I'm going to be doing a variety of riding including some duathlon/triathlon type stuff and riding around wiltshire on the sunny days frequenting some of wadworth's finest ale houses...

It's Ultegra 6600 kit and I'm a rather average sized person, i hope that helps!

 

[PatrickPending]

I find 12 - 25 or thereabouts suits most of my riding - that's with a 53/39 on the front. I ride mostly in Leicester though so its pretty flat - I'd struggle on a 10% or higher hill for any distance

 

[RedBike]

The crank arm length depends on your height / leg length. Depending on who you ask and what you're doing then you'll probably get a different answer regarding the best length. Smaller cranks allow you to spin more whereas longer cranks allow more torque.

Average, height (5-10"?), In which case you'll probably be looking at 172.5 cranks. Although you can happily ride all three lengths without too much trouble.

 

[RedBike]

My option on the cassette ranges from 12-23 to 16-27 with a bunch of options in between.

Depends on the hills. A smaller outside sprocket (ie the 12) will allow a higher top gear, a larger inside sprocket (the 27) will allow for a lower bottom gear.

If you go for a cassette with a big range of gears then you end up with bigger jumps between each gear. I would go for the 'standard' 12-25 unless you live somewhere very hilly, in which case go for the one with the largest range possible.

 

[jimboalee]

Crank length has got no relationship whatsoever to your standing height.
It is neither a function of your leg length.

The simplest explanation is the one given by Peter White.

Its based on Hip height.

 

[SimonC]

Is hip height not a function of leg length??

 

[jimboalee]

If this Ribble bike is going to have Ultegra, my guess is it's less than 20lb.

Fit a gear that's less than 50" and you'll be OK.

39 x 25 is 42", so you're laughing. There will be 39 x 23 as well, which is 46". 39 x 21 is you're 50" gear.

If you struggle on these, a lot more training will make them easier.

 

[ColinJ]

The option on the chainset is a little more academic seeing as I want a double not a compact - but does crank length make a huge difference? I can choose from 170 - 175 mm.

I'm 6' 1" and I have been riding with 175 mm cranks on my MTB and 2 road bikes for about 20 years.

I just bought a bargain new chainset for my MTB. They were only available in 170 mm, but were cheaper than buying the chainrings separately so I bought one anyway.

I thought I might as well give the new cranks a go at the weekend - I could always put the old cranks back on later if I didn't like the new ones. Well, I know people who swear that the difference between 170mm and 175mm cranks is really noticeable, but the only thing I noticed in 5 hours of riding was slightly more clearance riding over rocks.

Perhaps crank length is more obvious on a road bike?

 

[jimboalee]

Is hip height not a function of leg length??

No.

Leg length is the distance from the ground to your perineum.
Hip height is the distance from the ground to the centre of your hip ball joint.

Not the same.

 

[dtp]

Cheers for all the replies everyone.

I've gone for the 12-25 on the back and the 172.5mm crank arms. whilst there are some hills about there isn't anything too serious.

Just got to wait for them to get the bike back in stock now....

 

[PatrickPending]

Cheers for all the replies everyone.

I've gone for the 12-25 on the back and the 172.5mm crank arms. whilst there are some hills about there isn't anything too serious.

Just got to wait for them to get the bike back in stock now....

sounds like a good choice.

 

[jimboalee]

Cheers for all the replies everyone.

I've gone for the 12-25 on the back and the 172.5mm crank arms. whilst there are some hills about there isn't anything too serious.

Just got to wait for them to get the bike back in stock now....

Did you measure your Hip Height?

 

[Tynan]

not sure he said same thing, just function of

unless hips are unrealted to legs, he's right enough surely

 

[hubgearfreak]

does crank length make a huge difference? I can choose from 170 - 175 mm.

3%. i wouldn't say it's huge

 

[jimboalee]

not sure he said same thing, just function of

unless hips are unrealted to legs, he's right enough surely

Seat height is determined from Leg Length. LL can be measured directly as RandoChap suggests ( but is subject to personal perception ) or indirectly by Seated Height Ratio.
Some LBS decide your frame size as 2/3 your LL, so if you've 33" LL, your frame is a 22".

Crank length is from the height of your Hip Ball centre.
The ratio of upper leg ( thigh ) and lower leg are used to determine seat tube angle.

The TRUE method of sizing a bike ( and cranks ) is to take the eight dimensions and work it out.

Start with the upper and lower leg. Calc the seat tube angle. Calc the Standover height from the BB height and LL and therefore the seat tube length.
Then the reach from Torso and arm length.
Draw the frame up on a big sheet of paper to get the correct BB to front axle distance.

Simples....