Opinions sought on this drivetrain change

[graham bowers]

The bike is a Trek alpha 2.5 with 34/50 ultegra chainset, 12/25 10 speed 105 cassette, ultegra mechs, 10 speed STI's and chain.

I need lower ratios for hills, Derbyshire for example. The CTC group I cycle with are mostly on triples!

I'm thinking of an 11/34 Tiagra cassette with a Shimano medium cage rear mech, either 105 or Ultegra and a SRAM 10 speed chain, probably PC1051.

From my reading, the rear mechs seem to have sufficient capacity so it should all be compatible.

I've done a humungous spreadsheet that compares the current gear inches given by each ratio, the proposed gear inches plus the gear inches on an old rigid slicked up MTB I still use. I reckon the lowest new gear will be 27 inches, and I know how that feels from the MTB (accepting that it would probably be better on the Trek due to it being lighter and lower rolling resistance). And I realise some of my current ratios will no longer be available. I just want to get to the top, I'm not chasing segments etc.

What I can't form an opinion on is how slick the changes will be taking in to account the lower "quality" cassette and bigger tooth jumps.

Thoughts please?

Graham

 

[lpretro1]

better sticking at 32t for lowest sprocket - 34t is a big ask on a road bike plus the front end is liable to lift very easily when you pull on the bars uphill. Even I can manage most hills on a 32t and I'm a little old lady

 

[Joffey]

Before you start changing all that kit over just stick a 32 cassette on the back. Your mech will take it (you might not be able to ride 50 32 but you shouldn't be anyway). I did that on my 10 speed 105 mech and it was fine. Then just keep going up hills and you will get better.

 

[lpretro1]

Just remember to adjust the B-screw for the bigger sprocket!

 

[Citius]

What I can't form an opinion on is how slick the changes will be taking in to account the lower "quality" cassette

'Lower quality' is irrelevant - and highly debatable anyway. In any case, the tooth profiles are the same, so shift 'quality' will be unaffected.

 

[graham bowers]

So, two recommendations for 32t. That gets me 28.4 gear inches.

Joffey
I'd like to dig a little deeper in to your experience of using a 32t rear cassette if I may, as it is certainly an attractive option:

My current Ultegra RM is the short cage RD 6700, SS suffix, from my reading has a max recommended rear sprocket of 28t and 33t capacity. The medium cage GS version has the same 28t max rear sprocket, but has 39t capacity.

There is also an RD 6700-A that has a max recommended rear sprocket of 30t, not sure of the capacity.

My brain hurts :-))

Any idea which yours was please? The difference between the SS and GS versions seems pretty obvious once you see a pic of the two together, and the 6700-A pics I have seen have "30t" stamped on the cage.

Graham

 

[Joffey]

Mine was a SS 10 speed 105 with a max I thought of 28 but I saw a block with a 30 on his so tried and it worked.

I have also seen a 32 on dura ace di2 which isn't meant to work but it did by adjusting the b screw.

For the sake of £15 and the potential answer to your problems just get the 32 cassette and try it. If it doesn't work then send it back to Wiggle etc and get a med cage mech.

There are so many thoughts an opinions on this subject when I was playing around with my gears I just bit the bullet and bought the bigger cassette... And it worked.

Good luck!

 

[Joffey]

Here's an example of what you can do when you put your mind to it!!!

Look at that cassette on a Di2 Dura Ace mech!!

https://instagram.com/p/_7oxxaHJwd/

 

[Ajax Bay]

6.00 into this video:

View: https://www.youtube.com/watch?v=c5GX7O_aruM

 

[Smurfy]

I've done a humungous spreadsheet ............. What I can't form an opinion on is how slick the changes will be taking in to account the lower "quality" cassette and bigger tooth jumps.

In my opinion, what you need to look at is the percentage jumps between each gear, as this is what determines how the shift feels.

You can do this by modifying your spreadsheet, or by using a gear calculator. If you use the gear calculator method, best to consider each chainring separately, as it's too much trouble to keep shifting at the front, so you should only be using the front mech when you reach the last sprocket of the cassette.

Here is 34-50, 12-25, where the jump is expressed as 'Gear Step %'.
http://www.gear-calculator.com/?GR=...16,17,19,21,23,25&UF=2125&TF=90&SL=2.6&UN=KMH

Edit: Looking at the jumps separately for each chainring
http://www.gear-calculator.com/?GR=...16,17,19,21,23,25&UF=2125&TF=90&SL=2.6&UN=KMH
http://www.gear-calculator.com/?GR=...16,17,19,21,23,25&UF=2125&TF=90&SL=2.6&UN=KMH

 

[graham bowers]

Another aspect, Smurfy, thanks for bringing it up.
With this to think of as well, I should be able to put off making a decision almost indefinitely ;-))
Graham

 

[Ajax Bay]

it's too much trouble to keep shifting at the front, so you should only be using the front mech when you reach the last sprocket of the cassette.

Sorry, I am afraid I disagree with the above assertion/suggestion, do not think it a good recommendation, and it should not therefore be used to inform cassette choices. Shifting to the other chainwheel should be normal practice and is no trouble. As a rule of thumb I will change chainwheels to avoid crossing the chain large to large and small to small and not use the two largest sprockets when I'm on my outer, nor the two smallest when I'm on my inner. If the jump which occurs is too much for you to 'cope' with (either more force or more cadence) then double change by changing the 'opposite' way on the cassette (even two up/down if running a compact or semi-compact), at the same time as changing chainwheels. The only time I don't follow this rule is when I can see that I'm about to go through a dip (stay on inner) or if I can get over a rise just ahead and then there's a downhill (stay in the outer).

If you look at the various gear length calculators you can see the validity in this approach.

 

[Ajax Bay]

what you need to look at is the percentage jumps between each gear,

Meant to add that I agree with this part of the approach you advocate 100%, augmented by the proper integration of the ratios offered by both chainwheels.

 

[graham bowers]

Ajax Bay, I too try to avoid crossing the chain in pretty much the manner you describe, so will be taking it into account. Smurfy's point about considering percentage jumps is a valid part of the evaluation.
Graham

 

[Citius]

as it's too much trouble to keep shifting at the front, so you should only be using the front mech when you reach the last sprocket of the cassette.

That's the sort of thing that uninformed beginners might do. It's completely the wrong way of looking at it.