105 Rear Mech

willbevan

Hi guys,

I've started looking on ebay, seeing what sort of price second hand kit goes for, to get an idea how much it would cost me to upgrade my bike.

Could someone explain this though please

105 Rear Mech

9/10-speed compatible
Max. front difference: 16T
Total capacity: 29T
Max. rear sprocket: 27T
Min. rear sprocket: 11T

Now reading it, i would of though the Total capactity saying 29T would be the largest rear it could take, but no wait, the max rear sprocket is 27T on the next line :S

Presuming I understand than the max and min rear sproket is the range it will except, i.e. it will take a 11-27T, but wont take a 11-30T or a 10-27T(dont know if they excist), then what do the first two mean please? Max front difference and Total capacity?

Thanks

Will

Gragi

Worked out as follows:

difference in front gears plus difference in rear gears.

The maximum that this number can reach for the 105 derailleur is 29.

To illustrate, with 52-39 and 11-27 you would arrive at this:

52 less 39 (= 13)

Plus

27 less 11 (= 12)

Giving

25 (i.e. 13 + 12)

If you had a 52-39 and 12-25 combination then it would comfortably go within the maximum, ie. would come to 25.

However if you tried a triple, e.g. 52-42-30 plus 12-25, this would be over the total capacity. i.e. (52 less 30) plus (25 less 12) which equals 33.

Gragi

Whoops.. I wish I could count :oops:

I was originally going to illustrate with 11-23 cassette..... but changed my mind but didn't change the figures above!!! So now it must be really confusing!!!

What I meant to write was, for 52-39 on front, and 11-27 at the back you arrive at the total capacity:

27 less 11 equals of course 16.
52 less 39 equals 13.
So, 16 plus 13 equals 29. Total capacity reached for your 105 derailleur

I need a beer.

Gragi

Maximum difference on front is big ring less little ring.
So you could in theory have a 55-39 combination as this is a difference of 16.
You couldn't have 52-34 for example as this is 18.

I've just spotted another mistake in my original reply.
52 less 30 = 22.
25 less 12 = 13
22+13 = 35.... not 33 as I had originally typed.

Sorry. Another beer I think...

htravis

Total capacity is the amount of chain the mech will wrap. You see that if you max the range for the front, 16t, with, say 53-37, the maximum remaining range on the front will be 29t-16t=13t. So, if the large cog on the rear is a 27t, the mech may be folded over onto itself when you (but why would you ever?) have the chain on the small chainring in front and a 13, 12, or 11 on the back.

In practice you can go a tooth or two beyond the max in the rear with most rear mechs, with slightly compromised shifting. And, in further practice, where would you find a front chainring pair difference of 16t? IMO, always have enough chain for the incorrect large chainwheel large cog combination, or you may seize. Inadequate wrap capacity is benign and readily and harmlessly escaped.

HPT
USA

Gragi

HTravis obviously drinks much better beer than me as his response makes complete sense!

willbevan

ah thanks, now makes sense!!!

With the compact crank i've just bought for example i have:

50/34 on the front... so thats:

16T

On the back, I have 12-28

So 16T

So thats 30T...

If that was on the 105, which is maximum 29T, it would be tight.....

So i need to go check that the sora mech i have is the long cage, so it will cope with the 16T difference at the front (its either 14T or 22T)

And also that it has enough capacity, is either 29T or 37T looking at the sora stats...

Hopefully ill be alright!!! or ill be getting a new rear mech lol, don't suppose anyone has a basic 8speed rear mech lol

Get that right someone please?

Think i'll be going to check what my rear mech can deal with :S is a sora!

Thanks

will

willbevan

hmm looks like my rear mech is a short cage, im guesing here, but the distance between the centre of the two jockey wheels is about 50mm, now looking at my mountain bike thats a triple, the distance is nearer 75mm.

Can't find any specs on the shimano website about the sora's distance between the wheels so if anyone could confirm this for me that would be much appriciated?

Don't suppose i could get away with using an old shimano gs2000 of a 7 speed could i lol, looks like a wrong fitting though

Thakns

Will

Gary D

Yes, the 50mm centre distance is the Short Cage.
75mm is the Long (or designated Medium I think?).
You can actually get "Long" cage mechs for Mountain Bikes that will take a 45T difference.
Hope this helps.
Gary.

maddog 2

the total capacity is rarely an issue though.

I think about it (and calculate it) slightly differerently than those above:

running big-big is 52 up front and 27 at the back (52+27=79 teeth)

running small-small is 39 front and 11 at the back (39+11=50 teeth)

difference is 79-50 = 29

But the key point is that you shouldn't be running small-small anyway, in my opinion.


The main issue is whether the mech can handle big sprockets.

John.T

If you are using Shimano I don't know how you have a 12/28 cassette as they don't make one. Check it is not a 12/27 as this is the largest road cassette they make. If so the 105 short cage mech will work fine with this and a 50/34 chainset. It may just go to the 12/28 if that is what you have but this will push it rather hard. Shimano used to quote 29 teeth as the max but this was before they got in to the compact act. I don't know if they have updated their information yet but the short cage mech will definately take 31 teeth, ie:- 50/34 with 12/27. Correct chain length is critical and should be as long as possible while still having tension on the 34/12. I hope this is helpfull.

maddog 2

12-28 is available in 8spd (XT, LX etc.)

it's borderline whether a road mech will stretch to 28t (short or long cage). Sometimes it will bo okay, other times you'll get rubbing.

willbevan

the rear mech is shimano sora (sorry if confusion as to the 105, as it was some stats on a 105 that got me asking questions)

The cassete is sunrace or something, some cheap cassette, and its definetly 12-28

My main concern is that its a small mech, and the sora will only take 14T difference according to the shimano website and im going to be 16T difference :S

Might be cheaper (as im not fit enough) to fit a 48T larger chainring than getting a new mech, any opinions please? or should i just fit it when it arrives (50/34T crank) and being careful make sure the chain has enough play at the large end, and still enough tension at the small end?

maddog 2

just set it up so that when you're on big-big the mech isn't overstretched

don't worry about about small-small - you shouldn't use this anyway.

John.T

With a compact I have found that you use more ring/cog combinations than with a 53/39 setup. I use everything from 34/13 to 50/24 on my 12/27 cassette and on occasion have used the 50/27. 50/19 and 34/13 are the same ratio (near enough), about 69". This is my most used gear ratio so I need the chain tension to be good on 34/13. To achieve this it needs to be reasonable on 34/12. This cross over is the only disadvantage I have found with compacts. On 53/39 I just wore my inner ring out and only used the 53 for racing. I also stuggled more on hills.
Willbevan. You should be OK with the Sora. I have used one with 50/34 and 12/27. You will need to set the 'B' pivot adjuster screw carefully to stop the top jockey catching the 28 cog. Try it anyway and if it catches get a Tiagra or 105 mech. It should cost about the same as a 48 ring would.

maddog 2

if you drop the big ring down to 48 you'll stay on that ring most of the time, dropping onto the 34 only for proper hills.

You lose a bit at the top end but it's a minor loss unless you race. I've still hit 60mph downhill so it matters not o jot in my book.

willbevan

John Tiffany wrote:

With a compact I have found that you use more ring/cog combinations than with a 53/39 setup. I use everything from 34/13 to 50/24 on my 12/27 cassette and on occasion have used the 50/27. 50/19 and 34/13 are the same ratio (near enough), about 69". This is my most used gear ratio so I need the chain tension to be good on 34/13. To achieve this it needs to be reasonable on 34/12. This cross over is the only disadvantage I have found with compacts. On 53/39 I just wore my inner ring out and only used the 53 for racing. I also stuggled more on hills.
Willbevan. You should be OK with the Sora. I have used one with 50/34 and 12/27. You will need to set the 'B' pivot adjuster screw carefully to stop the top jockey catching the 28 cog. Try it anyway and if it catches get a Tiagra or 105 mech. It should cost about the same as a 48 ring would.

Cheers, thanks for typing up your experince with them, is very much appriciated! Will wait for the compact to arrive and give it ago! If it doesnt work then ill probably get a 105 mech (or an ultegra if its cheap lol), do the 10 speed rear mechs work with 8 speed cassette?

maddog 2 wrote:

if you drop the big ring down to 48 you'll stay on that ring most of the time, dropping onto the 34 only for proper hills.

You lose a bit at the top end but it's a minor loss unless you race. I've still hit 60mph downhill so it matters not o jot in my book.

Hmm i did notice today when i was on my larger 52T chainring that i could stay in it for the flat and some minor short uphills, but then i went to the 28T cog at one point :S before switched to the 42T front.

I realise its potentially bad using small small, is big big, also bad?


Thanks,

Will

John.T

It is best to avoid both the big/big and small/small combinations because of the chain being so far out of line. This will increase chain and ring/cog wear and also slightly increase chain friction. You should not need them as these and 2 other gears are duplicated. I only use the big/big combo when I get it wrong on a climb on the 24.
If you want to check your gear ratios have a look at Sheldon's gear calculator. http://sheldonbrown.com/gears/

willbevan

maddog 2 wrote:

if you drop the big ring down to 48 you'll stay on that ring most of the time, dropping onto the 34 only for proper hills.

You lose a bit at the top end but it's a minor loss unless you race. I've still hit 60mph downhill so it matters not o jot in my book.

Well im that unfit that unless im going down hill i can only use the largest 3 cogs on the 52 at the front and thats with a cassete that ends on 28 (unless you count the down hill sections) and thats 48ish-65ish gear inches. That will be smack dam in the middle of the compacts small ring so should be good, till i get fitter and are further up, but then i can always put the double back on.

When i say fitness i should say power in my legs, as i have a good areobic base from running :S (have been incactive for like 3 months though due to injury)

John Tiffany wrote:

It is best to avoid both the big/big and small/small combinations because of the chain being so far out of line. This will increase chain and ring/cog wear and also slightly increase chain friction. You should not need them as these and 2 other gears are duplicated. I only use the big/big combo when I get it wrong on a climb on the 24.
If you want to check your gear ratios have a look at Sheldon's gear calculator. http://sheldonbrown.com/gears/

Thanks for the info John, being a bit of a newbie, i have been running on the large chainring and using the large rear cog trying to stay on the front pushing myself to get fit, which to me makes sense, but if it increases wear, as long as i know where they overlap I can affectively do the same.

Thanks for the link to sheldon's gear calculator, made a spreadsheet thats all graphed so i can see the differences between my current crank,the compact and how each of the chianrings overlaps

Thanks

Will

John.T

Glad I could help. Don't forget your legs don't know what ring you are on, only what gear you are pushing. It is a good idea to spin a smaller gear most of the time. Too much big gear work can be counter productive. Aim to use a cadence of about 80 to 90 rpm most of the time but do some work either side of this.