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Buying a new rear derailleur.
Hi all, I'm a bit new to the maintenance of a bike so would really appreciate any help. I have a road bike that needs a new rear derailleur fitted.

My current rear derailleur just says shimano on it but my gears are shimano sora triple deck. I've looked online for a replacement shimano sora derailleur which are all 9 speed but mine is an 8 speed. Will this cause a problem?

Also, I've seen that you need to know the total tooth capacity. Can you just confirm how you work this out?

Thanks for any help!
Any 8 or 9 speed Shimano or Microshift RD with adequate capacity will work fine.

Total tooth capacity:
A = number of teeth in your big chain ring; for example 54
B = number of teeth in your smallest chain ring; for example 26
C = number of teeth in your largest cog of your cassette(or freewheel); for example 34
D = number of teeth in your smallest cog of your cassette(or freewheel); for example 11

Total TakeUp = (A+C) - (B+D)
in the example = (54+34) - (26+11) = 51 teeth
The example above is our Trek T50 tandem, we never use 26/11 combo, and would not go past 26/21;
= (54+34) - (26+21) = 41 teeth
so the RD only needs a take up capacity of 41 teeth.

The other number of teeth you see with RD's is the largest cog of the cassette or freewheel that it will work with. Most road RD will handle 28 teeth, some a bit more. MTB and Hybrid RD are usually rated for at least 34T.
Thanks for the help!

I'm also fitting new gear shifters but can't afford to replace like for like so was going to get some shifters that just fit onto the handlebars like on a hybrid or MTB. Do you think there'd be any problems with doing this? At the moment I'm looking at shimano Altus shifters and a shimano Sora rear derailleur.

Any help appreciated!
Road FD have a different cable pull ratio than MTB/Hybrid FD. Indexed shifters need to match. There are flat bar road bike index shifters that match road FD.

You can find 8 speed Shimano drop bar shifters on Amazon.

Thanks for all of your help so far - really appreciate it!

I'm looking to buy the Shimano Sora RD.

And then get one of the following flat bar shifters:


Those are the only ones that I've been able to find on Amazon that should be suitable. Would these work together ok and do you have any recommendations?

Many thanks,

Hi Laura;

The Sora and Claris combination will work with your current front derailleur.
Thanks Nigel, much appreciated!

Hi Nigel,

I've had to move to a flat bar handle bar due to the shifters not quite fitting my old handlebars. Due to this my brake levers won't work on the flat bar due to their orientation.

I need to get some new levers that will just work the brake for me and fit the flat bar. I've looked for Sora brake levers to keep the group set but I can only find the ones below on Amazon.


Do you know of any problems I'd have going from the road bike STI shifters to separate gear and brake levers?

Thanks in advance,

Hi Laura;

LOTS of problems coming your way.

First get us some detailed pictures of your bike; both overall and the handle bar area.

The brake levers in the link work only with V-brakes (aka linear pull) and SOME disc brakes. They will NOT work properly with other kinds of brakes.

The flat bars you pick need to have the center section the same diameter as your current road bars; or you will need to replace the stem. Common diameters are 25.4, 26 (drop bars only) and 31.8 (latest).

You are going to need all new cables, and probably new housings too.

You have gone down a very expensive road. Replacing the orginal STI levers would have been less money - and still might. Look for some Microshift SB-R483 8 speed road levers.

Instead of going the flat bar route, it may be less expensive to convert to 9 speed.....

You have given us a very small percentage of the information required to properly advise you. Your questions have been answered, but if you had given us a complete picture from the start, the answers would probably have been different.
Hi Nigel,

Firstly, thanks for your help so far, it is massively appreciated.

Secondly, please accept my apologies if I've made anything confusing.

To give a little background, this is exactly what happened. I originally had to replace my gear cable, when I did this I found a loose piece inside my STI levers. I tried to fix this as best as I could but it wouldn't work and another piece broke in the process. I also had to replace my rear derailleur which was broken and really needed to be replaced. So I bought new gear shifters and a new rear derailleur. It was my intention to keep the drop bar handlebars and just fit on the new levers keeping the STI levers just as working brakes. I didn't see any size on the website for them to fit but in honesty I should have emailed them to check before buying. When I went to fit the shifters I found that they were ever so slightly too small to fit the drop bar. It was then that I decided to see if the spare flat bar I had would fit my handlebar stem, which it did.

So....to sum up, I already have a flat handlebar that I managed to take off an old unusable bike that fits perfectly. I have the shifters on the flat bar all set up and connected. All I need now is a way to apply the brakes - i.e. new levers that work with my current brake system.

My current brakes are Cantilevers with the model number BR-CT91. If it is possible I need to buy some levers that will work with these brakes. I've seen that most levers come with new housing and cables, if they don't I'm happy to buy them as they don't cost too much.

I can take some pictures as you suggested if you would find this helpful but I'm going to be without internet from midnight Wednesday until 17th November at the earliest due to changing our internet provider so it may take a while to progress any further with this.

Again, thank you for your time, patience and help. I really do appreciate it.


Hi Laura;

No apologizes needed; unfortunately, you are the one taking the loss.

Brake levers: Tektro MT2.1 Brake Levers - they have two positions for the cables, one for V-brakes and most cable operated disc brakes, and one for other styles of brakes. I have these on a several of my bikes - encompassing V-brakes, cantis and dual pivot side pulls. They work well for all. They do NOT come with cables or housings. Please also note that "road" brake cable will not work, "universal" and "mountain" will. Universal have a road fitting on one end and a mountain fitting on the other end.

Chain Reaction is one source, shop around.

These levers are not for drop bars, and will work with most other styles of bars. If for some reason in the future, you have problems with one or both of your canti brakes, you can replace with much less expensive and more powerful V-brakes.
Hi Nigel,

Thanks for the reply. At the moment I've spent about £50 total and the cheapest replacement levers I could find were £95 upwards so at the moment if nothing else goes wrong it will be cheaper than replacing with STI levers.

That looks like a really good way to go with the brake levers as it'll make any future problems easier to deal with. Is there any particular brand of brake cable that you would recommend? Clarks seem to offer quite cheap cables with the housing included. I'd prefer to get something about mid-range and upwards in terms of quality. Also, I've seen a few offer different lengths on the cables, presumably this is just the difference between the front and rear brake cable lengths?

Again, thanks for your time and helpful advice.

Hi Laura;

Cables; I currently purchase Jagwire in bulk because I have a large number of projects. In bulk it is very inexpensive. In the past I purchased Bell brand - cheap kit with front and rear brake and derailleur cables. Clarks is similar.

Lined housing is bit smoother.

Make sure you lubricate the cables with a drop or two of light oil. I use Tri-Flow, which is a light oil with PTFE particle suspended in it, mixed with a solvent, so that much of it evaporates, leaving a very slippery film which also provides some corrosion protection.

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