8 speed Front Mech Triple Mountain Bike 44/32/22

[deliquium]

I'm wanting to return a 1991 Raleigh 'Blueridge' 531AT frame to its mountain bike duties, but with more modern bits - specifically a smaller ringed chainset. Will use one of my custom 13-32 8 speed cassettes and 8 speed bar end shifters on brackets as thumbies (friction front changer)

Thinking of getting a Spa 44/32/22 square taper chainset, but wondering how critical are the stated angle limitations of an Alivio FD-T4000 front mech?

https://www.spacycles.co.uk/m2b0s166p14 ... Mech-T4000

This mech is specced at 63-66º - presumably that's between the seat tube and chainstay?

Just measured the 'Blueridge and it's getting on for approx 68º. (the seat tube angle is a rather steep 'modern' 73º)

Also the T4000 chainline is stated as 50mm, which is more than I would like by about 5mm!

Any advice or comments would be most welcome

[Brucey]

the angle thing isn't usually a big problem (mostly it alters the capacity of the FD, such that if the frame angle is greater than specified, the capacity of the FD is reduced slightly) but the chainline is more likely to give trouble.

IME a FD like this



(i.e. a bottom swing one) will cope better with a 'road' chainline.

If you get one with a braze-on mount, and a band-on adaptor, the interface between the two can be doctored so that any angle issues can be allowed for and overcome. You will get the best shifting if the chainring interval you are using is close to that originally specified for the FD's intended chainset. IME Older MTB FDs were often used with 46T chainrings, so 44T is close enough in most cases.

BTW you can run into another kind of trouble with small chainrings and a smaller chainstay angle than specified (eg fitting a MTB mech to a 700C wheeled frame); the tail of the FD may hit the chainstays. I don't image that will be a problem in your case though.

Because you are using bar end shifters you don't have to worry about shift ratio, which is a whole can of worms by itself.

cheers

[Sweep]

This mech is specced at 63-66º - presumably that's between the seat tube and chainstay?

Found this thread trying to figure out something similar for something I am doing.

So seemed the best on-topic place to post rather than starting a new thread.

Can someone give a direct answer to del's question so that I can see if I have got it right?

If possible with a link to an online angle diagram to avoid any confusion?

I ordered a spare front mech to stock up for my Ridgeback Expedition.

I take it it is the same as Del's - the Alivio T4000.

On the box mine also has DS3 after that number code.

It's this one:

https://bike.shimano.com/en-EU/product/ ... 0-DS3.html

The other two (non ticked) tick boxes on the box are the T4000 TS3 and the T4000 TS6. Both are top swing, the first with a published angle (meaning to be clarified) of 63-66 and the second with a published angle of 66-69.

When ordering the thing I couldn't actually read the name on the back of my existing mech (damn Shimano's unecessarily tiddly metal printing) but selected the one I did (down swing) as it appeared to be the only one in the range which was down swing and would take a 48 big ring. So bingo, that must be the one.

The odd thing is that the box says that "angle" on this mech is 63-66 and when I last measured what I took to be the angle in question on this bike I got a number substantially above 66.

Yet all, as originally supplied by Ridgeback, works wonderfully.

Have I got something really basic wrong or has the mighty Ridgeback worked some magic that is beyond my O level maths?

[hamster]

The angle number is for the seat tube angle on the frame - not sure if it is directly measurable on the mech.

[NetworkMan]

The chainstay angle is the angle between chainstay and seat tube. A 26" wheel mountain bike used to be reckoned to have a chainstay angle of 66-69 degrees. A road bike will have larger wheel and may have a lower bottom bracket both of which will reduce the chainstay angle w.r.t. the mountain bike so the chainstay angle is more like 63-66 degrees. I don't know about 29ers.

Fitting an FD with the 'wrong' chainstay angle may not matter very much, apart from a lack of shifting precision, however some issues may arise. If the chainstay angle is too big for the mech the chain will run low in the cage and may rattle against the bottom of the cage when on the smallest ring, reducing capacity. Conversely if the chainstay angle of the frame is too small the chainstay may interfere with the bottom back of the cage although you may get extra capacity.

Hope that helps, most of it is from a 2003 article in Cycle by CJ which I couldn't find on the CTC site when I last looked.

The FD that Sweep linked to is presumably intended for a hybrid of some sort (presumably with 700c wheels) since it has a mountain bike chainline of 50 mm but a road type chainstay angle.