Gear41

I recently purchased a 1988 Trek 400T. The gearing on the crankset is a little strange to me (it's not the original crankset). It is a triple with 52t, 48t, and 26t chainrings.

Anyway, I changed the shift cables today and I'm having trouble adjusting the front derailleur to accommodate the innermost chainring. Could it have something to do with the large range between the largest and smallest chainring, or am I just doing something wrong?

Either way, I really want to replace either the 26t chainring. Now, I've just got to find a replacement.

JReade

Half step with granny!

w98seeng

Derailleurs have a max tooth range they will accommodate. The difference on your bike is 28, probably too big a difference for your front derailleur (probably a double derailleur). If the derailleur changed properly before the cable change, it might be a double derailleur adjusted right at the outside limit of the derailleur and was able to change gears.

What model is your derailleur?

Here is an explanation from bikepro.com...
Front derailleurs have a rating known as "capacity". Capacity is a rating for the difference in the number of teeth between the chainring sizes. If the "capacity" of the front derailleur was 14 teeth, the difference between the largest and smallest chainring sizes must be 14 teeth or less. For example, if you had a road racing double crankset that was 52-40 (52 teeth for the large outer one and 40 teeth for the small inner), we would find that 52 minus 40 is equal to 12, we have a 12 tooth difference between the two chainrings. Because 12 is less than the 14 tooth "capacity" of the front derailleur, it would work with no difficulty. If, however, it were a 54-36 crankset the result would be (54 minus 36=18) 18 and this would exceed the front derailleur's "capacity". The front changer couldn't lift the chain comfortably to the large chainring. In the case of a triple crankset, the same principle applies, however you would use the difference between the outermost and the innermost chainring.

Gear41

Okay, that explanation from bikepro.com made a lot of sense. I remember hearing something about capacity in the past, and had a feeling the jump from 48t to 26t was quite large.

I am pretty sure the front derailleur is a triple (although there is a chance it could have been replaced). I've spent a lof of time on vintagetrek.com , and all the specs match up with the 400T (except the replaced crankset). The derailleur was changing properly between the three chainrings before I replaced the cables, however there was a lot of rub when on the outer rings.

Since I really do not like the current gearing (half-step with granny – thanks JReade), I may not play with it much more. Maybe I can work out a crankset trade at the co-op or find some replacement rings.

illwafer

lower your derailleur. it looks like you can squeak by...

T-Mar

+1. The subject derailleur appears to be a mid-1980s Shimano FD-Z204-GS (designation stamped on back of cage). This derailleur has an advertised capacity of 27T, so you're pushing the limits, but it should work, probably grudgingly.

ColonelJLloyd 

https://www.parktool.com/blog/repair-...ur-adjustments

Puget Pounder

How accurate are front derailleur capacity ratings? I've run 26 grannies and 53 max before with no problems. I've also consitently exceeded the capacities on doubles as well.

ColonelJLloyd 

Well, you can't blame manufacturers for placing a conservative rating on them.

Puget Pounder

True. I've just never run into problems exceeding a triple FD's capacity. I have with a double, but never a triple.

Gear41

Thanks for the advice on lowering the derailleur. Will do. Also, thanks for finding the info on the capacity.

Captain Blight

Also? Back the limit screws out like all the way after you lower your derailleur and before you clamp the cable down tight. I don't know what it is about that particular mech, but I've always found them to be a PITA to adjust.

BluesDaddy

My $.02
Detach cable from mech*.
Lower mech until it just barely clears the big ring as it swings out.
Leaving the cable off, put the bike in a stand or otherwise raise the rear wheel.
To set the limit screws, pedal and cycle the mech across its travel to get the limits right.
Put the shifter and mech in low gear and attach cable with minimal tension.
Test ride.
*"mech" is easier to type/say and I would love if we could switch over, just like we did with Metric.

IthaDan 

^^ I've become a fan of "FD" and "RD"

Yeah, check out the chainline, no wonder you're having issues. This is a VERY rough approximation of your "granniest" gear. See how it barely clears the bottom of the FD cage?

Lower that FD. Set it so it just barely clears the large chainring.

AZORCH

I'll betcha this will do the job.

noglider 

Front derailleurs can be trickier than rears!

It doesn't look like a triple FD.

jjhabbs 

easy. Change the middle ring to a 44 or so. Then lower your deraileur to about 2mm above the top ring.

IthaDan 

Serious question: Why do you have to change the middle to lower the FD.

EDIT: OH, I get it. Because the inner cage will hit.

Those giant 110BCD chainrings look kinda odd the more I look at them.

JohnDThompson 

Half-step with granny doesn't like the modern front derailleur designed for modern compact crank; the dropped inner plate interferes with the middle chainring. Replace it with a vintage front derailleur without the dropped inner plate, e.g:



Then lower the derailleur so the outer plate clears the large chainring by only a couple millimeters.

himespau 

Probably a stupid question, but what's the value of the halfstep? I mean I guess it can give you tight spacing for cadence but it seems like it'd be rather limiting to your terrain choices and then you'd have a huge drop to the granny, no?

JohnDThompson 

The half-step with granny set up allowed for a large number of closely and evenly spaced gears for most riding, and bail-out gears when the going gets steep.

Modern freehubs with ten or more cogs and a compact crank provide a similar range of gearing, but that wasn't possible when you were limited to only 5 or 6 cogs in the back.

jjhabbs 

That campy front deraileur wont work well with that granny. The one on the bicycle is fine. You dont have to replace the middle to a 44t in order to lower the front deraileur.. You just want to lower it to make it more responsive..

JohnDThompson 

Don't tell that to my wife's bike:

Puget Pounder

Owned.

Gear41

Okay, I see the difference between this derailleur and the one on my bike. When I lowered the FD, I noticed that the inside plate was uncomfortably close to the middle chainring when shifted into the outer ring.

Switching to an appropriate FD is a good suggestion, but I think I will instead replace the 48t middle ring and possibly the 26t ring also. For now, though, I'll just stay out of the granny gear – no big hills or heavy loads in the foreseeable future anyway.