Shrevvy 

I have a Trek 1500 with a complete 7400 Dura Ace group except for thumb shifters. The thumb shifters are Shimano Light Action. The rear shifts just fine, but the front is having problems. The LBS can't figure it out either. In the stand, the front shifts smoothly and easily. In the real world with someone riding, the chain does not want to go into the big ring. The thumb shifter become stiff and it is much harder to work the lever to get the derailleur to move. The shift is very clunky and it will not complete the shift every time. The pressure needed to move the lever is bad enough that it hurts the thumb. This is an upright bike for my wife and she will not be able to shift with that much pressure. Why would it shift easily to the big ring in a stand, but not when on the road? We swapped the cable for new one even though we had just installed a new cable. I'll post pictures if that helps.

The Golden Boy 

Is the inner derailleur cage hitting the chainring?


I would expect there to be rear shifting problems if you're using SIS shifters- 7400 Dura Ace has different pull ratios-

Shrevvy 

I just checked again and the cage is not hitting the chain ring. It is 8-speed and the rear is a 7402 and the front is a 7410. We have it shifting friction and not SIS. The rear shifts great. I don't understand why the front shifts so well on the stand, but not when actually ridden.

texaspandj

Are you pedaling when shifting?
Are your chainrings straight (no run off)?
Does it actually shift but just more difficult on the road?

Shrevvy 

Are you pedaling when shifting?

Yes. When it struggles to shift, it seems to help to light pedal a little as if you were attempting to shift under load. On the stand, it shifts crisply when pedaling.

Are your chainrings straight (no run off)?

Yes, I believe they are straight. I don't see a wobble when pedaling. Not sure what no run off means.

Does it actually shift but just more difficult on the road?

It does not always make the shift. If it does, it is with a lot of pressure on the thumb shifter. Sometimes, you have to back off and try again.

texaspandj

I had the same exact problem with my grip shift with 105 7 speed group. I put inner cable line on the bottom bracket shell eyelets then loosened the front derailleur cable (leaving a little more slack) and presto It worked. However not sure which one contributed to solving problem.

As a side note, I always make the front derailleur cable pretty tight on my other bikes with no problems, but for some reason this one would do exactly what you're describing.

Reynolds 

The lever moves easily while on the stand but becomes hard to move while actually riding? That's unusual.
I assume you checked out the limit screw, right?

clubman 

I think front DRs are always harder to shift under load, we just don't notice it much. There's no tension on the chain on a stand.

New chain?

wrk101

Shift early before you are under load. Shift before the hill. My wife tends to shift late, puts a lot of pressure on the system.

Don't test it on a work stand, test it on flat pavement, a parking lot perhaps.

squirtdad 

not having the setup to fiddle with, this is theory only.

one possibility is that the cable pull of the thumbie is right on the edge of what is needed.... I would put the lever in the the low (small) position as far as it will go and then tighten the cable. This should give the max pull when going to the big ring.

also I find that with friction I sometimes have to hold pressure on the shifter until the chain is fully on the big ring, otherwise the shift does not work


the light action shifters are 7 speed, but that should not make a difference for the front.

Good luck

fietsbob

Friction shift levers work great, but you have to pay attention ... indexing takes over for you..

like cars with clutches and standard transmissions .. vs Automatic trannys..

blamester

Fd too low or cable on the wrong side of the pinch bolt?
How is the chain line?
How does it shift going back down to the small ring?
When you shift ease up very slightly and with no hesitation pull the lever all the way. One swift movement.

Put the end of my thumb on the top of the lever and push down and back , never misses.

Jeff Wills

Yes, please post a picture. I'm not familiar with Shimano "Light Action thumb shifters", so the terminology may be confused.

clubman 

'Light action' was imprinted on most all the Deore thumbies, even XT. Could the OP's problem result from the odd spacing on a DA cassette or was that resolved on the 7400? I've never owned any DA except for 1st gen.

fleslider 

With the bike sitting on the ground sit on it and reach down and grab the front derailleur cable between the cable braze on and the bottom bracket , is it stiff pulling there?
if you get off the bike (eg no weight on the bike) and pull the cable in the same spot is it still tight?

curbtender

Yes, I was thinking the same. The cable should be over the top of the pinch bolt tab and not under/behind. Picture would help.

Shrevvy 

Finally got around to taking a few pics.

Shrevvy 

Entire bike. Ignore the lack of dust cap on the pedal. It was what I had on hand to test the shifting.

blamester

FD looks too low. Raise it a couple of mill.
That setup should work beautifully.
It may actually work better in friction.

canklecat

^^This^^

The front derailleur might need more clearance to cope with the frame flex under a rider's weight. That would explain needing to horse and force the shift while riding, but not on the work stand.

curbtender

Looks OK, but still can't see if the cable goes over the tab on the back side of the bolt. It makes a difference on the cable leverage. It also looks like the cables cross. Usually not a problem, but you are picking up drag somewhere.

Shrevvy 

Have a really early trip tomorrow. I'll try moving FD up and bit when I get back and see if that works.

clubman 

Good eye. The routing seems easier but it's not what you want.

IthaDan 

You ready for this?

7400 is an island into itself- the RD cable pull is unique to the generations contained within the 7400 umbrella (6,7,8 speed, freewheels, uniglide, hyperglide)

Unfortunately, if you want this to work right, you'll be relegated to friction shifters until you can track down some 7400 shifters.

Edit: wow, the front is what's being problematic? Woof. Those shifters are friction, methinks you might need an exorcist.

mountaindave 

I wouldn’t think this would be a problem unless the cross places the FD cable on the drive side of the BB.

The pictures seem to show new cables and housing, so that shouldn’t be an issue.

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