front mech replaced??? higher speed needed to change up?

benjii19

Hi all I've just been to the LBS to pick up my girlfriends bike (she hasn't looked after it) I took it in to have a cheapo front mech added (Shimano Acera) as the old one was knackered. I tried it out after and found that whilst pedalling relatively slowly it would struggle/take a while to jump on to the top wheel of my triple.

I took it back in and the mechanic kindly set it up on his stand and changed up and down and it worked fine. He then explained it's like changing gear on a car you need the speed/revs to do it.

In essence you need to be pedalling faster for it to jump to the top cog. I'm just dubious because I'm sure on my bike it just hops up to the top cog at relatively low speeds. So have I been had, or is this something I should really know.

EKE_38BPM

I don't think that you need to have a higher cadence for the chain to move up onto a bigger chainring as the derailleur physically moves the chain and this should work at any speed as long as the chainring is actually turning, but, if you are riding at a low cadence when the chain does go onto the big ring, you will find it hard to keep turning the pedals as you are now in a higher gear.

So, to answer your question, no, you don't need to be pedalling fast for the chain to move onto the big ring, but you will want to in order to keep the pedals spinning.

jeepie

Possibly the cable is a bit too slack. I think you should be able to change at any speed, but as EKE mentioned you may not want to.

bunter

I think it's more important to be able to take the pressure off the pedals but keep the cranks spinning through the gear change. Pushing hard + gear change = lost chain

cyclingprop

And, if you pedal slowly it will take longer for the chain to securely engage...

CiB

I'm with the mechanic on this one. We know the theory is fine that the mech should shove the chain across but my exp is that pedalling in the zone results in a sweet change, and if nothing else he's proved it by hoiking it up on a stand and showing you that it works at a reasonable speed.

Show her how much you love her and buy her a Di2 setup. They change at any speed, even if you're dancing up a hill on the thing trying to wrestle the bars out of the steerer.

DonDaddyD

CiB wrote:

I'm with the mechanic on this one. We know the theory is fine that the mech should shove the chain across but my exp is that pedalling in the zone results in a sweet change, and if nothing else he's proved it by hoiking it up on a stand and showing you that it works at a reasonable speed.

I'm glad you said it first because I am also with the mechanic on this one. I've experienced this on a bike that needed gear indexing and certainly one that needed both a combination of high cadence and pushing the lever right the way in to engage the gear.

It's surprising that you should need to do this on a bike that has just been dialled right, but certainly I've experienced it on numerous occassions on different gears (when the bike gears need indexing).

il_principe

bunter wrote:

I think it's more important to be able to take the pressure off the pedals but keep the cranks spinning through the gear change. Pushing hard + gear change = lost chain

I wanted to say "Not with EPS" but I see Mr. Di2 got in there first. Dammit.

benjii19

So if the front mech has been replaced it should have no problem changin at alow cadence??

EKE_38BPM

benjii19 wrote:

So if the front mech has been replaced it should have no problem changin at alow cadence??

In an ideal world, there shouldn't be a problem, assuming it has been indexed correctly (which the LBS should have done). The issue I mentioned about wanting to spin faster when going into a higher gear still applies though, no matter how well it is indexed. Even with Di2 or EPS.

So, in the real world, I'm with the mechanic, even though in theory you should be able to change onto the big ring at any cadence. Using the car analogy, you wouldn't change into third gear whilst creeping along in first.

flimflam_machine

If the chainrings have ramps and pins to help the chain up from middle to top then a higher cadence will result in a quicker shift simply because the pin and ramped section will come round sooner. I can't think why a lower cadence should prevent shifting altogether; however, I can remember times when shifting would get very sketchy if I was turning the cranks too slowly. Perhaps the real cause is chain tension, which is often related to cadence. If you're honking too big a gear up a hill at low cadence your chain is under too much tension to shift, if it's essentially unloaded (as on a workstand) then it will shift easily at any cadence.