Electronic front shift question

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mbrider
Posts: 34
Joined: Fri Mar 12, 2010 10:49 pm

by mbrider

Can anyone comment on if it is possible to swap chainring sizes, without having to adjust the front derailleur?

Specifically I am looking at getting SRAM E-tap and would like to be able to swap out 52/36 summer gears for 50/34 winter gears. Would be super nice and convenient to not have to adjust the front derailleur.

Anyone have experience with this?

I know maybe too soon for E-tap users, so any Shimano Di2 users who can share experiences like this?

goodboyr
Posts: 1487
Joined: Wed Jan 10, 2007 10:56 pm
Location: Canada

by goodboyr

I guess I would just ask why? It's a pretty easy adjust, and as a bonus you learn to do your own wrenching.

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oldnslow2
Posts: 266
Joined: Wed Feb 03, 2016 1:50 pm

by oldnslow2

I installed etap and you will have to move the FD if you change chainrings.

sungod
Posts: 1702
Joined: Fri Dec 24, 2010 9:37 pm

by sungod

irrespective of system, manual/electric, you'll need to adjust fd for best performance, though you might be able to get away with a single position for the combinations you describe, you'd need to experiment

the advantage with electric is you need only move the fd up/down, whereas with mechanical you have to redo cable tension

should only take a few minutes, less time than it'll take you to switch the crankset/rings - if you want to switch only rings you'll need a crankset that allows it, etap works ok with non-sram cranks/rings so you're free to choose here

mbrider
Posts: 34
Joined: Fri Mar 12, 2010 10:49 pm

by mbrider

Thanks for the replies thus far guys.

I understand that it would not take much effort to loosen the bolt and slide the front der up or down the tab.

I guess for me, its that when I would switch to 50/34, it will be so that I can do climbs and keep my heart rate in a lower range on steep stuff, and I would not be using this combo for a race, so the shifting does not have to be perfect.

For the 52/36 combo, the shifting would need to be perfect. That is why I would want to set it up for this combo, and then not touch it and have it be perfect for when I put the 52/36 back on.

The cranks in question are the SRAM red BB30 new style - so pretty easy to pull off just the drive side and have access to all chainring bolts etc.

From reading installs of E-tap, it looks like the wedges that go behind the front der might be one area that could make it a little more involved to slide the der up or down on the front der tab.

Thoughts on how the tab would effect the moving of the front der?

goodboyr
Posts: 1487
Joined: Wed Jan 10, 2007 10:56 pm
Location: Canada

by goodboyr

For Di2, you would also need to unwind the support bolt that bears against the frame, and then reset at the new position. You may also need to reposition the reinforcing place to match the new contact point for the support bolt, although it might be possible to position the plate to cover both spots.

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2lo8
Posts: 551
Joined: Thu Mar 31, 2016 10:32 am

by 2lo8

The big problem is having too big of a gap between the cage and the chainring. A 2-tooth difference is a 1-inch change in circumference, and about a 4mm change in the gap. There's generally only a 2mm allowed tolerance, between 1-3mm. You would have to make sure it is as close as possible, then you might be able to get away with it. You might also end up throwing your chain. If you have a chain catcher, you might rather try to experiment using the 34t with the 52t.
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