Front mech-chainset clearance

[Kit Wolf]

Hi,

I'm trying to find a front mech suitable for my bike, but I'm not sure whether the one I have is
suitable. I currently have an Ultegra 6503 front mech (braze-on fittings, 27 gear) and a Deore 48
tooth chainset. This setup works fine on the bench, but the clearance between the cage and the teeth
of the large chainwheel is about 7mm - far more than the 1mm recommended. I can't lower the front
mech any further as it is already at its lowest setting.

There aren't any obvious functional problems - the chain doesn't foul the cage, even when it is on
the smallest chainwheel. The setup shifts fine on the bench but I don't know how it would cope in
the real world. I'm loath to try, as I would like to return it if it is not suitable, though I'm not
sure whether the replacements are likely to be any better.

So, should I expect any serious trouble if I keep this mech or will it probably work OK?

Thanks,

Kit

 

[Sheldon Brown]

Kit Wolf wrote:

> I'm trying to find a front mech suitable for my bike, but I'm not sure whether the one I have is
> suitable. I currently have an Ultegra 6503 front mech (braze-on fittings, 27 gear) and a Deore 48
> tooth chainset. This setup works fine on the bench, but the clearance between the cage and the
> teeth of the large chainwheel is about 7mm - far more than the 1mm recommended. I can't lower the
> front mech any further as it is already at its lowest setting.
>
> There aren't any obvious functional problems - the chain doesn't foul the cage, even when it is on
> the smallest chainwheel. The setup shifts fine on the bench but I don't know how it would cope in
> the real world. I'm loath to try, as I would like to return it if it is not suitable, though I'm
> not sure whether the replacements are likely to be any better.
>
> So, should I expect any serious trouble if I keep this mech or will it probably work OK?

I've said it before: "Front derailer braze ons are the work of SATAN" and now you know why!

With the cage that far from the big ring, you'll likely need to "trim" the front mech to get it to
work with some of the rear sprockets without rubbing.

If you have bar-end or (yuck) downtube shifters, it'll probably be fine, because those are not
indexed for the front mech, so you'll be able to "trim" the front derailer as needed.

If you have STI brifters, you'll likely find it impossible to get all of the rear sprockets to work
with the big front ring. You may be willing to live with this.

The problem isn't the derailer, it's the frame, and I doubt that any other front mech will work any
better with this setup.

Sheldon "Clamp-On" Brown +-------------------------------------------------------------+
| I often quote myself. It adds spice to my conversation. | --George Benard Shaw |
+-------------------------------------------------------------+ Harris Cyclery, West Newton,
Massachusetts Phone 617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts
shipped Worldwide http://captainbike.com http://sheldonbrown.com

 

[Phil Brown]

>I'm trying to find a front mech suitable for my bike, but I'm not sure whether the one I have is
>suitable. I currently have an Ultegra 6503 front mech (braze-on fittings, 27 gear) and a Deore 48
>tooth chainset. This setup works fine on the bench, but the clearance between the cage and the
>teeth of the large chainwheel is about 7mm - far more than the 1mm recommended. I can't lower the
>front mech any further as it is already at its lowest setting.
>
Lots snipped

This is arguement #1 against braze on front derailleurs. If it shifts OK it won't cause any
problems. If it doesn't your only real solution is to put a bigger chainring on or find a different
FD that hangs a little lower. Phil Brown

 

[Mike Jacoubowsk]

> I'm trying to find a front mech suitable for my bike, but I'm not sure whether the one I have is
> suitable. I currently have an Ultegra 6503 front mech (braze-on fittings, 27 gear) and a Deore 48
> tooth chainset. This setup works fine on the bench, but the clearance between the cage and the
> teeth of the large chainwheel is about 7mm - far more than the 1mm recommended. I can't lower the
> front mech any further as it is already at its lowest setting.

The most-likely issue will be a reluctance to shift off the middle chainring onto the small. The
problem is caused by the height of the front derailleur above the chainring; the higher it is, the
more difficult it becomes to shove the chain (due to the flexibility of the chain; as the length of
chain between chainring and derailleur increases, shifting will be less and less positive).

If you have bar-con or other non-indexed front shifting, you may be able to overshift enough to
overcome this problem to a significant degree, but normal STI shifting won't be very friendly.

--Mike-- Chain Reaction Bicycles http://www.ChainReactionBicycles.com

"Kit Wolf" <[email protected]> wrote in message
news[email protected]...
> Hi,
>
> I'm trying to find a front mech suitable for my bike, but I'm not sure whether the one I have is
> suitable. I currently have an Ultegra 6503 front mech (braze-on fittings, 27 gear) and a Deore 48
> tooth chainset. This setup works fine on the bench, but the clearance between the cage and the
> teeth of the large chainwheel is about 7mm - far more than the 1mm recommended. I can't lower the
> front mech any further as it is already at its lowest setting.
>
> There aren't any obvious functional problems - the chain doesn't foul the cage, even when it is on
> the smallest chainwheel. The setup shifts fine on the bench but I don't know how it would cope in
> the real world. I'm loath to try, as I would like to return it if it is not suitable, though I'm
> not sure whether the replacements are likely to be any better.
>
> So, should I expect any serious trouble if I keep this mech or will it probably work OK?
>
> Thanks,
>
> Kit

 

[Kit Wolf]

Sheldon Brown <[email protected]> wrote in message
news:<[email protected]>...
> Kit Wolf wrote:
>
> > I'm trying to find a front mech suitable for my bike, but I'm not sure whether the one I have is
> > suitable. I currently have an Ultegra 6503 front mech (braze-on fittings, 27 gear) and a Deore
> > 48 tooth chainset. This setup works fine on the bench, but the clearance between the cage and
> > the teeth of the large chainwheel is about 7mm - far more than the 1mm recommended. I can't
> > lower the front mech any further as it is already at its lowest setting.
>
> I've said it before: "Front derailer braze ons are the work of SATAN" and now you know why!
>
> With the cage that far from the big ring, you'll likely need to "trim" the front mech to get it to
> work with some of the rear sprockets without rubbing.

Thankfully the mech is for a recumbent that has an infinitely long chain; the one advantage of this
is that whatever gear combination the bike's in, the chain angle remains exactly the same.

> If you have bar-end or (yuck) downtube shifters, it'll probably be fine, because those are not
> indexed for the front mech, so you'll be able to "trim" the front derailer as needed.

Bar end shifters.

> The problem isn't the derailer, it's the frame, and I doubt that any other front mech will work
> any better with this setup.

I shall stick with the parts I'm using, and hope that all goes OK. I really hadn't been relishing a
mail-order search for the one perfect mech with the correct clearance that must exist somewhere, so
I'm grateful for your advice.

Regards,

Kit

> Sheldon "Clamp-On" Brown +-------------------------------------------------------------+
> | I often quote myself. It adds spice to my conversation. | --George Benard Shaw |
> +-------------------------------------------------------------+ Harris Cyclery, West Newton,
> Massachusetts Phone 617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts
> shipped Worldwide http://captainbike.com http://sheldonbrown.com