Half shift

javidr

Hi

I have been testing a Cx bike today and there is something that took my attention. in the crank shift, there is something called "half position" that moves the derraileur without changing any gear


How does it work? Do all the Cx bike bring this system?

Thanks

bobley

Shimano 105 certainly has the "trim" feature. I think everything from Tiagra to Dura Ace (mechanical) will too. Its not a CX thing.

If you're on the little/inner/left ring on the front and the little/outer/right cog on the back and the chain is running on the diagonal then you find it'll rub on the side of the front deraileur. If you nudge the front shifter a touch it trims out the front derailleur to prevent the noise/wear. Ditto if you use the big front and big rear together you can trim it out. Its not good form to run cross chained as such but the trim helps for a short blast.

If you have a posh DI2 system then it'll move the front mech to compensate.

What were you riding?

javidr

i was in the norco a1 threshold

my question mainly is if i dont use it, what would happen?

if i need to use it, at what point do i have to use it? after 5thcassette position?

tgotb

bobley wrote:

If you have a posh DI2 system then it'll move the front mech to compensate.

Apart from the eye-watering cost of replacing rear mechs, Di2 could be brilliant on a CX bike; no issues with sticky gear cables, the ability to put climbing shifters on the tops, and the mech would self-adjust (within limits) if you bent the hanger. If you hacked the firmware you could also add cool features like a separate button for double up-shifts off the start, and a "return to favourite starting gear" button which automatically selects the right gear coming into the hurdles or a run-up :-)

bobley

JaviDR wrote:

my question mainly is if i dont use it, what would happen?

if i need to use it, at what point do i have to use it? after 5thcassette position?

All that happens is that it'll make noise. As you say, you'll need to give it a tap to keep it quiet as you go across to the last 3 I'd say.

I think the trim is required as the 10 speed chain is narrow and the cassette is wide. Therefore the front derailleur is also narrow and requires moving slightly to keep it quiet. I'm not sure what Sram do but I'm guessing something similar.

TGOTB - I hope you've not been thinking about that too much....

tgotb

bobley wrote:

I'm not sure what Sram do but I'm guessing something similar.

Yep, it's the same.

bobley wrote:

TGOTB - I hope you've not been thinking about that too much....

If Di2 cost the same as SRAM Rival I'd probably give it a go; it's the sort of geeky project I enjoy, but the thought of having to blow £200+ on a new mech if I crash and bend it is quite enough to stop me going down that route...

Monty Dog

Front mech trim function is needed for modern transmissions with narrow chains - it comes with all mechanical groupsets. You only have to watch pro CX racing to realise that Di2 isn't infallible - not really practical for unsponsored riders.