Kids twist shift to thumb shift

larkim

In the near future I may be buying a decathlon bike for one of my kids which comes with a sunrace derailleur, cassette and SRAM twist shift (MRX Comp).

(Potential purchase is http://www.decathlon.co.uk/20-racing-boy-320-kids-bike-red-id_8327591.html with the sunrace info pages at http://www.sunrace.com/en/products/family/m20 )

From experience, my kids don't get on well with twist shifts, and I'd probably quickly convert this to a thumb shifter if the option was there.

As they are a 2:1 ratio shifter, and the Sunrace website also seems to point towards that derailleur being a 2:1 ratio, would I be able to use any 2:1 thumb shifter? Would I be setting it up so that gear 5 took all of the tension out of the cable (with therefore redundant clicks for gears 6 and 7), or would I have the hi / low screws set so that gear 7 is the smallest cog, and then gear 3 the largest cog, with no more clicks possible simply because the mech had shifted to the max position?

(I only ask as I haven't yet tracked down a nice-enough looking 5 speed trigger shifter - the sunrace ones look terrible - so I can see me having to get a 6, 7 or 8 speed shifter and putting up with the mild incompatibility)

The Rookie

SRAM MRX and Sunrace are Shimano compatible I believe and not 2:1, certainly all the MRX I've come across have been changing Shimano rear mechs.

I'd suggest Deore shifters.

larkim

I thought SRAM's modern stuff was 1:1, but Shimano-compatible implied 2:1? I read that 2:1 isn't technically 2:1, its more 1.7:1 (but then 1:1 is also 1.1:1 technically)? Confused!!!

e.g. https://en.wikibooks.org/wiki/Bicycles/Maintenance_and_Repair/Gear-changing_Dimensions

I think Deore might be overkill for a £100 kids bike though!

larkim

Oops, also just correcting myself about the speed ratios. I see that the pitch on a "shimano compatible" 7 speed is different to that on a 6 speed (can I assume 6 speed and 5 speed pitch is identical?) so it would have to be a 6 speed shifter like this http://www.ukbikestore.co.uk/product/397/sltx506pa/shimano-sl-tx50-sis-thumb-shifter-plus---6-and-3-speed-pair.html

The Rookie

6-7 is different but you can 'get away with it' as the difference is small, same as you can 7-8 which is slightly different again (and SRAM and Shimano used slightly different gear spacing as well on the same number of speeds as well as not all Shimano having the same spacing to boot!)

I'd prefer a nicer used shifter (classifieds or ebay) to a cheap new one personally.

larkim

Thanks!

The Rookie

Some useful info here on spacing - prepare for a headache!
http://www.sheldonbrown.com/cribsheet-spacing.html