Pull Ratio: The Ramblings of a Mad Man

Hello
I am in quarantine in Argentina. Have been for 3.5 weeks. It's pretty boring, not even allowed out for exercise here.
So I did some maths.
My plan is to eventually have a Mavic Groupset with Modern STI Levers doing the shifting.

Mavic 840 Derailleur Pull Ratio = 2.25x (found someone who calculated it online)
Shimano 10s STI Cable Pull = 2.3mm
2.25*2.3mm = 5.175mm

Calculated that JTek Shiftmate 4 reduces cable pull by 0.8x

5.175 * 0.8 = 4.14mm

Campagnolo 10s Cassette Spacing = 4.15mm

Therefore we should be all systems go right?
840 Derailleur + 10s Shimano Shifters + 10s Campagnolo Cassette = success.
Please tell me if I've gone mad.

I am trying to understand the reasoning for the first part of your equation; so for every click of the STI shifter, the cable moves 2.3mm, and for every 1mm of cable pull, the Mavic moves by 2.25mm - is that what you meant? (I haven’t checked the accuracy of the numbers myself - I can’t really be bothered on a Sunday morning in bed but it sounds about right).

Anyway, if you’re correct then go for it, what could go wrong? Although it could be that the 0.01mm difference might be your undoing. Worlds have been destroyed by smaller discrepancies.

And if that works, then I’m going to try controlling a 1925 Cyclo 3 speed derailleur with a SRAM eTap shifter.

Yeah that's it. I don't think 0.1mm will matter so much. Derailleur guide pulleys have more play than that anyway.

Sounds like a great project. Looking forward to more updates

That’s 1mm over the range of the cassette though. Not a lot but enough to cause so iffy shifting at the extremes perhaps

Agreed. I'm not sure if the Mavic guide pulleys on the 840 have play (don't have mine to hand). But I could always replace it with a shimano one that does.

Not quite, the difference in the calculation is 0.01mm. So 0.1mm over 10 speed but I agree that if this was a 100 speed cassette then 1mm may make a difference. ;-)

I think that might be one step too far for this project - you're quite right

Email Jtek engineering if you want assistance related to the shiftmates

And if adjusted correctly at the middle of the cassette, it'd only be 0.05 mm at each end

Yep, I hadn’t read your comment, only the one I replied to and assumed he meant 0.1 per gear

It’s ok. It does say The Ramblings of a Madman so I was replying in kind, though on reflection, I think he’s a genius!

I would hold judgement until I have it working.

The other option is to use a shiftmate 3 which should be 0.85x pull ratio.
5.175*0.85 = 4.39mm of movement per shift.
Shimano 9 Speed Cassette is 4.35mm spacing. Therefore 0.36mm over the range of the cassette, or 0.18mm if adjusted correctly in the middle as @JesperXT pointed out.
This way I could stay away from more expensive Campag 10s Cassettes and 10s chains.