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I have a Shimano Sora 3500 full groupset (FD 50-34, Crankset 170mm length with BB RS500 (BC1.37 x 24), Sprocket 12-28) on my road bike.

I want to make a partial upgrade to 105 set due to my budget limit, by:

  • changing both shifters to 105 R7000;

  • changing the rear derailleur to 105 R7000 GS (long cage);

  • changing the cassette to Shimano 105 CS HG700, 11-34T (fortunately my wheelset can accommodate the 11 speed cassette);

  • using Shimano 105 SLX CN-HG601 11 speed chain; and

  • still keeping the current installed Sora 3500 FD, crankset, and rim brakes.

Please kindly share your thought whether is it possible or I need to go with a full group upgrade?

I believe the Sora's crankset and front derailleur should be fine, right?

Thank you.

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    If you want to upgrade from Sora to 105 level you should really think about a whole new bike. You won't see the full benefit of 105 with Sora level frame, wheels etc.
    – Argenti Apparatus
    Commented Apr 24, 2020 at 11:46
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    @ArgentiApparatus I know of bikes sold where the only difference is the Claris vs. 105 groupset. And the same frame and wheels go on to even one level above that in their line-up (SRAM Apex). The manufacturers typically have only a limited set of frames and the wheels are often soon upgraded anyway.
    – Vladimir F Героям слава
    Commented Apr 25, 2020 at 9:10

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The Sora front derailleur is not compatible with the front 105 shifter. Shimano changed the actuation ratio (ratio of cable pulled to cage movement) on 11 speed road groups.

The Sora crank is not compatible with a narrower 11 speed chain. The chainring spacing is optimized for a wider 9 speed chain.

YOu can look up Shimano component compatibility on their product information page.

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No, in general it is not possible to do a partial upgrade when the number of speeds changes. Even the cable pull ratio between 9 and 11 is different. Even if you changed just the rear and kept your cranks, the shifting with the 11 speed chain would be bad. And the 11 speed casette needs an 11-speed chain. In general one has to upgrade everything and that commonly means buying a new bike where all other parts may be better too.

If you are willing to change shifters, RD, cassette, you will also have to change the cranks. The FD is then a minor piece in the budget. Your BB also needs a change if you have the 345 Octalink but not if you have the 3500 Hollowtech II cranks.

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You might achieve part of this, by changing everything for the rear and right hand shifter, but leave the front mech and chainrings and left shifter alone.

The downside of this is your brake/shift levers will be visually mismatched, and could have a different hand-feel. Could even have a different mechanism for changing gear if your sora shifters have the thumb click and not the two levers.

A second somewhat historical answer might be to change both levers, but to also move the front mech to a downtube shifter on the left side only. This was done in the 90s when big gears were in, and shifting the front was less common.

Downside here is that your bike probably doesn't have a downtube shifter already, and the cable stop might not work as a boss.

By the time you buy a complete replacement groupset, even less the brakes, you're still going to be spending a lot of money. Do figure the whole cost, worst case, and compare that with a 105-equipped bike.

Add in the value to you of a warranty and see how the two costs compare. Remember that there's nothing wrong with having 2 bikes, or you could even sell the old one as-is to offset the costs.

Do the numbers and make an informed choice based on your location and situation.

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    In my answer I did not recommend your first point (I may not have explicitly written the shifter would have to be kept, but it is implicit and natural). I am really afraid of bad shiftimg and possible dropped chains. I think it at least should be mentioned as another possible downside).
    – Vladimir F Героям слава
    Commented Apr 25, 2020 at 8:01
  • @VladimirF thing is, if the chainrings are left original, and the spider is unchanged, and the front mech is the same, and the left shifter is unchanged, then the only thing that really has altered is the chain width drops from 6.6-6.8 mm to 5.62mm This is almost nothing and can be compensated by narrowing the FD's cage. So the front mech of a 9 speed will work fine with the rear mech of the 11 speed.
    – Criggie ♦
    Commented Apr 25, 2020 at 11:38

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