When it was a 2x10 speed flat handlebar setup, the rear and front derailleur were from the Shimano 105 5700 series. After upgrading, the rear and front derailleurs have been upgraded to the latest 105 5800 series. One thing to note is that the 105 5800 series components are designed for 11 speed, but I am using it for a 10 speed set up on this mini velo. Not recommended, but it still works.
105 5700 series components are 10 speed components, while the latest 105 5800 components are 11 speed components. As they are one generation apart, the design and construction of the rear and front derailleurs from these two groupsets are very different. The latest 105 5800 components are modeled after the Dura-Ace 9000 and Ultegra 6800 series, and are targeted at bikes with a lower price point.
Let's start by comparing the rear derailleurs and see the similarities and differences. Both are black in colour (also available in silver) and are short cage versions (also available in mid cage versions for larger cassettes).
Upon first look, there does not seem to be any big differences between the 105 5700 RD on the left and 105 5800 RD on the right...
...however, the cable fixing point is much further towards the front for the new 5800 RD on the right side. This is due to the change from 10 to 11 speed construction. The cable fixing bolt is also flatter for the new RD.
Bottom view. Notice the different guide spring hooking points between the 5700 RD at the top and the 5800 RD at the bottom. This gives a more consistent spring force across the gears. The inner link of the new RD is also black instead of silver colour for the older RD.
The cable adjust bolt area is also quite different. There is an extra sleeve on the new 5800 RD to minimise the rubbing on the polymer coating of the new low friction polymer-coated inner cables. All black cage set for the new 5800 RD, unlike the 5700 RD which still has some silver coloured parts.
From the back view, both of them look quite similar
The short cage 105 5700 RD weighs about 223 grams
The new short cage 105 5800 RD weighs a little bit more at 226 grams
To tell the difference between the old 5700 RD and the new 5800 RD, look at the colour of the inner link and the cage set. The new 105 5800 has an all black colour construction, which is different from the 105 5700 RD.
Next, we will look at the 105 5700 FD and the new 105 5800 FD. It is very easy to tell the FD apart, as the look is very different.
105 5700 FD on the left, 105 5800 FD on the right. Both are brazed on type and not clamp band type. As shown here, they look very different.
The size of the outer link is very different. The new 5800 FD has a long arm construction which gives more leverage for a light shifting feeling. It is also hollow in the middle, both for reducing weight and also for adjusting the support bolt.
One of the downsides of having a long arm construction is the frame compatibility with some small wheeled bikes. For example, the long arm will interfere with the frame when installed on a Dahon Mu frame or Tern Verge frame, as the tall seat stays are very close to the FD mounts. This rules out the use of long arm FD on these frames.
I suspect this is one of the reason why there is only the Tern Verge X18 (2x9 speed, with old short arm design), instead of a high end 2x11 speed Tern Verge, as the new long arm FD is not compatible with the frame.
5700 FD has a sticker for the logo, while the new 5800 logo is embossed onto the chain guide. Not as nice in my opinion as it is not very visible and looks cheap. Also note the additional plastic skid plate on the chain guide which minimises chain rubbing noise when the trimming function on the shifter is not used correctly.
The difference in arm length is very evident here. Note the extra pin near the cable fixing bolt on the 5800 FD, which is used to adjust the path of the inner cable to suit different frames.
The guide spring on the 5800 FD has more coils, which will give a flatter spring force across the working range. There is also the support bolt sticking out from the middle of the 5800 FD. When adjusted from the other side, this bolt will contact the frame and make the whole FD more rigid for better shifting.
5700 FD on the left has a longer chain guide than the 5800 chain guide on the right.
This different length is partly due to the different bending design used, as shown here. It caused some chain rubbing issues on the Wheelsport frames when the FD mount is not designed correctly. For more details check out these two links below.
Wheelsport Ultegra Di2 2x11 Speed: Part 4 - Installation Issues and Solutions
Wheelsport Fantasy 2x10 Speed: Part 7 - Solving Chain Interference
Weight of 105 5700 FD, about 86 grams
Weight of new 105 5800 FD is slightly heavier at 89 grams