Adding a longer RD to a Tiagra 10 spd bike to be able to use a much wider range cassette

[Dan Lotus]

Afternoon all,

I'm looking at a Laura Trott RD2 2nd hand bike for my 12 year old daughter - review here: RD2

It's a 2016 model, and has a friendly 50/34 up front, but a rather less friendly 12-28 at the back :-(
I'm not sure what the stock RD would allow up to, probably 30 or 32 at the very most.

On the last road bike I bought her, that also came with a 12/27, but I fitted a 9spd RD, alivio or acera, I forget which, and was then able to fit a 12/36 cassette, that gave her a phenomenal range.

I could in theory do the same with this one, but then I recall there were some issues with the shift ratios on a specific version of Tiagra 10spd - does any more intelligent people than me recall which version that was, and what the issues are?
I presume it's the amount the RD moves per shift, which then adds a ton of other complications.

If this is likely to be an issue, then I think I would be best to avoid this bike and carry on searching.

 

[scragend]

It's the Tiagra 4700 that has different pull ratios. This came out in 2015 - I think that's what the RD2 has but that's only from a quick web search so not sure.

There are two Tiagra 4700 rear derailleurs, short and medium (or is it long?) cage. The longer of the two - the GS version - goes up to a 34 largest sprocket officially, and you might get away with a 36 in practice. But officially 34.

So if the bike currently has the shorter cage (SS) it should just be as simple as swapping for the longer cage, within the same Tiagra series.

 

[fossyant]

My 5700 GS is officially 32 but goes to 34 easily, so the Tiagra should too.

 

[Dadam]

As posted above if it's a GS cage it goes up to 34T, I have a bike which came from the factory with Tiagra 10 speed, 50/34 and 11-34.

It should be visually obvious if it is the SS cage.

 

[Dan Lotus]

Looks like a SS no? You reckon this is 4700 Tiagra?
Also, if it is the 4700 that has the oddball ratios, would that mean I can't just buy 'any' 10 spd cassette, and would need a compatible Tiagra version? If so, that could cause issues in future years.

And this is the bike as a whole - carbon forks as well: I've talked him down to 200 Euros - little bit steep, considering it's age and the fact I need to change something out at the rear, but it is fairly near to where my parents live in Spain, so that's saving having to try and fly with a bike in a bag, plus that added cost.

This from Google:
The Shimano Tiagra 4700 series (10-speed) is the version that features unique, non-standard pull ratios compared to previous 10-speed Shimano groupsets.
Key details regarding the Tiagra 4700 10-speed:

• Unique Cable Pull: Tiagra 4700 shifters and derailleurs use a different cable pull ratio (1.4 actuation ratio) that is not compatible with older 10-speed systems like Tiagra 4600, 105 5700, or Ultegra 6700.
• 11-Speed Compatibility: The Tiagra 4700 10-speed rear derailleur and shifters actually share the same, or extremely similar, actuation/cable pull ratio as Shimano's 11-speed road groupsets (5800/6800/R7000/R8000).
• Components: This unique ratio applies to the RD-4700 rear derailleur and ST-4700/SL-4700 shifters.
• Incompatibility: You cannot mix Tiagra 4700 shifters/derailleurs with Tiagra 4600 or other older 10-speed Shimano components, as they will cause poor shifting performance, specifically overshooting or undershooting gears.

In contrast, the older 10-speed Shimano groups (Tiagra 4600, 105 5600/5700, Ultegra 6600/6700) all share a common, standard cable pull ratio, making them interchangeable

 

[roubaixtuesday]

Looks like a SS no? You reckon this is 4700 Tiagra?

If you want absolute confirmation there will be a model number stamped somewhere on the brifter and I think also the derailleur

 

[Dan Lotus]

If you want absolute confirmation there will be a model number stamped somewhere on the brifter and I think also the derailleur

I've compared pictures on the Halfords website, and it's clear it is the SS - GS on the left, SS on the right.
The bit I have circled you can see is matched on the picture from the actual bike.

The cost of the RD is not much, that's not an issue, or the fitting, my concern is whether the stupidly awkward and non standard ratios are going to complicate things in years to come, well unless I change shifters and everything.
I'd need to match it with a Tiagra 4700 10spd cassette presumably, so the spacing matches the pull ratio?

Is a Tiagra 4700 compatible cassette also called an HG500.....?

 

[roubaixtuesday]

Is a Tiagra 4700 compatible cassette also called an HG500.....?

All 10s cassettes are cross compatible (as long as derailleur can cope). Some are pricier/lighter/allegedly harder wearing than others

 

[Dogtrousers]

You might find this interesting. Or maybe just confusing

https://productinfo.shimano.com/en/compatibility/C-454

 

[Dan Lotus]

Thankyou both, so I think I misunderstood, and the oddity with the 4700 shifters is that the pull ratio is only macthed by the 4700 Tiagra Rear derailleur.
So that does mean I couldn't change to an Alivio/Acera RD, but it seems that the GS Tiagra one that I can get for around £25 would allow up to a 34 - probably, but definitely a 32.
Cost for cassette and RD in that case something in the region of £50, which isn't bad, and I'd probably bung a new chain (I think I have a nos ultegra one kicking about) on as well so they can all get to know each other at the same time.

I'm becoming more positive about the idea.

My parents have said they would be happy to collect as well.
The bike and parts would then set me back about £225.

What do you reckon to the bike as well - not the lightest or flashiest, but would be fine for a youngster to enjoy some riding in Spain?
£225 for a decent fitting bike, with a reasonable groupset, and a 34/34 bottom gear seems like something approaching decent vfm?

 

[fossyant]

Cassettes are standard. You only need to match the shifters and mech with that model Tiagra.

Good price overall, factoring in shifter and larger cassette.

Nothing new, my Dura Ace 7400 8 speed doesn't work with any other shifter/mech combo.

 

[Dogtrousers]

You may be over thinking the "issues in future years". If you get it working now, a 12 YO is not going to subject it to Audax-like mileage and wear out the components any time soon. In fact she may well outgrow the bike before the parts come up for renewal.

Looks great to me, but note that I am not a 12 year old girl.

 

[roubaixtuesday]

What do you reckon to the bike as well - not the lightest or flashiest, but would be fine for a youngster to enjoy some riding in Spain?
£225 for a decent fitting bike, with a reasonable groupset, and a 34/34 bottom gear seems like something approaching decent vfm?

Certainly looks like a nice bike - I'd be very happy to ride something of that spec, though agree you'd want lower gears for a youngster. No experience on prices/value.

 

[Dadam]

it seems that the GS Tiagra one that I can get for around £25 would allow up to a 34 - probably, but definitely a 32.

Definitely a 34

Here's my Felt VR40 with Tiagra 4700 (GS cage) and 11-34 SG500 cassette as it came from factory

 

[roubaixtuesday]

Shimano spec gives 34 as largest sprocket. So you'd probably get away with 36!

https://bike.shimano.com/en-NA/products/components/pdp.P-RD-4700-GS.html