Croix de Fer: Tiagra/GRX swap

Even a modest bridge ramp puts me on the big end of the casette

There is no getting away from it: the CDF is a heavy machine. In running order (less panniers but including D lock, lights, bidon and bar bag) it’s 17.5Kg. In a ride ready state (dressed, shod, fed & watered) I’m 75.5kg so the mass to shift is around 93kg. No wonder I can count on the fingers of one hand the times I have used the 11t cog!

In 3,500 miles of riding the stock Tiagra groupset I’ve spent most of my time on the bottom 3, at tops 4 of the 10 cassette gears and when I need it the little ring/big cog combo is only just low enough to shift me up the modest climbs I meet riding to adjacent towns. Having replaced the chain and cassette I imagined all I had to do next was buy myself a smaller set of chain-rings. A drop from 50/34 to say 46/30 would reduce the knee buckling effort and be a simple case of unbolting the fasteners, fitting new rings and riding away. Not so, sadly 34t is as low as you can go with a Tiagra crankset.

My bike has to ‘do it all’ including shopping adventures, kite flying and photogrammetric projects. I’m looking for an easy ride with enough leverage to shift me and about 50kg of kit if required.

Oh happy were the days when we had just one gear to worry about!

Although the new fads of ‘bike packing’ and ‘gravel racing’ are not my bag, there are some wonderful crossovers from road racing and MTB into leisure riding these days. I love the disc brakes, brakeshift levers, sensible tyres and big wheel clearance: what a joy it is to be a rider in the early 21st century! The closest ‘style’ of riding to what I do would be somewhere between ‘touring’ and ‘gravel’. I use an unfashionable rack to attach luggage to the bike as opposed to pretending I’m a Tour de France rider who has to strap overnight ‘adventure’ kit to the bike and my routes involve awful sections of rutted droves which have thrown me off on occaision: gravel? huh, half the year it’s mud.

The gravel fad has given my bike enough wheel clearance and disc brakes to cope with the dreadful droves and pitted roads of the fens.

My CDF is a 2018 model and I notice the current (2021) version uses a much smaller chain-ring set up: the Shimano 2x 10 GRX groupset. So I decided to go for the ‘gravel designed’ outfit in the hope it would give me a drop in ring size as well as the XRT pioneered clutch system.

Out of fear of messing up the indexing I avoided mixing ‘non standard’ parts with OEM. I want to keep the cassette, shifters and chain. The rear mech swap (not essential but to better deal with the new chain tension) was achieved with the old cable and shifter in place with very little compromise to the indexing:

It will never be this clean again. One thing that surprises me about the GRX deraileur is the complete disregard for cleaning in the design: the thing is a veritable dirt trap. The Tiagra was much easier to clean.
Roughly speaking the GRX chainrings will shift the ratios up by one cog and I gain an inverse ratio bottom gear.
The option of an 11-34 hyperglide cassette is yet to be explored.

On inspection the GRX crankset has bolt on /bolt off rings so further tinkering should be possible. The BCD are different for the outside and inside rings and the asymmetric bolt centres limit what’s available so, even though I have dropped the ratios a bit 30t is as low as you can go with GRX. I had thought there might be a granny ring option but no.

The 46/30 GRX crankset. The bolt centres are different for the 2 rings.

Although compatibility with Tiagra is a given the one thing that isn’t is the front derailleur: it is recommended to fit the FD RX 400 as the chainline for the new crank-set is shifted out from the frame by 2.5mm putting it beyond the reach of the Tiagra unit. This also means my feet will be 2,5mm further apart too. The Shimano 2 piece crank-set system makes the swap simple and there is no need to change the bottom bracket bearings (they are easily the best I have ever had) making the swap a doddle. A bonus of the FD RX 400 design is the fine adjustment possible using the angle out screw and the cable tensioner, details missing from the Tiagra unit.

The chain was shortened by 2 links (the minimum) to keep the tension for the little ring /little cog end.

Getting the front derailleur to run at the cross chain limits (which worked happily on the Tiagra setup) proved impossible without chain rub at one end or the other. This is, of course, due to the offset chainline.

Fortunately there is excellent help on the web in setting up the derailleurs:

Although the reduction in gear ratio is relatively small 30×32 feels very low on the level, I may yet be caught out by things like the Parkeston Quay ferry ramp with a fully loaded bike but so far it’s nice to know I can climb easier.

A few short rides around town verify the fit and let me get used to the new noises. Back pedaling makes a new clicking noise, presumably because of the clutch.

As I’m refreshing the drive train I thought new pedals are in order. The nylon toe clips have survived but the original pedals have taken a bashing so, after looking at cheap replacements, realised there’s benefit in a pair of clasic Mikashima originals.

MKS RX-1:simple, sealed, lightweight and frictionless.

A track pedal might seem like an odd choice.
The Mikashima RX-1’s reputation for axle stiffness and low maintenance durability puts them at the top of the list for this toeclip rider. The strap stays where it’s put thanks to the extra threading eye.
Beautiful artwork from Zefal to show how to thread Eugine Christophe’s toe straps.

I am indebted to Jef Sharp at Life on a Bike who kindly torqued up the crankset for me.

A 25mile ride to Burwell revealed the benefit to the smaller chainrings, the close steps at the high end of the cassette are now in reach, this, coupled with a brisk cadence (80 to 90rpm) gives me a fine tuning of the effort which jsut isn’t there at the low end where the steps between gears are much steeper.

Cutting some of the weight might be possible. Dumping the pannier racks, muds and swapping the wheel set are options. Given the punishment I give the machine I think a carbon wheelset would be wasted money. Clipping panniers on and off racks without faffing around with an Allen key is by far the best method of shifting kit when I have to. Mud guards are essential and the constant fiddle to keep them clear of the tyre is largely avoided by use of rigid aluminium ‘splasher’ fenders from Brick Lane Bikes , they keep most of the water off me from below. In truth I avoid the rain but, getting caught out is inevitable and dodging the arse stripe is a must. There is a weight penalty for all these luxuries but, for me at any rate, it’s a price worth paying compared to the alternatives.

Getting caught in the rain: detail from ‘A storm over our town’ by Dame Laura Knight 1959


About billboyheritagesurvey

Heritage worker
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