Gear Ratios

[MacB]

Well the height and the diameter of the new chainrings will be different, I think it's 2mm per tooth but can't remember if that's radius or diameter, if the latter then it would be 10mm lower and the former 5mm lower...for the front derailleur. Lowering the FD to match the new big ring should take care of the small ring automatically. I've messed about with a Tiagra FD a few years ago, took off a 52/42/30 triple and put on 48/36/22 and it worked fine after a bit of height and angle adjustment. But no guarantees the version you have will be as accomodating, but a new FD isn't expensive if needed.

As for cranks and BB, is this an external BB or an older style?

 

[Ajax Bay]

the height and the diameter of the new chainrings will be different, I think it's 2mm per tooth but can't remember if that's radius or diameter

I noted the need to lower the FD in my post above. Using a 50t chainring as the median, each tooth more or less has a 2% (one tooth / 50t x 100) effect on the radius of the chainring (and thus the height of the FD). A 50t chainring has a radius of about 100mm. So a 48t chainring has a radius (centre to tooth tip) 4mm less.

My CDF has a 53/34T chainset and 11-34T, 10sp Cassette.

The OP suggests he has a 53/34 crankset at present. I doubt this (sorry Steve) - I reckon you have a 50/34 (and the CdF spec from 'Evans' states:
Chain set:Shimano Tiagra FC-4700 / 50-34T ). A 53/34 would have been customised by you (and since you rarely use your smallest two sprockets I cannot imagine why this would have been done). Also the difference (53-34) is 19t and that's well beyond the shifting spec of a double FD.
So the move from 50t to 48t will mean dropping your FD about 4mm. But you'll be setting the new height of the FD by eye, not by measurement.

 

[bpsmith]

No. It's only more effort if the rider pedals at the same cadence. Pedalling a 40" gear at 72rpm requires the same 'effort' (power) as in a 36" gear at 80rpm.

I beg to differ. Are you suggesting that grinding or spinning makes no difference as the effort is the same then?

Also when you are in the lowest gear you have, my point stands, it is harder to push a bigger gear inch chainring than a smaller one.

 

[MacB]

I beg to differ. Are you suggesting that grinding or spinning makes no difference as the effort is the same then?

Also when you are in the lowest gear you have, my point stands, it is harder to push a bigger gear inch chainring than a smaller one.

My understanding is the energy expended would be the same but the peak power different. Like eating small but often and never feeling overfull compared to binging and feeling sick. It's cardio as well, I found spinning uphill much harder at first than grinding a bigger gear out of the saddle.

 

[bpsmith]

My understanding is the energy expended would be the same but the peak power different. Like eating small but often and never feeling overfull compared to binging and feeling sick. It's cardio as well, I found spinning uphill much harder at first than grinding a bigger gear out of the saddle.

Exactly.

Like pressing 20kg x 20 times doesn’t mean that you can press 400kg once.

 

[Ajax Bay]

the further you travel per stroke, the more effort is required.

I said: "No. It's only more effort if the rider pedals at the same cadence. Pedalling a 40" gear at 72rpm requires the same 'effort' (power) as in a 36" gear at 80rpm."

I beg to differ. Are you suggesting that grinding or spinning makes no difference as the effort is the same then?
Also when you are in the lowest gear you have, my point stands, it is harder to push a bigger gear inch chainring than a smaller one.

If you are climbing a hill of a given gradient at the same speed, 'grinding' up it or 'spinning' up it requires the same power - fisics init? The rider puts more force on the pedal when 'grinding' but does so at a lower cadence than when 'spinning'. In my comment I identified your 'effort' with the term 'power'. Maybe this is not what you meant by effort.
In your second sentence, it depends what you mean by 'harder'. If you mean more force then 'yes'. But if you mean more power (force times angular velocity times crank length) then 'no' (assumes same speed of ascent). In fact you can 'rest' on a hill, by standing out of the saddle, changing up a gear (from the lowest) and climbing very slowly for a short while, before sitting down and 'spinning' up to the top.
HTH

 

[bpsmith]

Well the OP is talking about making hills easier, so we would be talking about Force here, rather than Power. I never mentioned Power.

 

[swansonj]

I said: "No. It's only more effort if the rider pedals at the same cadence. Pedalling a 40" gear at 72rpm requires the same 'effort' (power) as in a 36" gear at 80rpm."

If you are climbing a hill of a given gradient at the same speed, 'grinding' up it or 'spinning' up it requires the same power - fisics init? The rider puts more force on the pedal when 'grinding' but does so at a lower cadence than when 'spinning'. In my comment I identified your 'effort' with the term 'power'. Maybe this is not what you meant by effort.
In your second sentence, it depends what you mean by 'harder'. If you mean more force then 'yes'. But if you mean more power (force times angular velocity times crank length) then 'no' (assumes same speed of ascent). In fact you can 'rest' on a hill, by standing out of the saddle, changing up a gear (from the lowest) and climbing very slowly for a short while, before sitting down and 'spinning' up to the top.
HTH

You are assuming the efficiency of the human engine is independent of force/cadence. That is not valid.

The power output to the road is independent of gear ratio but because efficiency varies, the power input required from the rider does vary.

 

[buzzy-beans]

Excuse me lads and lasses but just as in plain English is encouraged on technically exacting forms, why the hell can't plain English be encouraged on subjects such as the one in the OP's question so that mere mortals can actually understand?

I am a newly returned cyclist after many a long year away when in former times my team coach would decide on what gearing would be fitted to my bikes, I never had anything to do with it, I was simply the gorilla who pushed the bloody pedals around!!! But now I am a 68 year old codger who has just bought an amazing classic bike off Bigssy which unfortunately doesn't have a low enough bottom group of cogs for me to comfortably push round on even slight inclines, so, and as such, I am in the same situation as the OP steveindenmark and would love to read some information that doesn't fly a zillion miles above my admitted age depleted levels of intelligence.

Many thanks

 

[buzzy-beans]

[QUOTE 4986111, member: 9609"]you will either need a smaller cogs on the front or bigger cogs on the back, or both. First of all what gears do you have now; two sets of gears on the crank and 7 gears on the rear wheel ?
How many teeth do you have on the front gears (particularly the smallest cog)?
And on the rear wheel how many teeth on the biggest and smallest cogs ?[/QUOTE]

Thanks for asking Reiver.

Up front there are two rings of 52/42
At the back the cassette is a 7 speed 13/19

 

[biggs682]

Thanks for asking Reiver.

Up front there are two rings of 52/42
At the back the cassette is a 7 speed 13/19

As mentioned before i think this will do the job

https://www.sjscycles.co.uk/cassettes/1128-shimano-cshg41-7-speed-cassette/

And should be a straight swap over apart from either extending or replacing the chain

 

[Ajax Bay]

I am in the same situation as the OP steveindenmark

So, like @steveindenmark , you're preparing yourself and your bike for the Transcontinental Race over 25 days, then. Chapeau.
@User9609 has pointed the direction. But be sure that your RD can cope with 31t difference (its specc'ed capacity will probably be less, and I suspect it will not manage a 34t sprocket. Just saying.
On my '81 ALAN, I had a 50/40 with a 5sp 14-25 freewheel on. I needed a bit more at both higher and lower end, so I swapped the 50t for a 54t chainring (SR Apex don't do a 39) and put a 6sp 14-28 freewheel on. This gave me what I needed - well as much as the components allowed. My RD (Nuovo Record Pat 1976) could not manage more than a 28t sprocket (and just about wrapped the 14 + 14 = 28t required).

 

[Ajax Bay]

why the hell can't plain English be encouraged on subjects such as the one in the OP's question

Afternoon. Please could you review my post #8 above (where I provided direct advice to Steve, the OP) and let me know which bits are not plain English. The power/force/effort stuff is peripheral 'chat' and Steve is quite able to ignore that.

 

[Tigerbiten]

My take on gears is ...........

Top gear wants around a 4:1 chainring:sprocket ratio as this gives you around a 100" top gear. Unless you are into ultra high speed downhill runs, a 100" gear spun at 100 rpm will give you 30 mph. So you'll spin out just sub 30 mph.

First gear wants the chainring to be around 4 teeth smaller than the sprocket as this gives you around a 25" gear. A 25" gear spun at 60 rpm is 4.5 mph. You can smaller but you start to get into triple chainring territory, not double.

The important range is around 70" because a 70" gear spun around 75 rpm is around 15 mph. It at this speed that wind resistance starts really to to kick in. So to go faster starts to take a lot more effort. This makes it a nice long distance cruising gear.

I think something like a 44/28 double matched with a 11-32 cassette will give you an almost ideal range of gears for most conditions. But in real life trying to get a 44/28 double will be tricky because it's not a "standard" size. The easiest way would be to make your own from the inner two rings off a triple and a small radius MTB front derailleur. But then depending on how you shift the front, a MTB derailleur may not be compatible.

In real life you're probably better with a 50/34 double and make up a 13-36 cassette. 13-15-17-19-21-24-28-32-36 looks ideal.

 

[swansonj]

My take on gears is ...........

Top gear wants around a 4:1 chainring:sprocket ratio as this gives you around a 100" top gear. Unless you are into ultra high speed downhill runs, a 100" gear spun at 100 rpm will give you 30 mph. So you'll spin out just sub 30 mph.

First gear wants the chainring to be around 4 teeth smaller than the sprocket as this gives you around a 25" gear. A 25" gear spun at 60 rpm is 4.5 mph. You can smaller but you start to get into triple chainring territory, not double.

The important range is around 70" because a 70" gear spun around 75 rpm is around 15 mph. It at this speed that wind resistance starts really to to kick in. So to go faster starts to take a lot more effort. This makes it a nice long distance cruising gear.

I think something like a 44/28 double matched with a 11-32 cassette will give you an almost ideal range of gears for most conditions. But in real life trying to get a 44/28 double will be tricky because it's not a "standard" size. The easiest way would be to make your own from the inner two rings off a triple and a small radius MTB front derailleur. But then depending on how you shift the front, a MTB derailleur may not be compatible.

In real life you're probably better with a 50/34 double and make up a 13-36 cassette. 13-15-17-19-21-24-28-32-36 looks ideal.

In deciding the ideal front/rear combination to achieve the desired gear range, presumably other factors are also relevant. Smaller (at both front and rear in proportion) saves weight, and a shorter rear derailleur arm is less vulnerable to damage. But bigger (again both front and rear in proportion) reduces forces and hence wear and hence presumably increases component life.