How many gearing inches is 4 teeth??

scooterboy

Im weighing up the old debate of triple versus compact. My heart says compact but my head says triple.

I live in South Yorkshire - Sheffield to be precise, so it can get fairly hilly. Im making a return to Road biking after a lay off of several years so im starting with fairly poor fitness.

Im looking at a Focus Cayo 2007 - Either the triple/105/Shimano R550 wheel spec or the Compact/Ultegra/Mavic Aksium wheel spec. Theres less than £80 in it between the two - i'd obvioulsy rather go for the better spec Ultegra version but i suppose i could almost buy a second pair of wheels (Aksiums) with the difference.

Whats the real effect of the extra 4 teeth on the 30 tooth triple/granny ring? Cassette ratios are 12 to 27, im aware of the gear overlap a triple set up gives.

In my Junior days i managed with a 52/42 set up no problem bearing in mind i grew up in Mid-Wales!!!

il_principe

Have a look here: http://www.sheldonbrown.com/gears/

Raph

Very little, I'd say. I've only ever used a triple on a mountain bike, and even then it's only ever been as a "cop-out" gear. Sometimes on a road bike I wish there were a granny ring but basically it's pure laziness. I used to get up 1:4s on a 42 front 21 rear, with luggage. Now I'm an old crumbly I have a 32 inner front and a 25 bottom gear at the back, it's got me up Wrynose, Hardknot and a few other corkers in the Peaks. I suppose if I were fussed about having a 52 ring on that bike I'd have a triple - 32 42 52, but it's the touring bike and top gear is 42-14 which is 81". I'm planning to put a 16 top gear on and squash up the rest of them cos I never use the top two gears. If you're "starting with fairly poor fitness", I'd go for a compact with a 34 inner, with a 27 rear that's 34", lower than I have on my bike known as "the Tractor". Even on one of my fast(ish) road bikes I have a 39/48 front and 13-26 on the back, so no really big gears.

In the middle of the range, in the gears you use most, 1 tooth at the back is significant, at the lower end it makes a lot less difference. I tried a 28 instead of 25 and I'm not sure I could tell the difference. I think in that part of the range, big differences make a difference, small differences don't. I suppose when you hit that killer hill where your nose scrapes the tarmac in front of you, every bit helps - I often go for really weird cassette ratios and my ideal is very close in the middle fanning out to big differences at the bottom - e.g. 14 15 16 17 18 19 21 24 28. I've had mates look at that and say it's stupid, but when you're hacking along the flat, one tooth is a big difference, when you're winching up a steep hill, three or four teeth make almost no difference. Also consider that it's logarithmic, as in the same 1 tooth is twice as much difference between 14 and 15 as it is between 28 and 29.

The thing about chainrings is that you probably want to avoid having to make lots of double shifts - if most of your riding is between the two ranges you'll be forever flipping between chainrings - that's ok if they're very close together so you only change at the front, but with 34/50 or 39/53 type setups you can end up with lots of double shifts - you run out of low gears on the outer ring, you then shift down at the front, and the rear mech right out - if you're doing that regularly it gets to be a real bore. I prefer to have most of my riding on one of the rings, and exceptional bits on the other - hence on the touring bike I'm mostly on the outer ring, and use the inner for hills, and on a faster bike I'm mostly on the inner ring and only use the outer for fast stretches on straight roads with a nice tailwind.

If you can live with the embarrassment of not having a macho big outer ring, I'd set the outer so that most of your riding is on that, and the inner so there's a bit of overlap. When you get fitter you can up the chainring sizes.

I suppose that lot might add up to saying you could get a triple with a middle ring you use most - but then would you use the outer ring very much?

Just my ha'pennyworth!

geoff_ss

Gearing is about ratios so 4 tooth differences depend on what the other gear is.

In this case you're asking about the difference between 30 and 34 teeth. The 30 tooth ring will provide 30/34 lower gear ie 0.88 or 88% of what ever you had before. If your previous set up gave you a 34" gear, then the smaller ring will be a 30" gear.

Measuring gears in inches is a peculiar, though popular, technique. I'm surprised it persists. To work it out you divide the chainring teeth by the sprocket teeth and multiply by the wheel diameter (27 is the usual one, but you need to be careful on small wheeled bikes)

Where does it derive? Well, it goes back a long way to before the invention of the safety bicycle. The gear in inches is the equivalent diameter of the front wheel of an old ordinary (ie penny farthing). In those days the highest gear you could get depended on the length of your legs. On the whole I think we have it easier - especially short legged riders

btw I also ride in the Peak District - if you're wondering what gears you need think of Winnatt's ... and get a low one. The hardest climb I've ever tackled is Arthur's Pass, west to east, in SI New Zealand. It's as steep as Winnat's in places, goes on for 10 miles and after a 100km approach in my case. I'm afraid it brought me to the 24" gear at one point and that doesn't happen often. I was riding my mtb on slicks with something like a 25" gear.

hth

Geoff

scooterboy

Thanks for the info guys. Most made sense

Im pretty much set on the compact route, Raph you made perfect sense.

Like i said in my original post i managed before Compacts and Triples were commonplace on Road bikes - The only thing now is im 15 years older and 2 stone heavier!!!

John.T

I have been using compacts for about 5 years and now have 50/34 and 12/27 on all bikes except for racing when I fit 12/23 (I may go for an 11 but can not find a cheap 11/21 10sp). I have found I do very few double shifts. In hillier country I use the 34 most of the time and on the flat I stick with the 50. 50/19 and 34/13 are just about the same ratio at 69" which along with 63" (50/21 or 34/14) is my most used gear. When I do have to double change I do both together with no trouble.
Compact works better with 10sp than it does with 9. My ideal would be an 11/27 11sp cassette with 50/34. It would cover all my riding.

Raph

"Measuring gears in inches is a peculiar"

Well, not all that peculiar - it gives one figure for the final resulting gear - it's irrelevant at that point what size chainwheel or sprocket, the resulting gear i.e. pedal turns : road distance, is all that's relevant. Given that you don't change wheel size during a ride, I agree the multiplying by 27 is a bit unnecessary, but since that's the way it's been done, that's what we're used to, a bit like currency - you know immediately what a tenner will buy but the moment you're thinking in euros or zlotys you have to work it out. Same with gearing, I'm used to inches - if I see gearing expressed as a ratio I have to work it out into inches to know what it means. One way isn't better or worse than the other - except that the inches system takes into account the wheel size.

Bearing in mind the "starting with fairly poor fitness" bit, I'd say a smaller outer chainring is a good thing - even with a 48T I'm finding it hard to stay on the big ring on the flat in a headwind with a slight uphill - with a standard cassette starting a 12 or 13 and increasing in ones up to 18-19 there's not much leeway till you have to change down. In fitter times I used to do whole 80-90mile rides on a 53 ring and only use the 42 for hills - that was a while ago!

It's difficult to find chainsets made up with sensible rings for those of us either less fit or more honest about our abilities - there's lots of 53s about, but precious little 46 or 48 outers as standard. What I've done on my tourer is used an old deore MTB triple and taken off the granny ring, and had to saw off the protrusions the granny ring bolted onto, to get it closer to the frame for a better chainline. However, a compact chainset will do just as well, only you'll probably have to buy it as a 34/50 and get a smaller outer if you find the 50 hard to push.

PS - for a 10-mile 1:4 I'd definitely want a granny ring! I've ridden in the Pyrenees a few times and managed on a 38-24 bottom gear (42"-ish), even on climbs of an hour or more - the sort where you get out of the saddle and just snooze off while honking side to side till you get to the top of whatever it is - those are rarely as steep as 1:10, but once you're into more than 1:7 or so, you're into either effort that can't be sustained for more than a few minutes or the need for a granny gear. I suppose once you get close to 1:1 ratio, it's close to a granny gear anyway, and you could get up fairly extreme things with a 34 front and 30 rear, especially if, as mentioned above, you're on 10speed.

aracer

Of course according to the great Sheldon, you should not only include wheel size, but also crank length in your gear calculations
http://sheldonbrown.com/gain.html

John.T

Adding crank length into the equation only complicates things unnessesarily. If you are riding the same gear measured in inches then you travel the same difference per crank rev whatever combination of cogs and wheel size is used. Any change in gearing is fairly noticeable but a change in crank length does not make much difference to the effort required.
Sheldon has a lot of good advice but I take him with a pich of salt on this one.

aracer

I'm afraid he is right. The crank length does make a difference to the force you have to exert (and also in how fast you have to move your feet) in exactly the same way changing gear does. The only reason it doesn't make much difference is that crank lengths only cover a tiny range (170 to 175 is the equivalent of going from a 35 to a 34 chainring).

Cajun

IMHO...
I think only you can evaluate how many & how steep your hills are on your normal route...In a 40 mile ride, one - ten hills will give your legs a work out with the double, but more (and steeeeep) hills will make your (my) legs toast....no fun when you are struggling or getting dropped by the pack and possibly have a steady diet of headwinds and more hills to conquer... I have a 30/40/50 & 12/25 which serves my purposes...I very seldom use the 30, but when I have to use it, it's a blessing....not to mention that when topping the hill, I have enough 'leg' left to begin a good pace without dreading more hills....

Raph

Totally fair point - in my case though, on the sort of ride where there are hills on which I'd use a 30, I wouldn't get onto the 50 often enough to justify it being there!

If you've got one bike for everything then perhaps a triple does make sense to cover all situations.

John.T

Aracer. As I said, an unnessesary complication as it makes so little difference. If we wanted to relate it to force required we would have to include bike weight, rolling resistance, mechanical resistance, gradient, wind resistance and body wieght. I have probably missed some but you see where I am heading. Gear inches is a constant for anything that gives the same ratio and is easy to understand as we all know what a penny farthing looks like.

scooterboy

I went for a compact in then end, also swayed by the slightly better spec on the Compact version of the Cayo 2007.

Lets hope i made the right decision, im also going to supplement road rides with some gentle off road stuff - im blowing the cobwebs of my '99?? Spesh Hardrock comp!!!

aracer

You're still completely missing the point. Yes bike weight etc. all make a difference to the force, however those are things you can't change for a given speed on a given bit of road, whilst the crank length is something you can change in much the same way you can change gear. You might just as well ignore the difference between 34 and 35 tooth chainrings. I'm not really sure why it complicates matters either (and struggling to imagine a 100" wheel ordinary).

John.T

Well I have 20 gears on my bike and can move between any of them but do not have more than one pair of cranks. Many of us use use different cassettes for different events but very few change cranks. Gear inches makes it very easy to compare different combinations.
One tooth on the chainrings does not make a major difference to gear ratio but one tooth on the small cogs does. Moving from a 34 to a 35 tooth ring makes a 2" difference on a 13 or 14 cog but only 1" on 21 or 24 tooth cogs. The system makes it easy to understand the % changes between different combinations.
Best to use the system that suits you best. I learnt gear inches when we only had 10 gears and you had to be pretty inventive to create the best gear set-up for any given event. I did some RRs on 52/43 with 14/15/16/21/24. It worked well on some courses but was rubbish on others. Now we can run close ratios over a wide range this is not such a problem.
The OP was about gear inches after all so perhaps we should just use what we understand best.