Baffled by chainset 'lingo'

[Framed]

Having now been successfully enlightened by other members re crank lengths, and on the point of ordering a new Stronglight or TA triple chainset, I am now completely baffled by all the accompanying jargon in adverts, (such as BCD with a long string of numbers underneath. What is this BCD?)
Also, having just been reading the replies to another member re problems on fitting a new triple to a 1981 Dawes Galaxy, this change-over seems fraught with perils, and I'm now afraid of making a complete mess of things, being a complete gear-ignoramus!
I want to convert my Hetchins tourer, which has up to now had a double, as lower gearing is advisable, to make things easier for a long-ago damaged knee. In the Dawes Galaxy responses, various geometric angles were mentioned; what are these, and what is their significance?
Also, what would be the lowest practical/usable gear?
Please can anyone advise?

[hubgearfreak]

AASHTA. here's what is meant by BCD http://www.sheldonbrown.com/gloss_bo-z.html#bcd

as for fitting dérailleur gears, i've no idea. i remove and bin them

[cycleruk]

What type of gear changers have you got on the Hetchins?
What size rings and sprockets do you have?

As for gearing, how low do you need to go?
You can go as low as 22 front to 34 rear but these suit MTB,s and very heavy touring

If you buy a complete triple chainset then you won't need to worry about BCD, except when you want new replacement rings.
What you will probably need are new front and rear derailleurs.
You may need to replace the bottom bracket and also the gear changers depending on what you currently have.

[PW]

Order your bits from Spa Cycles, over the phone. (01423) 887003. Tell them what you want to do and they'll sort out the compatibility problems, if any. They won't rip you off either. (Usual disclaimer, been a customer for years and keep going back, others will tell you the same).

[Framed]

Thanks hubgearfreak, for that brilliant link; -it's answered questions I didn't even know I needed to ask! (Where's your dustbin? I may need to pay it a visit by night!)

Thanks also to cycleruk; -1979 Hetchins had a 52T/42T drilled-rings double SR 'Apex' chainset, with 170mm cranks plus DuraAce front & rear derailleur and shifters, 6-block now on another bike but can't remember no. of teeth here and now (am in Library), and Hetchins has been out of use for a long time, so will need a new block for it anyway. This set-up had always worked well, except for my dodgy knee! Am going to try and transfer the old Hetchins set-up to a day bike, but now need heavy work-horse gearing to get the Hetchins into easier touring mode.
Gear ratios? Well, I need as low as is practical, but not to the extent of being like an egg-whisk, going nowhere fast, and falling off sideways! What would you consider to be the most efficient heavy work-load gear ratios?

PW, -many thanks to you too for the recommendation re Spa Cycles; -it was their ads. I was reading when I came up against the 'jargon' and was then afraid to order. Now I'll certainly go ahead with confidence, once I have sufficient practical info from you 'whizz-kids' out there too!

[Graham]

Hi Framed,

Let us know how you get on. I'd be interested to hear what SPA cycles recommend for you.

I have currently backed out my triple upgrade and am back with the double 40T/28T on the front ( Stronglight 80 ) and a 11 - 30 T cassette on the back. . . . . which might be a totally adequate setup for my loaded touring requirements.

[gilesjuk]

BCD = Bolt circle diameter.

http://en.wikipedia.org/wiki/Wheel_sizing#Bolt_circle

If you imagine an imaginary circle that intersects each chainring hole/bolt then the diameter of this imaginary circle is the BCD.

Mountain bikes use different BCD to road bikes, track bikes use a different BCD also. On road bikes there's compact drive which again is different.

[hubgearfreak]

Thanks hubgearfreak, for that brilliant link; -it's answered questions I didn't even know I needed to ask!

true, i don't think i've ever had a question that sheldon hasn't already answered.
hence the AASHTA (As Always, Sheldon Has The Answer)

[CJ]

Gear ratios? Well, I need as low as is practical, but not to the extent of being like an egg-whisk, going nowhere fast, and falling off sideways!

Don't worry, it's impossible within the limits of a readily available chainrings and sprockets, to gear a bike (with normal sized wheels) that low.

Let's do the maths. Gear (inches) = Wheel (inches) × Chainring (teeth) ÷ Sprocket (teeth). Taking that 22 chainring and 34 sprocket (that somebody suggested was only for mountainbikes or shockingly heavy touring) and a 700C wheel, we get a 17in gear.

The next formula is less well-known. You can derive it for yourself or take my word for it that: Riding speed (mph) = Gear (inches) × Cadence (rpm) ÷ 336.

Okay, so lets assume your knees are hurting on a steep hill, so you select that 17in bottom gear in order to reduce the force you have to apply to the pedals, but keep them turning at 80rpm in order to maintain power. So you'll be doing 4mph. I humbly suggest that any reasonably competent rider will happily pedal at 80rpm and can balance perfectly well at 4mph. Ergo: a 17in bottom gear is perfectly useful.

A skilful rider should be able to balance down to walking speed, i.e. 3mph. In which case (pro-rata) a 13in gear should be usable. To get a gear that low one would need to combine a rare and unusual (but obtainable with some difficulty) 20T chainring, with the soon-to-be-released 12-36T cassette that is intended for 29ers and fit it instead to a nominally 26in MTB-sized wheel that is shod with narrow road tyres and hence actually only some 24inches tall.

So whenever I hear someone claim they can't use a gear lower than (insert number bigger than 13) inches, I take it as an admission that they're actually not much good at balancing a bike!

[Big T]

I was riding my 17" gear up Winnats Pass recently and was actually doing 3.5 mph, so must have been pedalling at slower than 80 rpm. I was balancing Ok until a freak gust of wind blew me to a standstill.

fascinating stuff.

[Framed]

Further thanks to all of you, for ongoing v.useful input.

Thanks to you Chris, for your extremely informative info re gear ratios, from which I now gather that I shouldn't worry about the falling-over-sideways aspect of low gearing, as I was brought up on fixed, which I used for years, and which seems to develop good balance, as do the rollers in the attic,(hence my total ignorance, of gear-fitting and operating mechanisms!)

Graham, I too now feel that maybe it would make better sense to stick with a new double, but with much smaller rings than I had before, rather than try to convert to triple, with my abysmal lack of knowledge and mechanical skills. That way, in cowardly fashion, I don't have to face a total dismemberment and re-setup of bottom bracket and a whole load of new front and rear change-mech stuff! (At least, I hope I don't!)

[Mick F]

any reasonably competent rider will happily pedal at 80rpm

I can assure you I am not happy at 80rpm. Far too fast for comfort for any length of time.

Until fairly recently, I had no idea what cadence I naturally rode at. Now I have a Speed/Cadence sensor for my Garmin 305 and I have a read-out and graphs to ponder over.

I'm happy at 60 odd to 70 odd.

Since the beginning of this year, my Average Cadence is recorded as 64rpm.

[cycleruk]

That way, in cowardly fashion, I don't have to face a total dismemberment and re-setup of bottom bracket and a whole load of new front and rear change-mech stuff! (At least, I hope I don't!)

If / when you go to smaller rings, you may have to change the front mech'?
But try your existing one first.
It's to do with the radius of the rings and lowering the mech'. The bottom of the cage could catch the chainstay.

And the same goes for the rear.
If you go for a larger cassette then there may be a limit to the biggest cog.
Shimano gives a limit of 27T for their road rear derailleurs but they are reported to work to 28T.
If you want to go larger than this then you will probably need one from their MTB range.

[Graham]

Graham, I too now feel that maybe it would make better sense to stick with a new double, but with much smaller rings than I had before, rather than try to convert to triple, with my abysmal lack of knowledge and mechanical skills. That way, in cowardly fashion, I don't have to face a total dismemberment and re-setup of bottom bracket and a whole load of new front and rear change-mech stuff! (At least, I hope I don't!)

Apart from :-
- the poor chainline on the outer chainring
- the restriction on what cassette sprockets I could use with each chainring
- the micro-critical adjustment of the new triple front derailleur mech
the ultimate folly was revealed after a careful comparison of the gear rations of the double (16-gears) & triple setups (24-gears).
- 8 of the ratios on the triple were duplicates or near duplicates. I know that in practice one does not use a triple c/s like that. But it was a harsh reality check.

[Framed]

AAAAAAAAAAAARGH!!!! -So it gets worse1 I still can't evade all the perilous technical hiccups! Thanks, all of you, for your further points. Will now try to check re potential clearances, etc (Anyone got a decent Penny-Farthing for sale? -I know THEY didn't have all this trouble!)