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 Post subject: Re: Old Dawes Galaxy
PostPosted: Mon May 25, 2020 12:52 am 
Dirt Disciple

Joined: Thu May 21, 2020 9:25 pm
Posts: 29
Well Z what a great reply, very much appreciated...
Well let’s go through them......the current gearing is just fine for me...12 speed the two front gears are 48 and 36 teeth, with a BCD of 86mm...
the area is a mix of long climbs not too steep and flats..
I’m in my 70s quite fit 5’9 with a 29 inch inside leg so this frame suits me perfectly, though I need to loose a stone to get my ideal weight of 11.5.
I won’t be adding any weight, at the moment im just trying to get fit again after injuries...
The crank size, assuming I’m measuring correctly is 165mm..centre pin to centre pedal!!

I looked up the model 99, that looks very similar to the fitted parts...

Ray


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 Post subject: Re: Old Dawes Galaxy
PostPosted: Mon May 25, 2020 11:30 am 
Old School Hero

Joined: Wed Sep 06, 2017 5:38 pm
Posts: 188
OK - Thanks Ray.
Please bear with me whilst I get vaguely technical. Cycle gear ratios are calculated with a slightly bizarre formula:
Chain ring teeth / sprocket teeth x 27.
What's bizarre about it is that the "27" is supposed to be a representation of the distance travelled in inches per revolution of the crank and it is not - but that's another story. If you calculate this for all the possible combinations (preferably with the aid of a spreadsheet as it saves lots of time and lets you play around with alternatives) you'd get something like this for the original gears supplied with the bike:

____14___17___20___24___28
52__100.3_82.6_70.2_58.5_50.1
42__81.0_66.7_56.7_47.3_40.5

Compare this with my guess of what have now:

____13___16___20___24___28___32
48__99.7_81.0_64.8__54.0_46.3_40.5
36__74.8_60.8_48.6__40.5_34.7_30.4

My concern about the 1st setup is that there is a fair bit of overlap; 52/20 is pretty close to 42/17 and likewise 52/24 is very like 42/20, meaning that for all those 10 gears, there are only really 6 ratios to choose from (remembering that you should avoid the 52/28 and 42/14 combinations). The second example is little better, as when you change down from 48/28, the next lowest gear is 36/24 - with only 2 more to go.
In practice, of course you don't need to know the ratios or even where the next highest/lowest is - you just change gear to try to maintain your pedal rhythm regardless of the slope/wind but my point is that it is common to end up with far fewer choices than is possible with a little thought. If we assume that the range 30.4 to 99.7 is right for you, consider the following:

____14___16___19___22___26___30
50__96.4_84.4_71.1__64.4_51.9_45.0
34__65.6_57.4_48.3__41.7_35.3_30.6

Not perfect, and I could certainly do better given some time but I think this would give slightly more useable, better-spaced gears with closer ratios around the area that you use most.

This of course assumes that you can find the ratios that you design, and since it's not really fashionable any more to build up your own block (=freewheel) ratios you are often limited by the sprockets that come with the one you buy.

My main point is that chain, block and chain rings wear together and should be replaced as a whole (less so large chainrings, but the principle remains true) so doing your homework whilst grinding the last miles out of what you have can mean that your new drivetrain is better suited to your needs.

I'd stick with 165 cranks if you can, but again choice may be limited. As you have found, integral chainrings designed to look superficially like decent ones are a false economy and a good, forged alloy chainset with a "popular" BCD (e.g. 86mm) will last you forever.
Check back on here before you buy as there are a small number of "gotchas" around like different bottom bracket taper angles, odd pedal threadings and so on, but there are quite a few people here with intimate knowledge of this kind of thing (far, far greater than mine) who will steer you clear of trouble.

Hope this helps and shout if any of the gobbledegook needs better explanation.
Z

PS/ Apologies for the tables - this software doesn't let you paste the kind of thing I need to make it all line up.


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 Post subject: Re: Old Dawes Galaxy
PostPosted: Mon May 25, 2020 11:46 am 
Dirt Disciple

Joined: Thu May 21, 2020 9:25 pm
Posts: 29
Thank you simply for putting in so much work to help me...I’ll have to spend some time reading and absorbing this...but if I get stuck, I’ll come back..
I appreciate the ideal situation of buying matched pairs and the forged gear set...I Imagine they are press forged..
I used to be very good with excel so sorting a table won’t be an issue...

Ray


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 Post subject: Re: Old Dawes Galaxy
PostPosted: Tue May 26, 2020 4:48 pm 
Dirt Disciple

Joined: Thu May 21, 2020 9:25 pm
Posts: 29
Hi again Z....I have been spending time trying to set up the rear mech...front one is great, no issues, however....it appears that the rear mech is ..pretty worn out..with lots of play in the pivots of the jockey wheel holder...and indeed within the jockey wheels...this has led to some issues when pulling the high gear chain slip etc...

Having compared my gear ratios with your tables above...there’s a slight difference..

Mine are ...14t_16_18_21_24_34......

I don’t know how these transpire into usability ...but they seem to suffice for my use...
The issue I’m coming across is which replacement rear mech do I require??

It’s a little confusing because the Shimano sora fitted has a max tooth size of 32…clearly slightly out of spec but it does seem to cover all six cogs!!!

The jockey wheels have approximate centres at 80mm so it puts the arms on the long side as opposed to short....

I don’t understand the differences and why they exist...so if I can ask a simple question:

What is the correct replacement for my rear mech...?

Ray


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 Post subject: Re: Old Dawes Galaxy
PostPosted: Tue May 26, 2020 5:56 pm 
Old School Hero

Joined: Wed Sep 06, 2017 5:38 pm
Posts: 188
Not sure there is such thing as "correct", other than that modern ones are pretty much all designed for "indexed" systems. Whilst they'll work just fine with your friction levers, you won't get the digital ("click and you're in the next gear") operation that these systems intend without also changing pretty much every other gear train component.
The original mech was a Suntour VX-GT and whilst these can still be had in pretty good nick would not cope with that 34t sprocket. For that you'd need the VX-T (as in "Touring" - "S" in VX-S was for "Sport" and "GT" in VX-GT for, er, "Gentle Touring" iirc). The pivot points for the jockey wheel cage on these models is repositioned to mean that as you "climb" into a lower gear, the top wheel pivots away from the block so that there is no chance of their fouling each other. The long arm simply allows the mech to take up more chain slack that is a by-product of having a wide range of gears.
So, what to stick on in place of the Sora? I'm biased and would go for a VX-T or a Suntour "Mountech" or Cyclone II provided it was in good nick. The pivot points are roll pins that can be replaced if you're keen but if they're worn, the chances are the bushing are in a similar state. My wife's road bike has a Campagnolo Veloce and there is a long arm version of this. Her's has been faultless but they're quite pricey. I don't know my way around Shimano's or Sram's latest offerings. I've never found the former particularly hard-wearing but others swear by them.

Z
PS/ If you haven't beaten me to it, I'll bung your actual (not guessed) gear ratios into a spreadsheet when I get a mo and see what you have...
PPS/ The climb from 24 to 34 is quite an ask of any rear mech and may well have contributed to the early demise of the Sora.


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 Post subject: Re: Old Dawes Galaxy
PostPosted: Tue May 26, 2020 6:07 pm 
Dirt Disciple

Joined: Thu May 21, 2020 9:25 pm
Posts: 29
Thank you....as an update, I’ve realised that wear in the high 14t gear isn’t helping the drive chain....from what I can assert...this is the most vulnerable gear due to its lack of teeth....so.....given that the same freewheel appears to be readily available at £21..or so...I’ll change both that and the rear mech...this will be better with the relatively new chain!.....however....bearing in mind your previous point about changing all the components to match......I do limited mileage so the front chain wheel can wait several weeks..until I can find a suitable replacement.

I’ll see if I can find the VX t....interestingly the current mech has an apparent limit of 32 teeth...but it still makes that big cog..

Ray


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 Post subject: Re: Old Dawes Galaxy
PostPosted: Tue May 26, 2020 7:33 pm 
Old School Hero

Joined: Wed Sep 06, 2017 5:38 pm
Posts: 188
You're absolutely right about the wear on the 14T. Would you believe they make 10T sprockets now? And the narrower gear spacing (as in width between sprockets) means narrower chains = even greater wear. The design of the mech can increase the wrap-around in the higher gears which helps, but yes, inherent problem. Incidentally, you can probably fit a 6 speed "narrow" freewheel in there without too many issues*. If you measure the gap between the inner faces of the dropout, this should give an indication.

*edit: Ignore me, I can't count. You have 6 already. Hamster's comment on the other thread about the 24-34 gap is spot on.


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 Post subject: Re: Old Dawes Galaxy
PostPosted: Tue May 26, 2020 8:25 pm 
Dirt Disciple

Joined: Thu May 21, 2020 9:25 pm
Posts: 29
Well..I guess I’m learning...however as you may realise, I’m just reading what ever I can, I have realised that even for £80 .....this thing can become a money pit...therefore I’m trying to be realistic and initially do the things that are essential repairs...
Im also aware that you have put in a lot to this thread..I really appreciate that.

My main hobby is astrophotography and I do know a lot about that...lol

Ray


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 Post subject: Re: Old Dawes Galaxy
PostPosted: Tue May 26, 2020 9:20 pm 
Gold Trader / MacRetro rider
Gold Trader / MacRetro rider
User avatar

Joined: Mon Feb 11, 2008 12:03 pm
Posts: 7137
Location: held captive by baby haggis in a cave in Scotland
What a great thread and lots of input to help with getting it best suited to yourself.
All good advice, and as for me I use a fair bit of the older Shimano stuff and I have found it every bit as hard wearing and durable as it's Campagnolo equivalent, though like most find, once it goes modern it does seem to get a bit less reliable and no where near as long lasting once you start getting to ten speed and tiny sprockets running very narrow chains. Maybe that's why we are all here on retrobike :)
Look forward to hearing more about the Dawes progressing and it can always just be a work in progress like a few of mine :)

Jamie


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 Post subject: Re: Old Dawes Galaxy
PostPosted: Tue May 26, 2020 10:55 pm 
Dirt Disciple

Joined: Thu May 21, 2020 9:25 pm
Posts: 29
O.k ...I’ve increased my knowledge a little more...(he claims with ignorance)
I’ve now understood how important the rear frame width is with respect to the rear freewheel..
The width is 135mm.......
Inso far as I can tell...this will allow me to fit a 7 speed freewheel ..ref: sheldon Brown.com
This then allows me to have a closer range 14-32 ....and reduce that jump to the big 34t cog....Giving the derailleur an easier time whilst retaining a good low gear!

Apparently...it seems that the difference in width...between the 6 speed and the 7 speed is negligible and doesn’t create problems fitting.....such as dishing wheels!!

Chain dimensions will hopefully be the same!

Any comment gratefully received

Ray


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