What are the disadvantages of scrapping the triple chainset...

[Twanger]

What are the disadvantages of scrapping the triple chainset on the hybrid I commute on and just going with an 11-34 tooth rear cassette and, say, a single 38 tooth chainwheel? This would give me 8 gears ranging from 30 inches to 93, which is all I'll need. I realise I'll need a long cage mech...I have an old long cage 105 touring mech from the late 1980s that I was thinking of resurrecting. It's still in good working order.

Why do I want to do this? Well, the elegance and simplicity of a single chainwheel is one reason. I need to replace the transmission on the bike (a three year old Specialized Sirrus) and I am investigating options. If I do go down this road, what might a suitable crankset be?

Thanks for all comments and ideas.

 

[Fab Foodie]

It means you will always have relatively large differences between each gear, but IMO unless you're hell bent on making very constant progress it's not a big deal. IMO it's a good solution often overlooked especially for commuting uses. I guess you just need to ensure that the B/B width and chainset combo gives you a decent chainline.
Triple chainsets can be bought pretty cheaply, probably not a lot more than a single.
Can you replace the chainrings on your existing chainset?

 

[GrasB]

Be careful that you've not put a large ratio hole in the gearing area you mainly use (for me this mean I want 4 consecutive gears centred around the 70-75" marker). As for what chainset, 38t precludes the use of track 144bdc but SRAM do a 130bdc single ring chainset (it comes with a 48t chainring though), from there you add your own chainring, or you could use any road double chainset.

Your rear mech problem isn't the tooth capacity, a short armed mech won't have any problems accommodating a 26t tooth difference. Your issue is going to be the maximum sprocket size, most road mechs run out of scope around 28t. Try the old 105 but you may need to go for an MTB mech.

 

[Twanger]

Thank you both. Food for thought. I'll need to work out the ratios.

 

[GrasB]

I'd better clarify my post: 4 consecutive gears with 1t difference centred around the 70-75" gearing range

 

[MartinC]

The obvious downsides are that you'll have a smaller range of gears (lower top and higher bottom) and bigger gaps in the ratios you do have.

The less obvious downsides are that you'll be running with a poorer chainline when you use the sprockets at either end of the cassette - more friction, noise and wear. You'll probably also need to leave the front mech in place (no cable or shifter and adjusted to be in the right place) to stop the chain unshipping at the front whem you're using these extreme gears.

This set up may work better in 7 speed 'cos the cassette is narrower and the sprocket spacing wider.

Crankset isn't a problem. If you're running 130mm or 110mm BCD cranks just get a set of single chainring bolts and a chainring of choice (Spa Cycles have a wide choice). Assuming you're running a square taper BB you'll need to find (calculate or trial and error) the bracket length that gives you the chainline you want. If it's an external shell BB then you'll have little choice.

As others have mentioned your rear mech capacity should be fine. If it's an MTB mech then 34 teeth won't be a problem but if it's a road one then you may be limited to a 28 or 30 largest sprocket - trial and error is how you'll find out.

 

[MacB]

I looked at this when I commuted on a triple and ended up getting a hub gear bike and jazzing the triple up for a weekend ride, instead.

 

[MartinC]

I looked at this when I commuted on a triple and ended up getting a hub gear bike and jazzing the triple up for a weekend ride, instead.

Good point, well made. If you're happy with 7,8 or 9 gears then a hub gear has much to recommend it.

 

[biggs682]

on my work horse commuter only ever use the last 4 gears , and i dont think it has ever seen the inner granny ring in over 15 yrs worth of use

 

[Fab Foodie]

The less obvious downsides are that you'll be running with a poorer chainline when you use the sprockets at either end of the cassette - more friction, noise and wear. You'll probably also need to leave the front mech in place (no cable or shifter and adjusted to be in the right place) to stop the chain unshipping at the front whem you're using these extreme gears.

This set up may work better in 7 speed 'cos the cassette is narrower and the sprocket spacing wider.

This shouldn't be too much of an issue. My middle ring on my road triple covers the rear 9spd block without much issue.

 

[Twanger]

I'm grateful for the range of answers!

A hub gear might be the way to go. I only need the lowest gear for the hill on my commute, as I am elderly (well, 52) and bronchitic (30 years of fags - gave up a few years past) and not into honking bikes up hills like I used to be. That's what attracted me to the 11-34 cassette. The 34t sprocket seems to be an odd granny-like outlier from the rest of the sprockets.

I had not thought of the problems involved in running an 8 sprocket wheel off one point. Yes, of course I can see the problems.

I'm not too worried about close spaced gears in the cruising range - I have a brompton M6L as well, and love the gearing. I like to pedal quite fast in low gears, and don't cycle too fast as I just get too tired to do a full days work properly if I do. Picking the cherries on the road of life.....

I took the bike in to Evans for servicing, and they have given me a transmission I absolutely hate. It's cheap and clunky. Oh, the work was well done, and I can't complain about the care and attention shown me, but in the final analysis they could only sell me what pre-set systems they had rather than what I really wanted. So I decided to do it myself.

 

[RecordAceFromNew]

IMHO you should be able to get a good idea if your existing rear mechs work before buying a new cassette.

As others have mentioned, there is a possibility that neither of your rear mechs can handle a 34T cog. The way to find out is by shifting the chain on to the largest cog at the back, and see if the mech's top pulley is likely to clash with that cog if the cog had a radius of 7cm. You will find that changing the chain tension and adjusting the mech's B screw will make a difference to that available clearance. A good explanation/guide is here.

If the spec of your bike is the same as this, the middle ring is already 38T so I would have thought you can just use the middle ring initially and see how you like it with a new cassette before switching to a single ring and removing the shifting system. If yours is indeed a Sugino XD then it has a bcd of 110mm and suitable chainrings are widely available and inexpensive.

 

[GrasB]

You can get 'narrow' single speed chainrings that have deep cut teeth, this will help keeping the chain on the chainring.

IME chain line arguments are theoretical arguments & in real world riding have little to no weight. Almost every rider I've followed, even those who are avid "triples for a better chain line" preachers, once on the road only actually change chainrings when they run out of gears or the chain starts scraping against something.

 

[zigzag]

i'm an expert () in these setups as have/had four bikes like that with total mileage over 15k miles. chainline is never a problem, and smaller gear range is also not a problem.

for cassette with low 28t gear you would need short cage road derailleur. for 30-34 low gear you will need short cage mtb derailleur. you will need to calculate the gear inches to choose the right cassette. i found that i prefer shimano cassettes better with top gears being 11-13-15 rather than sram 11-12-14. you should aim to set the chainline for the cruising gear (38/14 or similar). regarding chainsets - i would use 110bcd 5-arm or 104bcd 4-arm, as there is the widest selection of chainrings in these sizes.

the problem can be keeping the chain on the chainring on fast bumpy downhills or during upshifts. therefore some sort of chain management device is needed (e.g. lowered front derailleur, single or dual chainguard, paul chain keeper, bash guard etc.). i'm not using any of these on my commuter bike, just shift a bit more carefully. and i use only two gears most of the time (acceleration - double-shift - cruising - double-shift - stop).

regarding hub gears, i've used a few (sa srf3, xrd5, sram dual drive), tested rohloff, but didn't fall in love with any of them. smaller ones lack the range, and those with bigger range are too heavy and bulky. sa/brompton wide range 3-speed would be my choice for hub-geared commuter bike, but these hubs are a bit exclusive to bromptons unfortunately.

 

[andrew-the-tortoise]

I run with a single front ring on one of my bikes, with an 11-34 cassette. To stop 'chain-offs' I use a chainring [a 42 tooth] specifically designed for single ring use - with no chamfering and deep teeth. Nice simple setup for my commute.

Another trick I found to eliminate 'chain-offs' on another bike I had, was to fit an old front mech with no cabling, and frig the L-H screws with an allen bolt to get it to sit in the correct position, the chain bounced against this rather than jump of the chainring during rapid shifting, highly un-orthadox but it worked!