Need help - modifying gears...
 

I just bought a Dahon Formula S18 with the plan to swap out the cassette to get lower gears (I live and work in hilly area - more concerned with climbing hills than beating speed records). It's being delivered tomorrow. I believe the largest cog has 27 teeth [EDIT - it has 25. The cassette is 11-25]. I was told by the salesman I will be able to swap it out for either an 11-30 or 11-32 cassette. However, I just read on Shimano's website that the largest cog this particular rear derailleur can handle is 27 teeth. Now I'm wondering what my options are. Are Shimano's specs conservative and can I likely get away with going up to 30t? Without negatively affecting performance? If not, what about changing out the small chainring up front for a slightly smaller one? How can I figure out if that would work (i.e. could the front derailleur handle it?)? The front chainrings are 56t/46t. Any ideas/suggestions?

Perhaps a simpler approach is to put a chainring set of 53/39T.

Thanks for the suggestion, jur. Is there any way I can calculate what the gear inch range would be if were to do that? I'm only looking to bring it down a little.

Use Sheldon's Gear range calculator:

Sheldon Brown's Bicycle Gear Calculator

My input:
Wheel Size: 20x1.75
Crank length: 170mm (does not feature in gear inches calculation)
Gear units: Gear inches
Chainring: 46, 39
Cassette: Custom sprockets
Next line: 27, 30
Internal hub: No planetary (internal) gears

Press "Calculate"

Output shows for the 46T-27T combination your gear inches are 31.8

If you switch your chainrings so that you have a 39T in there your gear inches can drop down to 27.0 (15% drop)

If you switch both your chainrings AND cog set to a 30T (at largest sprocket) then your gear inches are: 24.3 (23.5% drop).

Thank you, Shahmatt, but I think something is off. The gear-inch range listed in the specifications for this bike on the bike-seller's website with the stock 56/46 chainring and stock cassette is 26-95. Plugging 56/46 for Chainrings in the calculator, I had to plug in 11 and 33 in Custom Sprokets to get 26-95 gear inch range, but I know for sure the largest cog isn't 33t. In other words, before trusting the calculator for theoretical chainring sizes, I first tried to confirm that it is calculating correctly for the stock chainrings, but it doesn't seem to be.

The Sheldonbrown calculator is accurate, you can trust it. Highly likely, the bike's specs are wrong somewhere.

Hmm... I just found a review that lists the stock cassette at 11-25. With the stock 56/46 chainrings, if the Sheldonbrown calculator is correct, the stock gear-inch range is 34.4 - 95.1. That seems like a small range for an 18-gear bike. Wonder where they got the 26-95 range in the specs. I'm still a little dubious. I'll play with the numbers some more. Thanks.

Yes the stock specs seem wrong. There are several websites that do the gear inches calculation (try a web search). Here is another:

BikeCalc.com - Bicycle Gear Inches Chart

Same result though.

Wow - this calculator gives me even higher results (and different from Sheldon Brown's): Stock chainring and cassette comes up with 36.8-101.8 gear inch range. I don't believe that's accurate.

Do you mind telling us what is the rim size and tire size you have used on that second link?

The difference between the calculators come from tyre size. Calculating the circumference of the wheel with inflated tyre is somewhat inexact. My personal favourite method is to take the rim bead seat diameter and add twice the tyre width to arrive at the ultimate diamter, from which circumference is calculated. Obviously if you use different width tyres, the gear inches will change.

A simple sanity check is to take the gear range, ie the ratio of the highest to the lowest gear:

95/26=3.654

and compare with the sprocket ratios:

28/11 * 56/46 = 3.099. clearly the two are different.

Assuming a 32 tooth cog:

32/11 * 56/46 = 3.54, still not the same as the spec.

Assuming a 34T cog:

34/11 * 56/46 = 3.76, still not the same.

I still say the 95/26 is wrong.

how about asking the seller whats on the bike first ?

Well, it turns out all of you who pointed out the specs for this bike are wrong were correct. I finally got to take the bike out of its box and do some calculations. I measured the distance travelled in one crank revolution in highest and lowest gear (and divided the results by 3.14). It turns out Sheldon Brown's calculator is spot on. The gear-inch range is 34-95, not 26-95 as published. This is very disappointing. Moreover, talking to a bike tech today, I discovered that changing to smaller chainrings in the front is not an option. Because of the way the front derailleur is mounted, it cannot be lowered. So, my only option would be to swap out the rear 11-25 cassette for a 12-27, a very slight improvement which would not get my low gear to where I want it.

I'm really sad about this, as I really love the bike. When I test rode it, it felt so much like my old full-size bike. It rides and shifts beautifully. I thought that because all the parts are standard, the gearing could be easily modified, and the salesman indicated that it would be, but now I know that's not the case. The bike is beautiful, and I think a great choice for normal riders, as opposed to riders like me who would rather downshift to maintain cadence than stand and plow through tough hills. A low gear-inch of 34 is probably just fine for most riders, especially those looking for a speed bike like this, but I'm not a strong rider. Even back when I bought my first full-size touring bike, I had the shop swap out cogs to lower the gears. Unless the salesman comes up with a suggestion I haven't thought of, I will be returning the bike. I think my best option now may be the Dahon Mu P24, which has a huge gear range, and from what I can tell, the lowest of the low gear-inches - substantially lower than the Formula. I'm open to any other suggestions.

Thanks for all your help and input.

[MENTION=137322]Jerrys88[/MENTION]... I just received the Dahon Mu P24 today, got it assembled and tuned. And the gear range is incredible! Super low and also very high. I ordered a second one for my wife today after a test drive. Looking forward to some destination rides without having to rack anything outside the car. :--)

The internal sram dualshift hub is great, no detectable resistance and shifts quick.

The bike is very solid. The only downside on the bike is the rather cheap pedals, but I will probably put some combo 'performance campus' flat/SPD pedals on it anyway. I also removed the kickstand and fenders, removed reflectors, etc. (I have bike lights for when I need them).

The Formula S18 is a pretty cool bike with the disc brakes and double up front ... shame to dump because you can't get low enough gears .. I'd be inclined to run a mega-range 11/34 or 12/34 ( if you don't need 96 g/i on top) and a longer cage rear der .. you'll cover the extra wrap and can leave the front alone (if it really is the case you can't put a 39 up there for some reason).. I'm running a 60/39 on my Moulton and the Shimano front der handles it just fine.. my first order of business would be to mount a 39 up front and make sure it won't work .. that would give you a 29 g/i and maybe low enough.. if not, then on to mega-range rear cassette.. I wouldn't give in just yet..

Yeah - I might just bring the bike to another tech to get second opinion. If I do, I'll bring up your suggestions (which are new to me, being a non-techie). There is something really appealing about this bike. I've ridden an MU P8 (I assume the MU P24 is same frame) and, while it's a nice ride, it didn't feel as sporty as the Formula. Still, I feel at this point I ought to try out the MU P24 for comparison since it comes with all the gears I could possibly want. I wonder what the downside would be to installing a longer rear derailleur. Would it not be dangerously low to the ground on a 20" tire bike?

I would get a rear 11 32 sprocket ....
than I would run the biggest gears in the back with the small chainring in front
and the smallest in the rear with the large one in the front

somehow I seem to do that anyhow lol
meaning the only thing u need is a different sprocket

sure no problem to change derailleur as long as it doesn't have a neos derailleur as standard

..ps talking myself out of a sale again lol as I do sell the mu p24

Old wife's tail .. err, tale.. long cage gets close to the ground with 16" wheels, but is still done in some instances... with Kojaks on 406 wheels, you won't have a problem on the street.. lots of 406 folding bikes used to come with long cage rear when they didn't need to for wrap.. that's when I would go to short cage (for a single up front and even an 11/34 rear).. another opinion could be good..

Thank you so much for contributing your comments to my thread, Thor. I'm not very tech-savvy and trying to educate myself as fast as I can. Just to clarify (so sorry if this is dumb question), do I understand that you are suggesting keeping the short cage rear derailleur which is spec'd to handle only up to 27t, installing an 11-32 cassette anyway, and getting around exceeding the derailleur capacity by avoiding gears that use the large chainring up front + large cogs in back? That's an interesting idea - will have to plug in numbers in SB Gear Calculator to see what gears I would be losing.

The Formula S18 comes with a Shimano Tiagra SS Rd-4500-ss RD. Stupid question - how do I know if it is "neos"?

It's not a Neos (which mounts ahead of the axle) .. just a standard off the shelf rear derailleur.. lots of options .. not so much with the Neos .. and I wouldn't exceed the actual derailleur capacity.. there will be times you just forget what gears you are in and you will bind it up.. this stuff is all pretty obvious once you have it on a bike stand to quickly see what works and what doesn't .. a good shop should breeze through this..

Check the specs of the Vector P20 11/36 in rear 53/39 and a mid cage derailleur.. you should be able to do something quite similar with the Formula..

If you can return the bike then maybe you should. Making these sort of modifications can be expensive, and unnecessary if there are other options out there. Also I'm not sure about warranty implications.

However if you are still interested in the modification route some options below:

1. Consider if you need the top end of 95 gear inches. If no then you could swap out the double rings for a single 38T perhaps. Omit the FD and shifter and save some weight. Your gear inches range will be 28 to 65.

2. You could swap out to a compact double of your liking, 38T,56T maybe, omit the FD, and switch gears manually (move the chain by hand to the larger ring).

3. If you replace with a single oval ring (maybe a 41T with eccentricity of 16%) your gear inches would be 30.6-69.6, but would feel like a 28-69.6 because the minor axis of the 41T would have resistance of a 38T chainring. Basically when you drive the 41T with the 25T sprocket it would feel like you are driving a 38T instead.

Well, good news is bike tech I trust says he can do all the modifications: Swap out short cage rear derailleur with Shimano Deore MB derailleur, install 11-34t cassette, and 39-53 chainrings (he says it'll work - to get low gear I'm aiming for I need to do both front and back). That'll give me these gears (any comments welcome - I'm not an expert on gear configuration. The only thing that I'm an little concerned about is gap between highest two gears, but not sure I should be):

http://bikeforums.net/attachment.php...hmentid=464957

Bad news is it'll cost about $250. I'm going to test ride the Mu p24 tomorrow before making a decision.

MU p24 chart, approx, best I could figure from specs, for comparison:

http://bikeforums.net/attachment.php...hmentid=464958

[btw, thank you, Shahmatt for your suggestions. I'm afraid I would not be happy with the gear inch ranges of your first and third suggestions. Re #2 - I rode a year on my Brompton switching chainrings manually (I added a second one), and am trying to get away from that.]

You can tighten up the top by running an 11-32 (11-12-14) instead of the 11-34 (11-13-15) .. and still retain a low 20's bottom.. since you now want to also change to a 53/39, you probably don't need the 34 over the 32.. at 40 rpm, you're at walking speed ..

I believe lowering the gear can be done. Like people here said, you have two options:
1. change to mid or long cage RD to accommodate a bigger cassette up to 34 or even 36 T cog.

2. change your crank to 53/39 or even 50/34, but you said the FD can't be lowered. If that's the case, that's must be the FD hanger too high. You can saw the FD hanger off, buy a band on FD, which you can put wherever you want on the seat post. But probably you don't need to saw the thing off, just need to enlarge the slot on the hanger a little bit so the FD can be lowered.

If you think this is too much work (which I think is), pay the $250 for the mechanic or change a bike (vector p20 seems a good one).

If wide enough rear .. 135, rebuild the wheel with an IGH-Cassette combination Hub ..

Sachs, now owned by SRAM, and now Sturmey Archer owned by Sun Race, both sell hubs of that type.

' triple crank' is in the hub.