Zara Sp00k

I have a 1996 Trek 2300 with full Ultegra, 8 speed 12-23 HG90 cassette, I have an extra wheel and would like to get a cassette for it so I could simply swap full wheels depending on where I am riding, and would like gearing for climbing, anybody have a suggestion for what the largest cog I could use without changing chains? IOW, how many extra teeth can I put on and still use the same chain? Yeah, I know I should probably get a new chain, but still would like to only swap wheels without running into chain length imcompatibility. From looking at available cassettes, it looks like I will likely end up with a 11 tooth smallest sprocket which will be nice for the opposite direction. 28, 30, and 32 seems to be the next available sizes.

shelbyfv

With the bike in big/big, eyeball your RD to see how much slack there is. It might be able to handle a 25. The larger cogs will almost certainly require a new MTB RD as well as a new chain.

Zara Sp00k

hmmm, it sounds like even a 26, if I could find one, would be outsized, so then the question is, if I put on a 11-28 with an appropriate sized chain do you think it might be possible to use the 12-23 without changing the chain? Maybe I'd just not have to use the small chain ring when the 12-23 is installed.

kcjc

Shimano RD-6401? 28, https://velobase.com/ViewComponent.a...d531f&Enum=108

shelbyfv

I think there is a good chance a chain sized for a 28 would be OK with a 23. It might be loose in small/small but you don't need to go there anyway.

Andrew R Stewart 

I am not going to do the google searches to find out the exact model Ultegra you have and then find out the largest rear cog it is rated for BUT IIRC your rear der has a 27tooth max spec. Depending on a few factors one can often get at least one more tooth and if you're lucky a couple more. If you don't ride in the small/small combo (or the next small rear cog/small ring) you can size the chain to handle the expanded past the spec rear large cog and the chain will droop a bit in those no-no combos. What you really don't want to do is ever have a chain that's too short for the large/large combo.

Generally when installing a new cassette the chain is strongly recommended to also be changed. Not doing so can cause chain skip (especially on those small cogs), more rapid wear of the cogs, more noise and roughness and retain the chain's old and worn shifting responsiveness. Adding new links to a worn chain is what one does when in a jam and just has to continue on till they can replace the chain. Are you in this position now?

I would install a current cassette with a 28 large cog and a new chain. Adjust the system as Shimano instructs. Then replace that wheel with the other one and see how the longer chain on the older cassette works WRT both chain skip and pulley knock when in that cassette's large cog. You might find out that some B screw adjustment is needed when swapping wheels, or minor der limit screw tweaks.

Are the two wheels running the same hubs? If not then there's one more area of possible "close but not quite right" aspect that would want some slight adjustments. Having installed many hundreds of wheels (especially with different cog sizes) over the years it has been the rare situation where no adjustments were needed and the index setting, the range limits, the chain length and the B screw setting were all perfect. Andy

Zara Sp00k

Thank you everyone for your input, lots to think about Andrew. How I got in this situation is my rim broke, I found a great deal on a whole new wheelset, front and rear (new bearings were sublime!) then I found a good rim at a cheap price, I used a local do it yourself bike shop to install the rim to my old Ultegra hub, so now I have a set of fairly new Mavic wheels and my original Ultegra Hubbed wheels minus a cassette. So I figure add a cassette for climbing.

I see Sunrace makes a 12-25 which I'm pretty sure I could get to work with the same chain, better, but not much over a 12-23 for climbing.

Bill Kapaun

Put the chain on BIG:BIG and measure how much "slack" you have. You want a minimum of 1". (one link SET)

maddog34

What are your Chain ring sizes? is the derailleur the RD-6401 mentioned? if so then it has just enough room for the 28 tooth low gear cassette BUT you then only have enough WRAP(slack take-up) for 11 teeth difference in the front... 28 total Wrap..

I'd set the chain length with the small/small method and then you can swap back and forth with no issue.. but only IF that front jump is 11 teeth or less... you MIGHT squeak by wit a tooth or two extra wrap, but it can be iffy... 53-42?.. you're good to go.. 52-39?.. not so fine.

Zara Sp00k

yes, it's a 6401

53/39 so what do you say now? I am guessing no

icemilkcoffee 

You can read the Shimano documentation for yourself:
https://si.shimano.com/en/pdfs/si/U-...000-00-ENG.pdf
If your crankset has 52/39, then you are limited to 12-27 in the back, to keep the total capacity <= 28 (total capacity = front difference + rear difference)

Yan 

If you don't want to change chains, then you cannot change to a bigger cassette. A bigger cassette requires a longer chain. If you keep the same chain, your derailleur will break the first time you try to ride your bike.

Zara Sp00k

thanks for that, they say the smallest cog in back can be a 12, since I have a 53/39 chainring that means I could go with a 26 cog in back, SRAM makes a 12/26 cassette, I see Shimano current 8 peed cassettes are 12/25 and 13/36, maybe there is an older one available somewhere, but I think I'm going to get a new chain, fit the SRAM 12/26 which basically gets rid of the 14 cog and adds a 26

t2p

back in the 90’s I ran 12-25 on one Ultegra bike and 12-28 on two other Ultegra bikes ...
53-39 chainrings (possibly 38t on one) ... all worked well

pictured is one of the bikes with 12-28 ... (XTR 12-28 cassette)

Calsun

Most rear derailleurs can handle 12t to 34t cogs at the rear and manage the chain tension throughout that range. I had a 11-42t cassette and when I changed it to a 11-34t one I replaced the rear derailleur to one with a shorter cage for crisper shifing.

A larger cog means more chain length is needed and the derailleur needs to cope with that extra chain and maintain enough tension to keep the chain tight enough to avoid problems.

alcjphil

If your derailleur has enough capacity to take up slack on an 11 28 cassette it will also have enough capacity to take up slack for the larger small cog on a 12 23 cassette
However, you may need a longer chain for the larger cassette. Going from 23 teeth to 28 teeth for your large rear cog will require at least one extra double link. Although you will be adding 5 teeth, keep in mind that the chain only engages halfway around the cog so you will be going from about 12 teeth engaged with the chain to 14-15 teeth engaged, a difference of only 2 to 3 teeth

Zara Sp00k

In looking at the info on my cassette, it says the max difference on the chain rings is 13, yet, my bike came with a 14 tooth difference. 53/39
it seems most cassettes nowadays have an 11 tooth small cog, the info says 12 is the minimum.
When all is said and done, I don't know that I need an 11 tooth cog, when I was younger I'm sure I could have used it but now nearly all my riding is on paved trails (old RR grades and city) RR grades are not steep, where speeds are not high. Typically the hills around here are at rivers where the altitude gain is about 200 feet over 1/4 to 1/2 mile at the steepest . If I go out west I can take my mountain bike which can handle the steeper hills over a longer distance. So I am thinking I really don't need more than a 26 cog anyway, the 23 has served me well for a quarter century, I was just thinking as long as I get an extra cassette, might as well get different gearing.

delbiker1 

I have nothing to add to answering the question. But, I have caught a lot of fish on Zara Spooks. And a fun lure to fish with.