Can I use chain rings that were on a 9 speed for my 11 speed cassette or do I need to get 11 speed chain rings?

Max length chain you can get: 500 foot from here http://www.bikeman.com/CH5052.html ... should be enough :D !

You may find shops reselling whatever length you want.

A 11 speed chain should work okay on 9 speed chainrings.

Keep up with frankenbiking, the fun is in the process and in the final result!

The possible problem isn't the thickness of chainring and inner width of the chain, but the gap between rings and outer width of the chain. If you have 2 or 3 chainrings on 9-speed crank (pure guess, but who would get an 11-speed crank and then retrofit old chainrings if they even fit?), the gap between chainrings is wide enough that 11-speed chain can fall between them and get stuck. So, it can work if you're careful enough when shifting.

I have recumbents that use ~270 link chains. I could buy those lengths by the foot in bulk, but for me its cheaper to buy three normal chains of about 110 links, and join them with master links, and save the ~60 links left over for any running repairs.

I also carry a spare piece of 4-6 links in my on-bike toolkit along with 2x master links. And yes I have needed them twice in the last 3 years.

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As for chain width - 11 speed is thinner externally. The rollers are still exactly half an inch apart.

Generally speaking, newer thinner chain will run on older chainrings fine. You may need to file off any egregious burrs or damage which could snag, but the teeth are always slightly narrowed by wear. Test carefully in the garage before riding of course.

Shifting can be a little more drop-prone on the front with thinner chain - you may need to "narrow" the cage of your front derailleur subtly, and make your shifts more snappy and with lower pedal pressure. However I've got a 3x5 bike running with 9 speed cassette/chain/rear mech, while retaining the orginal 15 speed chainrings and shifter. Mostly this works because I use the stock friction shifter.

9 and 11 speed chains will have the same internal width and different external width (9 speed being thicker), so they will both fit snugly on 9 or 11 speed chainrings but shifting between chainrings may be problematic as spacing between two 9 speed chainrings will be optimised for a 9 speed chain.

I recently mixed a 9 speed double chainring crankset into a 10 speed groupset. Shifting from small to big is fine but when shifting from the big chainring to the small one, the plates of the (10 speed) chain tends to sit flush with the side of big chainring and skate over the teeth on the smaller, rather than slip nicely on to it, this is because of the slightly-greater-than-optimal spacing. Pedalling too agressively before the chain engages can cause it to slip off entirely (dangerous at a busy junction). I find that when shifting like this, I need to just pedal gently until I hear the chain engage before applying any real pressure. This would be absolutely unacceptable if I was using the bike for anything techinical or competitive but it's a commuter so I just make sure to read the road ahead give a little extra thought when chainring shifting.

I suspect that if I mixed the 9 speed crankset into an 11 speed groupset, it would be unworkable and the chain would either fall between the chainrings or skate indefinitely. You might well have better luck though and if you do, it would be worth doing some testing and getting familiar with the behaviour of the configuration before going out in traffic or into any off-road/high speed situations etc.

Not sure about maximum chain lengths available but most chains should be fine, provided you aren't running absurdly big sprockets anywhere.

Best of luck.

Shimano and KMC 11 speed chains sold for road systems typically will come with 116 links of chain (in this case the 116 refers to each pair of inner plates as 1 link and each outer pair as 1 link, when technically, those are half links as one pair of outer plates and the adjacent pair of inner plates is one chain link). All KMC 11s chains come with the proprietary "missing link" which, of course, is the half-link of outer plates that join the chain. That brings our count to 117 for KMC. Earlier models of Shimano 11 speed chains came only with a connecting link pin, which has been the Shimano method of joining a chain for 9, 10 and most of 8 speed generations. Beginning again with the newer reiterations of 11s chains having a "QL" in the model name, Shimano developed their proprietary "Quick-link" master link-type joining link which is included with their newer models of 11s chains. Some models are packaged with both a Quick link and link pin included with the chain. So Shimano lengths are 116 or 117 depending on the presence of a Quick link in the package. SRAM 11s chains, both the higher end, Red22, and lower tier, 1130, models are packaged with 114 link chains and their proprietary, "Power-lock" master link for 115 links. Within the 11s chain category are the fully compatible, 11s, e-bike chains. Shimano offers 126 link chains plus Quick link in their e-bike compatible models, CN-601 & 701 QL. One can buy bulk chain and custom cut lengths from that. Probably fairly economical since most chains, however packaged, will require cutting down to size and the associated Quick-links are sold separately in various count configurations as are 11s link pins should one prefer that system.

Because the INNER spacing of chains remain the same for different speed classes of chains, an 11s chain will run on 9s chainrings just fine. The wider spacing between 9s chainrings compared to the rings of 11s cranksets is the source of concern when running a higher speed chain with an overall narrower profile on a crankset designed for a lesser speed drivetrain which has a bit wider spacing between the chainrings. Rougher and slower shifting and the elevated risk of a horribly jammed chain if the narrower 11s chain falls in the wider gaps between 9s chainrings are the big concerns. These types of jams are not of the type where one hops off the bike, takes a finger and lifts the chain out of the gap onto one of the rings. They tend to be severe, very tightly jammed into the gap, and if the chain isn't damaged going in jammed under power, it likely will be by the force required to yank it out. A patient and equipped rider may rectify such a situation by getting lucky with disconnecting the quick link and easing the chain out parallel to the the gap it's jammed in. That's a no go as often as not which then may require loosening or even removing an adjacent chainring's fixing bolts to free the chain. Trying to do this roadside with a multitool's 2-2½" long hex or torx wrench with the crank yet mounted to the bike is tricky at best. You may have the knowledge to know what to do and the prepper foresight to have brought a multi-tool, but you'll surely be challenged figuring out how to get the multitool's flat head screwdriver on the grooves of the inner nut of the chainring bolt at the same time as the hex or torx wrench part of the bolt's outer aspect. If it's coming to all that, the likely reward will be a freed but damaged chain which will now be highly susceptible to derailment and jamming if it can continue to be used at all.

With all that disclaimer out in the open, it is very common to run a front drive of a different speed generation from the rear drive with excellent results. Entirely normal operation can be expected if that difference is one speed generation difference, as in 10s crankset with 9s cassette and chain, or 10s cranks with 11s rear end and chain. It's best, for numerous reasons, to run the correct speed chain as the rear cassette requires, so their will be a degree of mismatch either way up front. Personally, I've run 10 front (2 & 3x) on 9 rear and 11 front (2x) on 10 rear, with flawless performance and quickly dissipating concern for the above risk. A 9s front with 11s rear will work nominally, but I don't feel as though the concern for mid ride problems like above could be brought down to an acceptable level and, given a choice, I would not seek that option for a drivetrain build.

Essentially I am asking if I need thinner chain rings for the thinner 11 speed chain. Also, what is the max length chain I can get in 11 speed.

The largest danger using 9-speed chainrings in 11-speed setups is that the chain "skates" over the small ring teeth, when the rear derailleur is at the smallest sprocket and you downshift in the front from the big ring to the little ring. It isn't reasonable to shift that way.

In contrast, if doing the reverse (11-speed chainrings on 9-speed setups), the largest danger is that after shifting to such a cross-chaining setup (small ring in the front, small sprocket in the rear), the chain can rub on the big ring.

Source: the bicycling bible.

So, if using the gears in any reasonable manner, you'll be fine. I have a bike with 10-speed chain and 11-speed chainrings and another bike with 8-speed chain and 9-speed chainrings. I have observed this to not be any sort of a problem.