Little update. A sram nx 12 speed 11-50 cassette will fit straight onto trainer without using xdr freehub.
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@Robert_Patton
As Shuji wrote, it’ll work. See the March 23 post above in this thread: 7 Speed Bike on Wheel-off Trainers:
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I ordered an XDR hub for my Tacx Flux so I could run my 12 Spd SRAM AXS group on it…6 months later I’m still waiting, they finally have them in stock and apparently it should arrive in June!? Garmin have taken over Tacx and the customer service leaves a lot to be desired here in Australia.
You can buy a 7 speed cassette. You might find a 7 sp with the right gearing would match your 6 speed free hub. You could set or leave the rear derailleur to not use the inner most gear. As your bike has down tube shifters and 6 speed its probably not indexed just friction so no issue. To use a 7 speed cassette on my kicker trainer I had to use a number of cassette spacers to move the cassette out on the hub.
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Great post, a lot of stuff in here i did not know about 
Hi all. Totally new to all this so forgive the dumb questions.
I’ve just bought a road bike with 14 speed shimano gears (7 speed cassette on the rear wheel). It’s a new bike ( Challenge Venture CLR 2.0 700C Wheel)
Looking at getting the Wahoo KICKR Core smart turbo trainer, and I understand it doesn’t come with a cassette. What cassette size should I get with the KICKR that best suits the bike? Do I also need to get a front wheel riser? Anything else (tools for installation etc?)
Many thanks!!!
Ricky
@Ricky_John: You need the same type of seven speed cassette on the trainer you’ll use with Zwift. You can likely find that at your local bike store where you bought the bike.
The hubs on modern wheel-off trainers like the KICKR Core, out of the box, are set up for the most common cassette/shifter type on road bikes today, which is eleven speed. The number of cogs on the cassette matches the specs of the rear derailleur and the gear shifter up front. You’ll have a seven speed shifter and a seven speed derailleur.
Your seven speed cassette will slip onto the Core hub. The notches that make the cogs line up with each other are all backwards compatible. The one thing you’ll need to adjust is the spacer that goes on the hub first, so that the total width of the spacer plus cassette matches the hub width properly.
Here’s a YouTube video showing the concept and the adjustment that is needed for a seven speed cassette versus what’s needed for an eight, nine or ten speed cassette. Those 8, 9 and 10 speed cassettes will go straight onto the 11 speed direct drive trainer with a simple adapter. For 7 speed use, you need to add a second spacer, 4 or 4.5 MM, and file a couple of small indents in the aluminum spacer. Our local bike shop provided a 4mm spacer for a couple of dollars and we used a 7 speed triple on a 2014 Wahoo KICKR for a long time.
Hi.
Thanks so much for the info. Really useful.
Do I just need to ask for (or find online) a ‘4mm hub spacer’?
I’m looking online for a 7 speed cassette, shimano, how do I need what ratio to go for, for example 11-28T. Is there something on my bikes cassette or chain that will tell me?
Cheers again!!!
That’s a good question. While any Shimano 7 speed cassette will fit on the hub, the number of teeth on the cogs will have a small but noticeable effect in Zwift. The smallest cog determines the fastest you can go in Zwift at a given cadence (such as turning over the crank set 90 to 100+ revolutions per minute). The largest cog determines how steep a grade you can climb at a reasonable cadence (such as 60+ RPM). Very low cadences (50, 40 …) uphill are harder on the knee, etc.
Zwift offsets that in a big way by having a “trainer difficulty” setting. By default, it’s set at 50%, so that the extreme 17-20%+ grades treat your knees, and your gearing, as if you were actually on 8.5-10%+ grades, and so on. You can adjust that anywhere from 0% to 100%, so you can effectively give yourself virtually very low gears, regardless of the physical cassette on your bike.
The easy answer is to get a cassette just like what’s on your wheel. One point there is that if the chain is the right length on your bike now, and your derailleur works properly across the range of the cassette, there will be no incompatibility with the cassette you put on the trainer. The chain, derailleur and the cassette need to be in “sync” with each other in that the chain needs to be the right length for everything to work properly from the big cog to the small cog. If too short you won’t get on the big cog, and if too long there won’t be tension for the small cog. Also, a much larger big cog could need a longer cage derailleur.
In most cases, there’s enough “slack” that a small change in the cassette won’t make a difference, say from 28 to 30 or maybe 32, or from 14 to 11. Off the shelf, they’re only available in certain combinations.
Sheldon Brown was a guru of bicycle mechanics. He died in 2008 but his web site has remained on line. You can read more about seven speed cassettes here:
https://www.sheldonbrown.com/k7-7.shtml
Count the teeth on the smallest and largest sprockets on your cassette. That will tell you what size cassette it is. E.g. an 11-28T cassette will have 11 teeth on the smallest sprocket and 28 on the largest.
Thanks both for the really informative info. However I think I have a problem. I’ve just bought this 2nd hand (but only used once and pretty immaculate) road bike that is only 4 months old, and following the above info have checked and the current 7 speed rear ‘cassette’ sizes are 14-28. I’m not at home at the minute, but googling for 14-28 shimano cassettes only return results for ‘freewheel’, so I’m pretty sure despite being a new bike it’s fitted with a freewheel rather than a cassette. I think from what I’ve read thats a major problem for compatibility with a smart direct drive trainer 
.
Any ideas as to what options I’ve got? Will I get away with a 11-28 7 speed cassette or will there be issued with the derailleur /gear changing and chain size? Can I get the 11-28 cassette and change the change size? Or am I looking at changing the road bike?
Thanks again for the help!!
Ps - I plan on using the bike 90% on the trainer and just 10% at most on the the road, so any adjustment I may have to make to the bike (derailleur /chain?), it would have to be for the trainer, it wouldn’t really matter if it meant I couldnt easily switch back to the rear wheel with freewheel for using on the road.
That said, doing some more reading, using the bike that had 14-28 7 speed freewheel on a 7 speed cassette would probably be OK, as the spacing between the cogs would be the same, all be it the shifting might not be as smooth on he 11-28 as it was on the 14-28 (that I haven’t even used yet lol so that wouldn’t be an issue to me). I’ve seen no mention of whether the length of chain matters, which I suspect means that the cogs are the same size on both 14-28 and 11-28, but just the teeth are smaller in the higher gears?
So seems like it might be jato just but a 7 speed 11-28 cassette, with necessary spacers, but any feedback on this stuff would be great, I just don’t want to spend £700 on a trainer and have it not work properly 
Any Shimano chain and rear derailleur would meet the same standards (I strongly suspect but don’t have a link for you) regardless of whether the bike they’re on had a cassette or freewheel. The chain width and the cog spacing aren’t different for a Shimano 7 speed cassette or freewheel. So you could use the same bike on a trainer with a cassette on the hub as well as with a rear wheel with a freehub/freewheel.
With that said (and subject to correction) a look at your rear wheel/cogs would go a long ways to settling this question. I wasn’t able to find the specs online at https://www.argos.co.uk/product/8031820 .
If you take the rear wheel off, and take and post an in-focus, close-up photo of the cog set from the side, we can probably see and tell you whether that’s a cassette or freewheel without any other research.
Thanks Steve.
I’m away for a couple of days but will definitely post the pics when I’m backk to confirm it is a freewheel.
That sounds promising though, even if it is. I was hoping the 7 speed cassette vs 7 speed freewheel would be pretty similar if not the same in terms of spacing and size etc, but was worried about different number of teeth, although that doesn’t sound a problem either 
Interestingly I’ve just found a Shimano 7 speed 12-28 cassette in Halfords - which is presumably even closer to my bikes 14-28 cassette/freewheel than an 11-28 would be?
Just an update. I bought the 7 speed shimano cassette 12-28 and fitted to my new wahoo kickr core. Attached my bike to it, but although some gears are fine, some seem to rattle, some worse than others, and lots of gears just jump. Feels like only about 1/3 or may 1/2 of the gears seem OK. So looks like the gears on my road bike (which ran a 7 speed freewheel 14-28) just don’t get on with the 7 speed flywheel cassette 12-28 
. I suppose it’s possible to adjust the gears (not that I know what I’m doing), it may have to bite the bullet and trade my bike for one that has 8/9/10/11 speed flywheel cassette… Unless anyone has any suggestions!!
Any chance your chain is worn? And is the cassette on tightly (you’ll need some (more) spacers if there is play)?
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The bike is next to new so chain should be hardly used. Yeah the spacers seem to fine and the cassette fits snug…