Building a FrankenRainer

I have a Kickr on order to work on my fitness/strength.
I’m primarily a mountain biker so going to buy a very cheap second hand bike to pull a apart and second up permanently in the man cave (1st floor).

The kickr comes with an 11 speed shimano road cassette, does anyone know if an 11 speed Shimano MTB derailleur would be indexed correctly or I’d have to use a road derailleur in which case would a road derailleur fit an MTB frame (Bearing in mind this is never leaving the house.

Does the front ring size matter?
I worked out that in ERG mode it shouldn’t make any difference but in normal day to day riding around will roadies with massive front rings come hurtling past me as I’ll in be in a spin class or does it not work like that?

Anyway, looking forward to the sweat fest and getting fitter/stronger :slight_smile:

Hi @Calzor_Suzay and welcome

I’m no mechanic but I believe the spacing between the cogs is different between shimano 11 speed road and mtb cassettes.

Changing the rear mech won’t make a difference as the indexing / amount of cable pull is determined by the gear shifter.

The easiest option would be to swap out the rear cassette with a cheap 11 speed mtb cassette.

**Edit - also the size of the front ring won’t matter if you intend to use Erg mode. If you keep the chain around the middle of your cassette you should find your kickr responds well to power / resistance changes

Sorry I wasn’t very clear.
I’d be buying a new derailleur, & shifter so that wouldn’t matter, you can also get ‘flat bar’ road shifters to match.

ERG mode I know and not sure I’ll use it much, perhaps a training session will have it but I’m more interested in normal riding about as it’ll be a 1x up front.

A road derailleur can be fitted to a mtb frame.
Cable pull is different so mtb shifters don’t work with road derailleurs and vice versa.
Mtb derailleurs are designed for wide ratio cassettes with big sprockets, whereas the supplied 11sp road cassette only has a 28t big sprocket, the guide pulley will be to far away no matter how you adjust the b tension and lead to poor shifting.

Sprocket spacing is identical on road and mtb cassettes but road cassettes are wider than mtb ones because the big sprocket is not big enough to be dished over the spokes.

In level mode you can increase the kickr’s power curve by selecting a higher level, from 0 - 9, which will compensate for the smaller front chainring.

Best gearing for normal ERG use with a road bike is small (2x) ring and a middle cog. This keeps the flywheel in a reasonable rpm range so the trainer can respond more quickly to power changes and cadence changes (a couple of second usually.) In most cases you just leave it there in ERG mode, but there are a few situations you might want to shift. With an mtb setup, you want roughly similar gearing. 11 speed mtb and road cassettes have the same spacing, but the jumps between gears and total range is much bigger on an mtb cassette and the rear derailleurs are designed to match this. As mentioned, the derailleur and shifter must match speeds and cable pulls. Basically, it’s best to use a road cassette with a road RD, and a road compatible (drop or flat bar) shifter. Or, use a mtb cassette and mtb derailleur and shifter.

That said I did run my 11sp mtb on the trainer with a 10 speed road cassette. I adjusted the cable tension so that RD lined up with an appropriate cog. I just left it there for ERG mode, but could in fact shift a couple of cogs either way before things got out of wack.

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