Hey everybody. The bike I’ve been riding for the last 18 months has had 170mm cranks. At 5’3" and a short inseam, that always felt too long to me, and a bike fit confirmed that. I’m going to be putting 165mm cranks on my race bike, and I’m wondering if I need to pony up the cash for the trainer bike as well. Will the 5mm crank length difference cause me any problems long term, if I train on longer cranks and ride outdoors on shorter cranks?
I have a permanent turbo trainer bike with 172.5 cranks, and three road bikes plus a TT bike each with 170 cranks. I can’t tell the difference and I doubt even 5mm difference would be any different.
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Just make sure as your workouts get more specific (as your event gets closer) you do more and more training on your “race bike”. This is good practice anyway, but especially when your fit is adjusted even a little bit. You don’t want to end up having knee problems for going all out in a race with something your body isn’t used to. As long as you follow recommendations though and get some time on it when possible you should be fine.
This is of course if you’re not the “n+1” type of person, in which case, you do need an extra training bike 
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If you’re like many people on this forum, spending more time on the trainer than outdoors, then I’d say it would be certainly beneficial to get the same length cranks for your trainer bike.
The nice thing is that you can get away with cheaper gear. Ebay it.
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It’s not going to be night and day difference that renders your training worthless or unproductive.
On the other hand, it’s not ideal. Being on a trainer usually exacerbates problems with bike fit, and if the race bike with 165mm cranks really feels a lot better, then that means the bike on the trainer feels worse. On top of that, crank length has a lot of downstream effects on your fit - different cranklength means different saddle position, which means a different angle to your handlebars. Maybe all those differences feel negligible and don’t bother you; maybe they do bother you.
I wouldn’t necessarily jump on changing things immediately if you don’t particularly want to spend the money, but I would pay attention to how the two bikes feel, how riding them makes you feel, and whether that difference is important, troublesome, or concerning. If it is, then making your trainer bike feel better (which may be the bike you are on the most!) is probably worth it.
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