Newbie question on numbers in Zwift on different bikes

Hello all!

I am new to Zwift and the forum, as well as to bike training. I am not new to biking, but the numbers game is totally unknown to me. So I do not really know how to ‘read’ the numbers. Maybe more experienced Zwifters can help me out?

A little over a month ago I got a smart trainer (kickr snap) loaded my XC mtb on it -only bike I had- and got onto Zwift. Loved it, so got a subscription and a cadence meter and heart meter.
My first few rides and an FTP test were fine for as far as I know. FTP was about 142 if I remember correctly. I am nearly 49 so do not know what to expect, what a good ftp is for someone my age who never took interest in training before. But I had a few one hour+ rides where my average watts were around 140-150 watts.

Then, when spring got really underway, I got a fifth-hand Trek Lexa roadbike so I could go outside on my own mtb. The Lexa is now my “slave” bike that is in the trainer permanently.

But my average watts and FTP dropped dramatically. I had a headcold at that time, nothing serious, just a blocked nose and forehead so I would expect some loss of power, but I am not sure if it should be this much? Before the change, I did the mountain climb in Watopia in 38 minutes. Now, I cant get it any faster then 41 minutes.

I did a new FTP test and got it on 134, nd my average watts consistently stay around 125 over the whole ride.

My question is: Is this normal? Or am I doing something wrong? If so, what?

Is it the change of bike on the trainer that gives such different output/readings?
Is it the headcold? Can that make such a big difference? (And shouldn’t it have cleared by now, two and a half weeks on?)
Did I maybe put the bike too loose or too tight in the trainer? (Rear wheel does not slip though…)
Does tire pressure count for so much? (pumped it up hard just to be sure. No dice. Still much lower output then what I saw before)
Or is such a drop normal and should I just get on that trainer more?
A combination of some or all of the above?


It can be so many things so I will start with the obvious ones.

Did you calibrate your trainer after you swapped bikes.
Different bikes could have different gearing so you may be not selecting or not have the same preferred gear as before.

How old is the “new” bike, it may need a service dry bearings and chain can cause significant losses.

Did you use clip less pedals with the MB and not with the “new” bike?

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Hi Gerrie, thanks for your input. If calibrating the trainer means doing a spin till 37 kms and then letting it fizz out, I have done that twice. I noticed it was a lot easier on the mtb to have zwift think i did 37 kms already. Is there a different way of calibrating the trainer?

But a mtb has indeed very different gearing from a roadbike, so that might be a thing.

I have clipless on both bikes. The lexa needs a tune up for sure. Maybe i’d better plan that a little earlier then i originally envisioned.

Maybe I am simply unacustomed to numbers and maybe I am underestimating the amount of power that a headcold can take away from me, but after a few rides this week where I consistently had my FTP upgraded by the app after the ride (in three steps up to 151 now) and feeling quite well, I am starting to think it was not just the bike.

Dang headcolds!

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Good news that the power is going up up up.

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I have a couple thoughts about the difference in wattage between the bikes. The 5th-hand road bike might have points of excessive friction in the drive-train causing a loss of power. Your planned tune-up might address that. Also, the road bike likely has smoother tires than the mtb, leading to more slippage on the trainer roller. You might try using a trainer-specific tire. Good luck!

hi Aaron, thanks for the input. I am now also hoping that the tune up will make things better, though it hasn’t happened yet.

My FTP is back to where it was, a bit better even, so I do blame the headcold more now then I do the bike. I am just still a bit flabbergasted to notice how much a simple headcold can eat away ones power! The wonder of training by numbers, I guess.

I did and do use trainer tires on both bikes, thanks for the advice. I have got some gruesome knobbly mtb tires (I ride alpine trails with it, and then every extra ounce of grip you can get is good…) and I dont see how that would have workedon the trainer - and would have made a real funny sound too, I am sure!

What did happen though, was that the trainer tire of the oradbike blew out last week. I think I had it on too hard. Put a new tire on, and took care to not overtighten. Seems good and on first sight, the numbers havent changed much. The slippage when I change gears has grown a bit but I am assuming that is part of riding on these wheel-on trainers?

For reference: the old set up

And the new set up;

(I know, that old fashioned hotel ironboard for my laptop is a bit wonky. It will get upgraded. At some point.)

That look like a sweet bike for the trainer.

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Well, maintenance has been done by now, whole drivetrain checked, adjusted, lubed. And my FTP dropped from 191 to 157. Ha!

So, I have three culprits right now:
Culprit number one: the headcold.
Culprit number two: the difference in ratio between mountainbike gearing and roadbike gearing (2x9 in a X9 high end drivetrain versus shimano claris 2x8 of unknown age)
Culprit number three: a not so nicely working drivetrain versus a freshly maintained one.

Bike set up - as in how tight I put the roller to the tyre - might be a factor here too. Since my first training tyre blew out, I took care to tighten it just so that it will not slip anymore but not more then that. I am not sure how much difference it makes if I tighten it a few turns more or less?

We do a spin down about every two weeks and an FTP test when the bike has been out of the trainer. Just to get the numbers to align a little with the set up.

We are learning, though. And are seriously considering upgrading for a wheel off smart trainer the coming winter or the next.