20%+ Lower Power on MTB versus Road Bike

After a long season of training for road racing, I’ve switched over to just riding my MTB for the fall/off season and I’ve been stunned by how much lower my power numbers are on the MTB. I’ve pulled a couple of efforts from road and mtb from the last month or so below, listed out below. Comparing the 50 minute road effort and the 20 min mtb effort, both felt like high tempo efforts, the road power was ~100W higher with a lower HR for over twice as long. Comparing the two 15/18 min max efforts, the road NP was 57W higher. The implication here seems to be that my endurance zone on the mtb is something like 80-100W lower and threshold is 60W lower.

I’ve ruled out a few of the obvious reasons my power might be so different. 1) I did a test on my indoor trainer at 200W and 300W with both my road PM (assioma duo) and mtb PM (garmin rally double sided). Both read about 3-5W higher than the trainer as you would expect for pedal based power. 2) It also isn’t the big power surges and drops required to climb on singletrack. One of the power tests I did, noted here, was conducted on my mtb on a smooth steady paved bike path with my suspension locked out. 3) I measured and matched the saddle height and seat setback on my mtb to my road bike, on which I did a professional bike fit at the beginning of the season.

Has anyone had any similar experiences or have any ideas why the power might be so wildly different? There are obviously some differences in the position (stack, reach, q-factor), but can those really make that much of a difference? Anything I can do to pull my power up on the mtb?

Road Bike Efforts
-20 mins, 249W avg, 250WNP, 153W avg HR (snippet of endurance training ride)
-50mins, 303W avg, 319W NP, 161 avg HR (long climb in a race)
-15mins, 367W avg, 368W NP, 181 avg HR (near max effort)

MTB Efforts
-20mins, 206W avg, 208W NP, 167 avg HR (paved bike path, felt quite hard)
-18mins, 300W avg, 311W NP, 181 avg HR (single track climb, near max effort)


Assuming your assessment on the accuracy of your PM is correct then I would say, I’m not super surprised but those are pretty far off numbers. Personal anecdotes - when I’m training a lot on my road bike and switch to MTB, it always takes me a few rides to remember how to put out snappy power and/or consistent power on an off road surface. Your fit sounds pretty dialed so I think it’s probably just a case of specificity and it’s crazy just how specific our training needs to be in order to flourish in a given sport. Keep riding the MTB and keep tracking the power numbers.

Are you comparing apples to apples?

Why not do a steady test on the trainer with your head unit measuring the power meter with both your road bike and then MTB.

It makes no sense that you all of a sudden sit on a different bike and put out 50w less at the same RPE. If your meters are not faulty, there should be a small difference ( they state an accuracy range).

I have stages PM on my road bike and MTB. Power is approximately the same (on similar surface - riding road to road)

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I’m not a MTB’er but there’s something about momentum on a smooth road that let’s me output more power. Add in any sort of resistance to break up that momentum and I just can’t reach the same power levels.

The gearing can change the sensations for some rides/riders. Depending on what gearing you have on your MTB.

I have never compared the numbers between my bikes. But based on perceived efforts I have never felt the need to. Power data always matched my perceived efforts. I would verify the accuracy of the meters.

This is between gravel, road, XC, and long travel enduro (170mm coil).

I have a very similar experience on my mtb as well, I just cannot seem to put out the same power as the road bike. I think for me its an ergonomics issue on the mtb: I’m on a pretty raked out Enduro bike, and it’s really not optimized for high output. I think the power diff on the road bike stems from the better geo.

Smaller front chainring requires less torque so this will naturally result in lower power. Ratios are just so different between road and MTB. Your power is likely to be closer on sustained climbs on a MTB to flatter efforts on the road.

I’d take both bikes to the same climb, maybe over 2 days, and test directly. Right now you’re comparing a race performance with a ride you say “felt hard”.

Similar experience, but on the road bike I have a quarq d-zero and on the mtb a power2max ng or eco (whichever was less expensive). The actual power and perceived effort b/w the 2 is due to different body positions and surface that I’m riding on. I put more training time in on the road bike, so when it comes to doing longer efforts, it’s feels easier to me. When I get on terrain where power isn’t as consistent due to wheel spin, sudden start-stops, etc…, I think the sampling rate has something to do with the actual variances that I’ve seen.

… wanted to add, I haven’t put the mtb on the trainer to do some back-to-back 3 min, 10, and 20 min efforts compared to the road bike… Something that I should get around to trying.