To isolate the difference, you need to ride the same road climb on each bike quite a few times. I had the same surprise when i added the mtb power meter after having the road pm for years.
Trail power is totally different because your body is doing a lot more stabilizing and so you have less energy available for pedaling only. Generally, the road bike will have more consistent cadence so testing on the same hill will normalize that. Gaps after that will be shoes[smaller cleats?] and position and also PM variance.
If you have faveros you might be able to run them on the roadie to get any gap between the pm’s. This will also normalise your shoe difference.
I would also say that the time periods in your original question are way too long to compare road to mtb power, because when you descend as the trail dictates that drains energy away that could be put into pedaling.
I can still so slightly less power on the road climb when on my mtb, but its so marginal i put it down to a less stable BB and also the variance between the PM’s.
Hey Tim, you’re probably right that I’d have to do the same climb a few times on each bike to really know what’s going on and how much the difference is. I also agree that trail power will be less due to the factors you mentioned, but in the OP it was a gravel climb after a short warmup on pavement so I expected to be able to hit something near my road FTP for ~45 minutes.
I raced on my MTB on Saturday (and Sunday) and the numbers were much more in line with what I’d expect. I did 273w for 40 minutes up the first climb at an average HR of 152, and it felt like an 8 out of 10 effort, not a max effort:
Maybe I just had a bad day when I did that MTB climb in my OP, or maybe I just got more accustomed to the position on my MTB vs road bike since I did two big days on it last weekend and then aso did every ride during my taper on the MTB.
Those difference are pretty significant but not wildly so. If you really want to separate the difference out I’d take both bike to a fairly steady climb and do like a 20min threshold or so effort with both bikes and then compare.
Fit. Not just static bike fit but MTB at 8+% incline vs road bike at flat or shallower gradients
Inertia. Pedaling forces are pretty different at 6mph than at 15mph than at 22mph
Cooling. Faster road riding or even a trainer with really good fans is going to likely have better evaporative cooling than a slow mtb climb. I even see that your trainer ride was at like 45deg. and even though the MTB ride wasn’t super hot (~63deg) the VA forest humidity will play a major role.
Surface. Well before the surface is so chunky that you have to stop pedaling you are still being bumped around, doing micro-accelerations over ruts, rocks, etc. Even just having to navigate different lines to avoid chunkier places
Balance. You will be using much more core and upper body on a slow MTB climb just balancing at low speeds.
So before you go crazy looking for differences, compare the power output apples to apples on the same terrain. Then narrow down which of the above (or others) are the reason for the difference.
I think surface, even for gravel on the MTB, really can’t be ignored. I have an old MTB set up on my trainer, so my position is pretty similar inside and out (I ride MTB ~95% of the time), and even when riding on gravel roads, my RPE is a little higher outside than inside for the same power. The stabilization and micro-accelerations add up.
Well done John! Given your race was 110klms id say you did well to pace yourself by not going to hard up the hill. Even an 8/10 might have been too hot. Funnily enough i did a 98klm race on Saturday here in Australia and totally over cooked it on the first hill, hitting a new max heart rate 30 mins into a 4.5 hr race. Needless to say there were some dark patches in there and barely held it together.
What was your average power for the last climb at 90klms?
A new max HR 30 minutes into a 4.5 hour race?! Uh oh! Did you manage to recover and ride strong for the rest of the race?
On the last climb of my race I did 263w for 46 minutes at probably an 8.5/10 RPE. Average HR was 150, a little higher than it would normally be, but I probably had a little HR drift and dehydration at that point.