I recently completed a training plan leading into a XCO MTB race and overall I was really pleased with my race result. I’ve been doing all my intense Trainer Road workouts on an indoor smart trainer with my road bike on it and then using some of my longer endurance rides to do on-course practice with my MTB. Even so I did feel a little like I was not putting down the power on the MTB like I do on the indoor trainer. Obviously I know there are huge differences in riding a MTB out door and a indoor trainer but I decided that I wanted to try a few sessions of putting my MTB on the indoor trainer instead of the road bike. The first session was an endurance session and I was pretty tired so I didn’t really think too much about that it felt much harder than it should have felt. The next session I tried was VO2 max session and one that I have completed before at the same power numbers with the road bike hooked up to the smart trainer and I came no where close to being able to complete it with my MTB hooked up to the smart trainer. I wasn’t feeling great that day so I kind of just chalked it up to a bad day. Today I decided to do some testing with swapping my MTB and my Road on and off the trainer and recording power numbers both from my power pedals and my smart trainer. When I switched the bikes and pedals I made sure to calibrate everything (both pedals and trainer). I did 4, 5 minutes sessions with the smart trainer in ERG mode set to 160 watts. The first two I did with the MTB hooked up to the smart trainer and the over the two 5 minutes sessions the pedals were recording an average of 185 watts at the pedals for the 160 watts at the smart trainer. For the second two when I switched the pedals and trainer over to my road bike the pedals and the smart trainer were matching exactly at 160 watts. I was so shocked I hooked the MTB back up with the power pedals and did another 5 minutes session and it was the same thing, avg 25 watts higher power on the pedals to produce 160 watts at the trainer. Am I crazy or is this about 25 watts of drive train loss going on with my MTB? Is that even possible? Is there anything else I could be missing here before I starting replacing things on my MTB like my bottom bracket, pulley wheels, and chain?
Do you have the same length cranks on both bikes? And if not, did you update settings when you swapped the pedals between the two bikes?
They are different crank lengths, 165 on the MTB and 172.5 on the road bike. I did make sure to update the crank length setting each time I swapped the pedals. A few other notes about my equipment. I’m using a Jetblack Victory smart trainer with a Zwift cog. My MTB has SRAM XO Transmission with a flat top T-Type chain. My bottom bracket is a standard SRAM DUB BSA, its not even a year old but it does sound a little loud. My chain is about 2.5 years old.
Do you have a rear shock? I would think that there would be some power loss there.
Yes, but I had it locked out when on the Trainer.
You could deep clean and good lube and retest, that’d clinch it
Joe
What is the difference in chain ring size? The smaller chainring on the MTB combined with the single speed of the Zwift cog reduces inertia and makes it feel harder to push the same power.
Are you running the same kind of BB on your road bike and MTB?
Thanks for the replies everyone. Ill try a good cleaning and retest. Chain ring on MTB is 32T. On the road bike while on the trainer the chain sits on the 34T chain ring on the front. Different BBs on both bikes. SRAM DUB BSA on MTB and I’m not sure on the road bike, its a Shimano drive train.
For anyone that’s curious here are some screenshots of the comparisons I made.
MTB 1 - Calibrated pedals and set to MTB crank length
Power at Smart Trainer:
Power at Pedals:
MTB 2 - Recalibrated Pedals and Smart Trainer
Power at Smart Trainer
Power at Pedals:
Road 1 and 2 - Recalibrated Pedals and Smart Trainer, set pedals to road crank length
Power at Smart Trainer:
Power at Pedals:
You might take the chain off the ring and spin the crank. It should rotate pretty freely. Then move the chain off each pulley on the RD and ensure each is rotating freely. You can do this without taking the chain off. Those and the chain are the only parts of the drivetrain involved when on the trainer. When you change the axle adapters on the trainer switching between the two bikes is there any possibility of something going wrong there?
FWIW: I find that pulleys can start to drag and putting a drop or two of light lube on each side where the bearing is can help a lot. I use Triflow for that and to lube RD pivots.
I took some pics and videos of the state of my MTB drive train with how everything is spinning right now pre deep clean. Ill post them here shortly. As far as changing the axle adapters, that is a good point. My road bike uses the quick release vs the through axle on my MTB but I’m pretty confident I’m changing the adapters correctly.
Here are some short videos demonstrating the current state of my MTB drivetrain
25W drive train loss seems quite a lot, but I find it equally as weird that you don’t seem to have any drive train loss at all on the road bike. (Or maybe there is, but by chance it matches the offset between the two power meters?)
The narrow wide chainring will be a bit, the clutch in the derailleur also adds more chain tension than would be without it (slower), those pulleys look fine but that bottom bracket sounds terrible. Though I’d doubt it all adds up to 25W, but maybe something close…
I’m not sure if you said which drivetrain is on the road bike, but the flat top chains are less efficient generally. So might have a couple of watts there depending on what it’s being compared against. Doubly so if it’s factory grease and/or dirty. I’ll also second that your bottom bracket sounds pretty bad. I know you said your MTB is locked out but I can’t help but feel maybe some of the power transfer is going into the suspension. Even locked out my SIDluxe moves a little.
Those vids all look good. BB seems kinda slow and should be addressed, but I think 24W would be much more noticeable.
Thanks everyone for the great feedback. I’m going to go ahead and replace the BB and chain and clean and regrease everything. Ill test again and report back when its all put back together.
This post motivated me to put my nice bikes with powermeters on the trainer. I always had a 15 year old trek madone on there without power meter. First up was the 2024 canyon aeroad with sram quarq power meter and force drive train with flat top chain on the trainer (tacx neo 2t). Chain was clean and waxed. The crank power meter read 10-12w higher consistently doing “james”, essentially a 1.5h high zone 2 low tempo ride with no power variation. Average power was 226 vs 235w. I figured this was reasonable. I’ll throw my supercal 9.9 with xxsl power meter on the turbo tomorrow and do the same thing.
A little shocked this hasn’t been brought up, but highly recommend using power match if you have an on-bike power sensor. I have seen pretty wild discrepancies across bikes when using the trainer power meter. The trainer power meter is a borderline random number generator depending on a multitude of factors. Use the pedal as your power source and keep the bikes clean - should solve all your issues haha
care to elaborate?
25W delta seems too big to be explained by drivetrain losses on one bike versus drivetrain losses on another bike. So I think there’s something else going on here.
I’m not convinced about recalibrating the trainer between your two tests. My guess is that this is introducing another variable. Since you are using the trainer power as a reference to compare the two bikes, I suggest not recalibrating it - it’s not like anything is changing from the trainer perspective when there’s a different bike mounted to it. It is calibrating to temperature or spindown resistance which should not be changing significantly.
Also, what powermeter pedals are you using? Are they sensitive to the installation torque when moving to another set of cranks?
Alright I’ve got some results and things are much improved. As you can see the delta between the pedals and the smart trainer with my MTB on the smart trainer is pretty much negligible now. Here is what I did:
- I replaced the bottom bracket with a new standard SRAM DUB BSA bottom bracket.
- I replaced the chain with a new SRAM Eagle Transmission GX flat top chain
- I cleaned the new chain thoroughly to remove the factory coating and then used Silca Super Secret Chain lube on it
- I removed the Eagle Transmission derailleur and took apart the cage and cleaned and oiled the jockey wheels.
- I reinstalled everything with fresh grease and to factory torque and specification
Data from Pedals
Data from Smart Trainer
Regardless of what the numbers show, which looks really good now, I could immediately tell from a rate of perceived effort perspective that levels of effort felt as I would expect.