Gravel Bike vs. Road Bike Watt Comparison

Mar 9, 20 14:53

Post #1 of 16 (4586 views)

Hey All,

I've been riding indoors my fair share this winter and got outdoors for the first time last week. My only bike at this point is a Parlee Chebacco, and while I ride gravel often (I'm located in Northwest Mass), I ride on the road with friends more. Last week I went out for some sweet spot reps on a local climb and came home after 2 hours and 15 minutes with a normalized power of 281. My threshold is around 332 per a Trainerroad ramp test, but I've never held a normalized power that high for that long, and while it wasn't easy, it wasn't that hard either. I felt fine and did a short run off the bike. I was hopeful that my power meter was accurate, but a bit skeptical.

Today I went out for a 3 hour ride (40 miles with 4000 ft of climbing) and my average watts were 225 (247 NP) and I rode with a friend's whose were 204 average and 225 NP, he rides a Felt AR3 with Zipp 404's, I'm on my Parlee Chebacco with aluminum mavic wheels and 40mm schwalbe gravel tires. We both weigh about 77 kg's.

My question is, does the difference in bikes make up for a roughly 20 watt differential or is my power meter reading wrong?

My bike was professionally fit, but beyond that, it's a gravel bike, so I have no idea what my cda is.

I use a stages single-sided dura ace crank arm.

Re: Gravel Bike vs. Road Bike Watt Comparison [tylerh] [ In reply to ]

Mar 9, 20 15:03

Post #2 of 16 (4575 views)

Way too many variables. The fact that power was read by two different devices can be at least half of the 20 watt difference, sometimes the entire 20 watts. Sure, bike weight and drag will account for some of it.

Re: Gravel Bike vs. Road Bike Watt Comparison [tylerh] [ In reply to ]

Mar 9, 20 15:14

Post #3 of 16 (4567 views)

Seems reasonable.

Unscientifically, when i do a group ride on my gravel bike, i typically see avg and NP is about 20 watts higher than when i ride the same route with the same group on my road bike. Sounds to me like your indoor training paid off this winter. Nice work.

It's a good idea to calibrate (zero offset) your PM every ride to ensure consistent results. Even though most modern PMs have temp compensation, the drastic temp shifts this time of year still seem to impact my numbers if i don't do a zero offset each ride.

Re: Gravel Bike vs. Road Bike Watt Comparison [tylerh] [ In reply to ]

Mar 9, 20 16:57

Post #4 of 16 (4492 views)

Tires could easily be 20 watts or more.

Re: Gravel Bike vs. Road Bike Watt Comparison [Brap7] [ In reply to ]

Mar 10, 20 4:32

Post #5 of 16 (4338 views)

Tires don’t change wattage

Re: Gravel Bike vs. Road Bike Watt Comparison [mvenneta] [ In reply to ]

Mar 10, 20 4:53

Post #6 of 16 (4315 views)

mvenneta wrote:

Tires don’t change wattage

They do change the performance achieved for the wattage which is the point under discussion.

Re: Gravel Bike vs. Road Bike Watt Comparison [tylerh] [ In reply to ]

Mar 10, 20 4:58

Post #7 of 16 (4306 views)

tylerh wrote:

....I use a stages single-sided dura ace crank arm.

Your last line makes everything else potentially irrelevant.

A single sided meter does not measure total power so unless you somehow know that it always represents your total power by some other means, you don't know with sufficient accuracy what your real power figure was. if the other rider was also using a single sided PM, the problem is compounded.

Even putting that fundamental issue to one side, there are still too many variables: tyres, pressures, drive train losses, choice of line, position on bike, clothing, drafting, hairstyles, etc, etc, etc....

Re: Gravel Bike vs. Road Bike Watt Comparison [tylerh] [ In reply to ]

Mar 10, 20 6:18

Post #8 of 16 (4258 views)

tylerh wrote:

I use a stages single-sided dura ace crank arm.

I've measured a difference at identical trainer speed between my two different Stages meters of up to 30w.

Being 77kg whereas I am 70kg, no idea on the gross or overall power measure. But, for only 13mph and 100ft per mile........it seems slow for that much power no matter the setup. I average around 13.5 to 14 riding my cross bike on gravel at around 80ft per mile elevation. I can only assume I'd be a couple mph faster on tarmac.

I'd say, borrow someone else's meter or smart trainer and do some comparisons if it bothers you what the meter reads.

Otherwise, if it reads consistent day to day..........doesn't matter THAT much. Because you can train by that.

Re: Gravel Bike vs. Road Bike Watt Comparison [tylerh] [ In reply to ]

Mar 10, 20 6:37

Post #9 of 16 (4234 views)

If you are in the same position and if you ride at the same effort level in the same environmental conditions, there should be zero power difference between the two bikes.

My guess is that your effort level varies, and that your power meters are inaccurate (many of them are).

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Re: Gravel Bike vs. Road Bike Watt Comparison [tylerh] [ In reply to ]

Mar 10, 20 6:45

Post #10 of 16 (4220 views)

The big unknown is drafting/position on the road. Drafting will save you at least 30% so everything else being equal the 10% difference in power is well within the margin of uncertainty. Now if you hammered the front the whole time and he sucked your wheel then thing are odd.

The difference between setups will be good for more than 20 watts. I wouldn't be surprised if its 20+ watts per tires when comparing 40mm gravel tires to a 25mm road tire on the road. But again if he was taking more of the wind then he would have needed more watts.

More generally this is why power meters are not very useful in pro racing. They can help domestiques pace a chase but once drafting comes into play you just ride with the group rather than by power.

Re: Gravel Bike vs. Road Bike Watt Comparison [TrierinKC] [ In reply to ]

Mar 10, 20 6:49

Post #11 of 16 (4217 views)

I totally acknowledge that there are way too many variables. I guess what I'm asking is just if it seems reasonable, or if I should immediately suspect that my power meter is way off!

Re: Gravel Bike vs. Road Bike Watt Comparison [mvenneta] [ In reply to ]

Mar 10, 20 6:50

Post #12 of 16 (4213 views)

mvenneta wrote:

Tires don’t change wattage

Are you for real? Rolling resistance of tires certainly affect the wattage it takes to go a certain speed. If you don't think so then take your mountain bike with knobby tires on your next scheduled fast roadie group ride. Let me know how that goes for you.

Re: Gravel Bike vs. Road Bike Watt Comparison [tylerh] [ In reply to ]

Mar 10, 20 7:13

Post #13 of 16 (4178 views)

tylerh wrote:

I totally acknowledge that there are way too many variables. I guess what I'm asking is just if it seems reasonable, or if I should immediately suspect that my power meter is way off!

You should suspect your "power meter is way off" simply because it's a single sided device.
A single sided device isn't really fit for the purpose of doing that sort of comparison. This is especially true if you've never gathered data as to your own typical power imbalance. The device only measures the power output from one leg and it then assumes that cyclists are symmetrical. That's a big assumption. Many cyclists have very significant differences between left and right and they're not stable either. The balance fluctuates with power, cadence, fatigue and position.
If you don't know what your power balance tends to be like then you're working in the dark. If you do have a reasonable idea, the values are still just estimates albeit you can use them more accurately. A single sided "power meter" should really be considered an power indicator rather than a power meter. It's not measuring your total power and it's not reasonable to assume real accuracy.

Aside from that, there are enough variables in this that you COULD have accurate power data, but I would not be expecting high accuracy regardless. the comparison is beside the point.

Re: Gravel Bike vs. Road Bike Watt Comparison [stonerider] [ In reply to ]

Mar 10, 20 8:25

Post #14 of 16 (4118 views)

I didn’t see the question as “could my tires make me go slower” (yes). Hundreds of variables could make you go slower than your friend ... just body position could do it

I read the question as why is my power different between these bikes

Last edited by: mvenneta: Mar 10, 20 8:26

Re: Gravel Bike vs. Road Bike Watt Comparison [tylerh] [ In reply to ]

Mar 10, 20 15:10

Post #15 of 16 (4004 views)

I'm going to respectfully disagree with all the folks that are saying a single-side PM is not accurate enough to tell you anything. I've used multiple platforms and my Stages single side is accurate enough to let me hit 5 minute repeats exactly on the second time after time over the same course. Now, when it is reading 180 watts could it actually be 184 or 176? Sure, but the number is not important as long as the reading is consistent.

I suspect the issues is tires, aero and drafting as others have suggested. The difference between a good road tire (say a 5000gp tl) and a good gravel tire could easily be as much as 10 watts per tire. I imagine if you could swap wheels with your buddy you might see a lot different result.

Re: Gravel Bike vs. Road Bike Watt Comparison [tylerh] [ In reply to ]

Mar 10, 20 15:12

Post #16 of 16 (4002 views)

tylerh wrote:

My question is, does the difference in bikes make up for a roughly 20 watt differential or is my power meter reading wrong?

Question seems pretty clear:
Does the difference in his (gravel) set up as compared the friend he was riding with (road set up) plausibly explain the difference in workload to ride together?

The answer is yes; the differences in a road and gravel set-up can easily explain the difference in power output per speed between a road and gravel set up, and therefore the difference between your workload and your friend's while riding together.

Assuming everything related to the rider being equal (if you were riding both bikes)
A switch from Gp4000 road tires to Schwalbe Gravel tires (guestimate based on Tom A's inference) could be ~17 watts at 20mph.
Felt AR with 404s vs. Gravel bike with box section wheels contributes even more difference.

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