6

I'm surprised this hasn't been asked before, but as a general rule, is a single rear pannier or a backpack (of similar sizes) more aerodynamic on a hybrid or MTB? Even on flat bars drag is much more than (road) rolling resistance for sensible speeds, and small changes could be significant.

I'm specifically considering a backpack narrower than my shoulders, and except on fast descents or into a headwind I have a fairly upright posture. I also have quite wide bars. My general preference is for a pannier but with a couple of longish (50--70 km / 30--45 mi) rides coming up I'd like to optimise for effort.

3
  • 3
    What makes you think the drag is the limiting issue? You don't ride in the same position with or without a backpack
    – Batman
    Feb 7 '17 at 16:47
  • 1
    @Batman I've seen graphs showing that on a road bike drag dominates over about 20km/h. I've got more drag than that, and over 70km average about 22.5km/h (and can sustain 30km/h on the flat for 10 minutes if I want to, which I wouldn't on a long ride). It's true that riding position will change, but for a small backpack probably not much (it certainly didn't feel like much when I tried it recently). An ideal answer would take that into account (though that's yet another bit of personal variation)
    – Chris H
    Feb 7 '17 at 16:53
  • I gong to do some wind tunnel testing on that. I'll post the results. Feb 8 '17 at 4:10
13

A back pack is faster than a single pannier. This exact scenario was tested in the Specialized "Win Tunnel" in their "The Brew Master" episode. They suggested the time savings was 2 minutes over 40 km, which isn't that much for casual cycling considering how uncomfortable a heavy back pack can be! Also see their "Areo at Slow Speeds" to understand how the results can be generalized to a wide array of average speeds.

Extrapolating to the "real world"

Answering @JPMay comment request

In the "Brew Master" video they don't really quantify things in the most helpful way (if you are not racing). In terms of percentages, the least aero option they tested is only about 2% slower than the fastest (assuming you go between 20-30 km/h). – JP May

The time difference would likely be more useful as a percentage so it could be applied to different speeds. Drag (what you have to overcome by pedaling) is directly proportional to the drag coefficient:

drag = CdA * p * v^2

where drag (and the power required to ride a speed) goes up by the square of velocity (v), but scales linearly by the drag coefficient (CdA). Whether you ride with a back pack or panniers impacts the CdA value only. For example, reducing your CdA by 5% will reduce your required effort by 5% whether you are riding at 20 kph or 40 kph. This is why aerodynamics still matter at slower speeds (its just harder to "feel").

What we don't know is how fast the hypothetical 40 km was ridden in the wind tunnel testing. Assuming a race pace of 40kph, this would work out to about 3.33% longer for a single pannier vs a backpack.

Therefore, to understand the impact on real world conditions take your ride duration with a backpack and multiple the time by the following to determine the time it would take to ride with pannier(s):

  1. one small rear pannier multiply the time by 1.0333 (3.33% longer)
  2. two small rear panniers multiply the time by 1.0667 (6.67% longer)
12
  • 1
    That's neat -- I can't watch the videos at the moment but will do so later. Having the saving quantified helps -- not worth it for the 50km ride with a big hill at the midpoint when I'd rather not get so hot, but worth it for the longer ride when I don't need to carry so much.
    – Chris H
    Feb 7 '17 at 16:57
  • @ChrisH - What is not in the testing are bike bags (e.g., frame bags). These can take the load off your back and onto the bike while still keep your frontal area down!
    – Rider_X
    Feb 7 '17 at 17:32
  • Good point. My GT frame isn't ideal for full size frame bags, and I already have a mini one for on-the-road tools and first aid kit, so I'm unlikely to get one (especially not in the next couple of days before the shorter of the rides that made me get round to asking the question). There's also the question of where to put water. Trunk bags are also interesting for small loads, but not on my current rack.
    – Chris H
    Feb 7 '17 at 17:52
  • 1
    @ChrisH - Yes that is correct. Anything in behind effects the flow of air coming off your body which adds drag. You want to be tear dropped shaped with larger displacement up front rather than in behind. This is why I am planning on switching to a roll bag off the front handle bars for commuting.
    – Rider_X
    Feb 8 '17 at 18:35
  • 1
    I like a front bag too, but haven't found a good way to get it to work with a cheap but bright light. Your links have inspired me to look again at options for the top of my rack
    – Chris H
    Feb 8 '17 at 19:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.