Koyote

Yeah, this is one of those threads that only gets started in winter, when a person (in my neck of the woods, anyway) can't ride outside. And starts going stir crazy.

In another thread, someone mentioned this Bike Calculator site, and I remember fooling around with that many years ago. The general conclusion is that, even in big climbs, weight (bike+rider) is not as significant as we often believe it to be. Yeah, lighter is faster, but not by much, at least according to the calculator.

But still, I am considerably slower on my gravel bike, which is about six pounds heavier than my regular road bike. This gets me to wondering: how much of the difference is weight, how much is rolling resistance (700x40 tires with significant tread vs 25mm slicks), and how much is aero drag from the gravel bike's somewhat more relaxed position. Does anyone know of evidence that looks at these factors in relation to one another?

No real question here, just wondering about people's thoughts.

Retro Grouch 

It's speed dependent.

At low speeds weight and all the various rolling drag factors hold you back. Once you pass around 15 MPH, aero trumps everything else combined. The faster you go, the more relatively significant aero drag becomes.

asgelle

Speak for yourself. Many people have a very good grasp of the significance of weight (and rolling resistance and drag) in various scenarios.
This calculator lets you see the contributions of the various sources to the ******ing force https://analyticcycling.com/ForcesSource_Page.html

Koyote

Quote:

Originally Posted by asgelle

Speak for yourself. Many people have a very good grasp of the significance of weight (and rolling resistance and drag) in various scenarios.

That seems a harsh response, given that I wasn't speaking for you. Maybe you should drink a beer and relax. And thanks for the link...I'll have a look.

ldmataya 

My friends and I have had this debate for years and our generally accepted conclusion - as a way of going faster, aero trumps weight when other factors are mostly the same, and you aren't riding a steep uphill time trial.

So, when posed as follows; given that my friends and I ride pretty similar road bikes, we are of similar ability, and we all want to go faster for the same effort, what should we spend $500 to improve? Our answer, after realizing that the first thing you should do is make sure you have tight fitting clothing, is to not spend it saving 450 grams (1 pound). We prefer an aero helmet, or upgrade to more aero wheels.

If I were to guess about the OP's speed difference, I would predict the factors are 1. rolling resistance and 2. what they wear in on the gravel bike vs the road bike.

Doge

On gravel the road itself provides resistance. Of course sand is worse. More weight in sand or gravel displaces more sand and gravel.
Rolling resistance can matter esp on big low inflated tires.

fietsbob

How do you plan on measuring them?


It's a worthy Post graduate study, with physics , engineering .. materials science

Read this book? Bicycle Science ..





...

HTupolev

Relative significance of different factors depends on lots of particulars.

When someone's gravel bike is significantly slower than their road bike, the first place to look is the tires. Tread pattern can make a difference, but the other big thing is protection: gravel tires often have very tough sidewalls, and some people also use tires with tough puncture-protection belts beneath the tread. These can dramatically increase rolling resistance, sometimes enough to cost a mph or two.
Tires being fat does increase the aerodynamic drag of the bike a little, but it doesn't necessarily mean more rolling resistance, and on the whole it's much less impactful than most people think. Part of the reason I've gone super wide on my gravel bike (53mm) is so that I can get away with using a high-performance tire, with less worry of sidewall cuts or pinches... it's a lot faster on the road than most of the 30-40mm tires that my friends use on their gravel bikes.

Aero effects of posture depend on both the bike and on you. There are a lot of different ways that people build up and fit gravel bikes, ranging from very reclined to road race. And it's hard to guess differences in posture without seeing the rider on the bike... some riders will move up a millimeter when you set their bars a millimeter higher, some riders just go lower on the bars to stay in the same posture.
Aero differences between postures can be astronomical when the postures are very different... beach cruiser versus triathlon postures can cost lots and lots of mph. But plenty of people cruise on the road on their gravel bikes in roughly the same position that the do on their road bikes, too.

Weight? A 6lb gain can cost a couple percent in speed on a sustained climb. The relative performance impact is bigger for lighter riders than heavier ones.

Gconan

may I ask what tires are you using? Thanks.

Retro Grouch 

Here's another thought:

A light bike will accelerate a little bit better. Since every bike ride starts at 0 MPH, as you accelerate to 15 MPH or whatever, a light bike will always feel faster.

On the other hand, once you surpass 15 MPH, aero drag becomes the dominate factor that's holding you back. It's not going to take very long for a more aero bike to gain back the time you saved in accelerating from 0 to 15 MPH. The more aero bike, while feeling less lively, is going to be faster over the total ride.

HTupolev

Most of the time I run Rat Trap Pass Extralight on my gravel bike. It's nominally a 2.3" tire according to the sidewall, but that was a tire mold mistake on Panaracer's part: the inflated width is generally in the ballpark of 2.1". The price is very high, which I justify to myself by thinking about the slow wear rate of wide slicks; in terms of $-per-mile, they're cheaper than some of the skinny tires I use on my road bikes. Compass tires need a lot of care to be set up tubeless*, but because I've had no trouble, I'm still using tubes. The high width and lack of damping results in extremely loud road hum, so you might dislike these sorts of tires if you prefer your wheels silent (or you might enjoy them a lot if you like a good roar).

*The sidewalls are pretty porous and need a careful application of Orange Seal to become airtight, and to avoid tubeless blowout risk, you need to be veeerrrrry careful to make sure that the bead sets up on the rim shoulder properly.

Gconan

Thank you HTupolev! The reason I asked is because I was thinking about purchasing the Snoqualmie Pass for better rolling resistance vs the Clement MSO's I have on pavement. Now I guess I have to decide tubless vs tubes in them. Thanks for the information!

Witterings

There's lots of talk generally about weight affecting speed especially up hills .... My friends who are lighter by a couple of stone are certainly quicker than me on climbs (although both their bikes have better climbing gears as well) but on downhills I seem to effortlessly leave them standing ... even if we're all free wheeling together it's me that's on the brakes to stop me going ahead.

This has made me question if the overall benefit from being lighter isn't quite as much as people perceive and if there is any benefit to weight doing down hills as they're probably more aero than me which again should theoretically be in their favour ..... especially when do some quite long stretches hitting 35 mph
Also isn't the aero affect meant to be exponential beyond 15mph so the impact should be greater especially at the higher speeds but doesn't seem to be.

Interested if anybody else has seemingly experienced similar and if there's any "science" behind it or of it's just something I have incorrectly perceived.

HTupolev

Yes. Weight is gravitational force pulling you downward. If you have more weight relative to your aerodynamic profile, you'll have an easier time sustaining higher speeds downhill.

But unless you're participating in a downhill-only time trial, this is rarely overall favorable to adding weight to a bike.
The extent to which weight losses improve climbing performance is considerably lower than that to which weight gains increase descending speed. Decrease a rider's weight on a steep climb by 10%, and they'll climb nearly 10% faster; increase their weight by 10% on a steep descent, and their coasting terminal velocity will increase by less than 5%.
Furthermore, the draft matters a lot more on descents than ascents. If you're someone who rolls downhill slowly, you can get sucked downhill quickly in the draft of a fast descender. But when you're a poor climber, you can't get towed to the top by staying in the draft of a good climber.

It's polynomial, not exponential. The force of aerodynamic drag increases with the square of speed, and the power required to overcome it increases with the cube of speed.

Nothing special happens at 15mph.

av1

Aero over weight and rr! I made over 20 short rides on a 14km mixed road, some flat, some up and down, on two of my bikes on equal terms with each bike. They have the same drive train , speeds , 28mm road slicks, same wheels. The difference was one is a couple cm more aero but the weight is 2,5lb more. In the end the results showed me that the more aero was faster in both aver. speed and max speed despite the more weight and beeing with the higher rolling resistance rated durano tires than the better rr rated grand prix tires on the other bike. Both durano and grand prix are foldable and weight around 260g per tire. No idea how it will go in longer climbing only hill maybe the lighter bike will shine more.

srode1

Its all very important and it's a matter of compromises. I can tell you even at 12 to 13 miles / hour, on our regular morning group ride the guy pulling on the climb is not the guy who is going to finish first most of the time - everyone is drafting and the sprint to the finish of the climb is won by someone that was behind regardless of who is pulling. I can tell you for sure, when I dropped 10lbs I went from not even being able to see the sprint to the finish most of the time on this climb because I was too far back, to winning it on occasion. You should have seen the look on peoples face when I passed them at the top of the hill - it was like "what the heck are you doing up here!". And that was with the same power output on the climb, nothing changed except weight and I was able to hold onto the pack drafting until the sprint. Now that was on a road bike, but the laws of physics apply equally to gravel.

The aerodynamics of a gravel bike frame vs a road bike is probably the smallest factor though, I've won 30mph sprints to city limit signs on my gravel bike vs road bikes when I had road tires.

KraneXL

Weight/thread

canklecat

Weight and tires matter. Just switching from my 30 lb hybrid with 700x32 or 700x42 tires (same bike, different terrain/seasons) to my 24 lb old school steel road bike with 700x23 slicks is good for a 2 mph speed increase without any extra effort. The riding position is less a factor since both bikes have the handlebar at or slightly below saddle height. The weight and tires matter. Especially if any climbs are involved. On mostly flat routes and downhills with a tailwind I've occasionally matched my best times even riding my heavier hybrid.

Leaving my water bottle behind helps, both weight and aero. On a nearby training route if I plan to ride only an hour I might leave the water bottle home in cool and cold weather. Helps on some climbs into headwinds. Ditto, stuff on the handlebars -- camera, lights. If it's daytime I'll leave off the lights. If I'm feeling lucky I'll leave off the camera, although the one ride I didn't take my camera last year was the one time I was hit by a car. But there is significant drag from all that stuff on the handlebar, especially on windy days (unless it's all tailwind -- no such luck around here with mostly swirling winds redirected by the terrain).

Aero kit -- jersey, shorts, shoes/shoe covers, helmet -- probably matters as much as most other factors, even compared with an aero bike. An aero bike and wheels may not help much in quartering or crosswinds, while aero kit will help in pretty much any conditions. Surprised me but that's the conclusion of a couple of articles based on wind tunnel tests. I shouldn't be surprised though. I don't have a skin suit but looking at my Strava data I'm usually faster in warm weather when I'm wearing minimal snug fitting jerseys. My winter jerseys and jackets are all casual fit and I can feel the wind catching around my collar and sleeves, unless it's cold enough to zip up tightly.

BTW, there's an app that can access our Strava data to factor in wind from the publicly accessible weather archives, dating back to any of our old Strava logs. I'm not sure whether the app is being upgraded or maintained. Last time I checked it had miscalculated some wind directions and gave me credit for riding into a headwind when actually it was a significant tailwind. But when the app works correctly it scores weighted data which is interesting for comparing my fastest time with a tailwind assist against a much younger, stronger local rider who had little or no tailwind or even a headwind. Puts things into perspective. We may have similar times, or mine might be a little faster, but it doesn't mean I'm faster in identical conditions. Just means I'm a good tailwind mooch.

BlazingPedals

Weight is important, but the relationship between weight and aero isn't always so clear. For instance, aero wheels have been shown to help climbing even when they're heavier, and at much lower speeds than you might imagine. But where's the break even point? I'm not sure a calculator can find that.

KraneXL

The reason why weight is so important is that it is constant, and therefore, always remains a factor. Rolling resistance on the other hand, delivers its greatest impact at the beginning of movement as you overcome the inertia from being stationary. Aero however, becomes significant only as you approach higher speeds (~20mph+). Below that its impact is minimal.

Rides4Beer

I've noticed this as well. I'm 185lbs and ride an aero bike, even sitting fully upright I am always gaining on other riders in the group going downhill, if I tuck it's way worse. I'm almost always on the brakes because of people in front of me, a lot of times I'll be freewheeling and they're still pedaling and I'm gaining.

wphamilton

Yes you gain from being heavier, going downhill. However, you don't gain enough time downhill to make up for the lost time uphill, given equal power. Just the way the math works out.

Witterings

Agggh ... so it's not me imagining it then ... I so felt it was the case and it really is quite noticeable.

Now I know it, the other guys I go out riding with don't and I've just bought a gravel bike which they're thinking is going to be way faster than their 29er's and is already putting them under a bit of pressure on the flat road sections .... armed with this extra knowledge though I'll turn up the heat on any downhill sections and can have some real fun

Wonder how long before they're researching gravel bikes

asgelle

The Martin model, https://www.recumbents.com/wisil/Mart...%20cycling.pdf on which the calculators are based lets you do this either analytically or iteratively through the calculators. For example, using analytic cycling.com and starting with the defaults: 250w, 75kg bike+rider, 0.004 Crr, 3% grade, speed = 16.69 mph., adding 0.5 kg slows the rider down to 16.62 (0.67%). To get back to 16.69, CdA has to drop to 0.49 (2%). For 7% grade, the same 0.5 kg increase requires a drop in CdA to 0.43 (14%).

asgelle

At 10 mph on a 4% grade, drag is 8% of the total force on the rider (17% at 15 mph). You might consider that minimal, I don't.

Rolling resistance and force of gravity are both independent of speed and proportional to weight. Power to overcome each is linear with speed. Therefore, their significance is exactly the same for the same speed.