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I have two bikes. One is a $7k+ Cervelo S-Series, and the other is a <$1K Trek Domane AL 2. With the same perceived effort applied, my Cervelo goes much faster. I am able to routinely hit above 20mph mark with ease, whereas with the Trek, I struggle even to get up to 17/18mph. The Trek is noticeably more difficult to ride. I feel like I need to apply so much more effot to the pedals in order to get the bike to move. I would like to upgrade the Trek so that for any given wattage, it goes faster.

My question is, what exactly is making the Trek so much slower than the Cervelo? A few things come to mind.

  1. Aerodynamics
  2. Weight
  3. Rolling resistance.

The Cervelo is more aerodynamic, but it doesn't seem like the resistance I feel is because of aerodynamics.

The Cervelo is lighter, but only by 3.5 lbs. Again, doesn't seem to be the biggest culprit.

I have Continentall GP 5000 tires on the Cervelo, and just the stock basic Bontrager tires on the Trek, so that could be part of it?

For me, it seems like the Shimano Claris groupset is the biggest culprit. It feels very clunky. Would upgrading to a Shimano 105 be something to consider for this bike?

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    Are both bikes are set up for the same riding position? Handlebar stem length, seat position etc. Your body is the source of most of the wind resistance.
    – David D
    Nov 6, 2020 at 19:07
  • The weight of the rider is by far the most important parameter and the cheapest to change!
    – Carel
    Nov 7, 2020 at 10:26
  • @Carel: It might be hard to believe but some of us are already at the lower end of normal weight or even in the underweight category.
    – Michael
    Nov 8, 2020 at 13:10
  • A few months late here, but no one's explicitly mentioned wheels as the difference. Does that Cervelo happen to have 50mm-or-so deep CF aero wheels while the Trek has lower-end AL almost-box rims? Aerodynamic wheels probably save 20 or so watts at 20 mph over shallow-rim wheels. Feb 20, 2021 at 2:14

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Most likely aerodynamics. Modern road bike vendors spend a lot of money improving the aerodynamics, especially for the racing top models. Mor for the so-called aerobikes, but even for the ultra=light bikes, up to the point the weight permits.

The weight is not too important on the flats and is less important than many think even in the hills as most of the weight is in the rider anyway.

Tyres will make some of the difference too. Obviously, top ricing tyres will be faster on smooth tarmac. I made a big improvement from the stock Schwalbe tyres to better G-Ones on my gravelbike. But aerodynamics of modern bikes cannot be beaten by tyres, the differences are usually smaller there.

The groupset may be more clumsy when shifting but when the gear is there, it does it job very well. If you want more gears and have the money (which you probably have if you have the Cervélo S), you can buy 105, it will be lighter, but do not expect to be any faster on flats. (I ride Claris regularly. Some would miss the gears, the shifting requires some power in your hands, but otherwise it does the job done well enough.)

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    BTW, I do not really understand how these numbers - "I ride 20 mph" "I ride 17 mph" are gotten. Either people live in very flat countries or I really have no idea how I could get the number. I can get average speeds for rides with 1% mean elevation gain, for 2% mean elevation gain, by I honestly do not know how fast I might ride in some hypothetical conditions the numbers often mentioned might correspond to. Also, one pushes harder on short time trials, other times one rides leisurely and looks around. Nov 6, 2020 at 17:44
  • I also doubt the numbers. At the very least one should use a power meter (preferably the same one) and the same test route and clothing. Several, alternating measurements in short succession and a circular route would help as well.
    – Michael
    Nov 8, 2020 at 13:18
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Weight doesn't make much difference unless you're climbing, but aerodynamics does, and so do tyres, but less so. Assuming both drivetrains are in good condition, they'll be similarly efficient, differing mainly in weight and possibly shifting speed. Fit is also important for your efficiency.

One bike is a more aerodynamic frame, but it's reasonably likely that it also puts you in a more aerodynamic position. This would mean a noticeably increased drag, but the thing about drag is you don't really feel it, you feel wind speed. A more aggressive position may help, as may clip on aero bars, but don't spoil the fit

Faster rolling tyres do help, but you have to make a significant improvement to notice it. There are rolling resistance figures for many tyres available online, with notes on how to apply them

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  • BTW I ride a heavy tourer with Sora components, and can maintain more than 20mph for over half an hour without too much trouble. It's my nicest bike. Riding your nice bike will give you a psychological boost that translates to speed.
    – Chris H
    Nov 6, 2020 at 17:36
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In a steady-state effort on level ground, there are three forms of resistance that you need to overcome. In order, these are:

  1. Aerodynamic drag
  2. Rolling resistance
  3. Mechanical losses

This page does a good job visualizing these.

Say, for example, that you need to produce 200 W to go 20 mph.

A good drivetrain in good tune can be 98% efficient, which is amazingly good. Even your Claris drivetrain is probably 95% efficient, unless it's dirty and the parts are worn out. There's not a lot of room for improvement there. This is soaking up maybe 5 W, maybe less.

Tires account for rolling resistance (weight figures in too, but the difference between you + heavy bike vs you + light bike is probably about 5%). Good tires might soak up 30 W. Bad tires can be considerably worse in terms of rolling resistance: a really fast tire might use only a quarter the power that a really low-quality tire would use.

That leaves aerodynamic drag soaking up the other 165 W. And what makes aerodynamic drag especially important is this: mechanical losses and rolling resistance scale linearly with speed, but aerodynamic drag scales in proportion to the third power of your speed. So going just a little faster requires a lot more energy; conversely, improving aerodynamics a little can have an outsize effect, and that effect gets magnified the faster you go.

Another thing to remember is that the biggest source of aerodynamic drag is probably you, not your bike. Two bikes could have about the same aerodynamic drag if you put them in a wind tunnel with no rider, but if one puts the rider in a more aerodynamic position, that system will have lower drag.

Also, FWIW, a bike can "feel faster" without really being faster. Deep-section aerodynamic rims are provably faster than lightweight box-section rims, even up moderately steep grades. But pulling away from a dead stop, you can feel the lighter wheel "spin up" more easily.

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    Technically aerodynamic drag scales with the square of speed; the power that is required to overcome it is what then scales with the cube of speed, since power = force * distance/ time,i.e. power = force * speed/
    – thelawnet
    Nov 6, 2020 at 19:47
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    I didn't consider the drivetrain. There, possible sources of drag are the pulley wheels, the bottom bracket, and the chain. I'm not aware of comparisons between Claris or Tiagra versus higher end stuff, but some people think the Dura Ace chain may be ~0.5-1W faster than 105/Ultegra. For pulley wheels, bushings may be 0.5W slower than steel ball bearings (DA has 2 bearings, Ultegra has 1 bearing and 1 bushing, 105 is bushings), with ceramic offering another incremental benefit (gotta be small, though). Not sure about the potential at the BB.
    – Weiwen Ng
    Nov 6, 2020 at 20:35
  • @thelawnet thanks, I'm always a little fuzzy about that point.
    – Adam Rice
    Nov 6, 2020 at 20:42
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For others' reference, the Cervelo S-Series is an aero road bike. The OP didn't state a model year, but the current generation S-series bikes all appear to have aerodynamic carbon wheels and fully internal cables. If the OP did indeed pay over US$7,000, then it seems like they have an S5, which may have some additional integration at the front end of the bike (handlebars, cabling, fork), which might improve the aerodynamics further.

In contrast, the Trek Domane is an endurance road bike. The Al2 is an entry level version.

In general, if we are talking about a bike's speed for a certain input power, aerodynamics should have the biggest impact, then rolling resistance, with weight being a distant third. Furthermore, weight mainly applies uphill.

Additionally, the placebo effect can't be discounted. This is not to say that the OP alone is gullible. It is rather that we are all gullible in unblinded tests between different products. The 3.5 lbs additional weight of the Domane alone would exert a nocebo effect (opposite of placebo), even if the objective impacts are relatively small unless you are on a steep climb. The groupset should also fall into this category. Provided the OP has enough gears to maintain their optimal cadence at most speeds, the groupset alone shouldn't actually make the Domane substantially slower. That said, Claris does only have 8 cogs, and the Domane's stock 11-32 cassette could have substantial jumps in gearing. Notably, the gearing is 11-13-15-18-21-24-28-32. On flat ground, I use the 13, 14, 15, 17, and 19 cogs a lot, and I frequently wish I had a 16t cog for more even gearing changes in that range. Claris' gearing jumps in that gear range are significantly larger.

Upgrading the Domane to racing tires and light butyl or latex tubes would definitely make a noticeable difference, and would be cost effective. If the Domane used disc brakes, you could potentially share the wheelsets, but the Al2 appears to be rim brake. Extensive upgrades to entry level bikes are not cost effective. If you want a fast endurance bike, then it's better to buy one from the start.

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  • The placebo effect can be big. With new tyres I immediately diminished my commute time from 60 to 55 minutes. But later I found I often go much slower even with the new tyres. It is strongly about how hard you push. A new fancy bike can one make to push really hard. Nov 6, 2020 at 19:26
  • @VladimirF In case it was not clear, I agree. I believe Josh Poertner made similar points to you when discussing the placebo effect on at least two of his podcasts (Marginal Gains).
    – Weiwen Ng
    Nov 6, 2020 at 20:32

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