With the ever growing offering of aero climbing bikes or lightweight aerobikes, I wanted to start a central thread to discuss what matters and what might be marketing bs. Especially with the launch of the Tarmac SL7, there has been a lot of chat about "now Aerobikes are obsolete", and how it may be faster than this and that bike.

For me, a competitive cyclist and a fan of bike tech in general, but not an expert on aerodynamics, physics or sports science, I was doing a bit of research, but would also love to learn things from you guys on this topic. Since we are comparing the lightest aerobikes, and the most aero light bikes, the results can only be very marginal, and they are. In the end, all of this is about saving a watt here and there, so if you think to yourself "why does anyone care about 3 Watts?", I highly advice against reading any further.

The conundrum that the whole industry is faced with is making a bike that is fastest based on the two variables of weight and aerodynamic drag. Obviously, the by far biggest factor will always be the rider. The rider propels the bike forward, the rider creates the biggest air friction (70 to 85% as I understand it), and the rider is by far the biggest part of the system weight (usually over 85%). I would refrain from discussing how the biggest gains are made north of the saddle, as that should be a given. Also, I am talking about the latest and greatest bikes here, that are clearly aimed at ambitious athletes, that already put in significant effort into becoming lighter, stronger and better at staying aero. Also, most of the lightweight aerobikes or full aerobikes nowadays are very similar in the terms of geometry, so the rider aerodynamics shouldn't really be affected by the choice of the bike. Feel free to disagree.

**Weight:**

This forum of course is a gold mine of knowledge about this topic. I'd say the weight difference on "aero bikes" is around 1kg on the top end, with the BMC Teammachine, Tarmac SW SL7, Emonda SLR9 being around 6.8kg, and the heavier aero bikes Cannondale SystemSix HM, Cervelo S5 Disc and TREK Madone SLR9 being between 7.5 and 8.0 kg.

So, how much does a kilo of system weight more or less matter?

In (male) road racing, system weights range from around 63kg (54kg rider + 6.8kg bike + 2.2kg clothing, shoes, water, helmet, spares) to just over 90kg (80kg rider + 8kg bike + 3.5kg of clothing, shoes, water etc.). That means a 1kg reduction will reduce the system weight by 1.6% at most, and 1.1% at least.

**Flats:**

On the flats, we know that the weight difference is making hardly any difference in speed. This will be the more relevant section when discussing aerodynamics. The two relevant topics here are increased rolling resistance and increased inertia (disadvantage when accelerating, advantage when free wheeling). According to this article on Training Peaks (https://www.trainingpeaks.com/blog/unde ... esistance/), a 5lbs (2.2kg) increase in system weight will increase rolling resistance at 20MPH by 0.8 Watts. Since the graph is fairly linear, we can assume it is probably

**around 0.3 or 0.4 Watts**for the range of bikes we are talking about.

The effects of weight on acceleration are a bit more complex than that. What I, as a physics noob know is, that the power put into the system is not lost until you brake. So while a heavier bike is more difficult to get up to speed, it should stay at a higher speed at a lower effort. Acceleration being a linear function of weight (correct me if I am wrong), a 1.6% increase in system weight should mean that for the same level acceleration the rider on the heavier bike would need to overcome 1.6% more power. When accelerating at upwards of 500 Watts for a short sprint out of a corner, that would be 8 Watts more. I am not a good sprinter and can probably use any help I can get, but I am sure that 8 Watts more or less here, for such a short period, are just not noticeable for me. Considering the difference between 1100 Watts (my approx. max power output at 70kg bw) and 1083 Watts, I think the same holds true.

Also, unlike wind resistance or rolling resistance, these 8 or 10 or 20 watts you have put more into the heavier bike will help you roll along for a little longer at some point. The only real advantage here would be a very tight angled Crit, where you brake a lot, and the energy is actually "lost".

**Climbs:**

This is where the 6.8kg aero-climber should really outshine the "fat" 7.8kg aero bike.

Again, not an expert, so I am quoting (https://mccraw.co.uk/2012/06/19/bike-we ... rformance/)

The speed at which you climb is dominated by the power you put out versus the work needed to overcome gravity – rolling resistance and other frictional losses are minimal and unlike riding everywhere else, aerodynamics isn’t getting a look in.

This means it’s possible to express the effect of weight on climbing, with a fair degree of accuracy, by dividing one weight with another using the following formula:

(System weight heavy)/(system weight light) * speed heavy = speed light

For my example the maximum gain should be:

(64kg/63kg)*15kph = 15.24kph (0.24kph advantage). Well of course the 1.6% weight loss results in a 1.6% speed gain in this example, where rolling resistance and aerodynamics are disregarded...

Since we are talking climbs that are steep enough, that wind effects are minor, speed is based on W/kg mainly.

I would like to go with 3 examples show the effects the 1.6% decrease in weight has in different scenarios.

Short climb at 64 v 63kg system weight, effort at 400 Watts: 400/63 = 6.35 W/kg. With 1kg more: 6.35W/kg*64 = 406 Watts for the same speed uphill. So this is

**6W saved**.

Longer sustained effort at 78kg vs 77kg system weight, effort at 310 Watts (around 4.5 W/kg of BW). 310/77kg = 4.03 W/kg. With one kg more, that's 314 Watts for the same speed uphill. So this is

**4 Watts saved**

Even longer sustained effort at 91kg system weight vs 90kg, effort at 250 Watts. 250/91kg = 2.78 W/kg. With one more kg more, that's

252.8 Watts and

**2.8W saved**

Mixed: Of course a mixed course is what is most likely what people are looking at, and it is very difficult to analyze, since every course is different and there are several variables to consider. What I found quite interesting, was the estimate from this article, that states during the Transcontinental race 2016 (3900km, 45km of climbing), that 1kg in saving would result in a saving of 1 minute per 1kg. Mind that this a "slow" ultra endurance race, so the effects of weight per distance are greater than in a shorter, faster race). This is around 1minute per 4 to 5 hours or 12 to 15 seconds over an hour.

Conclusion on Weight saving: the weight saving of the lightest aero climbing/ light weight aero bikes over top end aero bikes is around 1kg, which is between 1 and 1.6% of total system weight,

**This results in a saving of just under half a watt everywhere, and at best 6.5 Watts on steep short climbs (including rolling resistance)**.

**Aerodynamics:**

So then, what is the aerodynamic gap between the bikes like the SL7, Emonda SLR, Wilier SLR Zero and bikes like the Venge, Madone or S5.

I have exhausted all my resources here and purchased TWO issues of the Tour Magazine, to get some Aero-Data

They are pretty much the only mag to have aero-tested the SL7 yet, and have measured it to create a drag of 210 Watts (with a pedaling rider on it). I don't really think this is a realistic figure for that speed, since it will easily take north of 300 Watts to maintain 45 kph, even on a TT bike with a disc rear wheel. I once read an article saying that "wind tunnel speeds" only simulate around 85% of the speed outdoors. Don't know if that is true, looking forward to your input here.

What I find relevant here, is that the difference, since it's a measurement of the entire system, not the bike alone, therefore can be applied to various speeds.

The 210 Watts of the SL7 are a fantastic value, with the Madone SLR in this very magazine even testing two Watts slower (probably this shows the limitations of wind tunnel testing). The fastest bikes they have ever tested are the S5 Disc (205 Watts), the Factor One (206 Watts) and the Cannondale SytsemSix (204 Watts). They are

**on average 800g heavier than the SL7**or Emonda SLR9, and test on

**average 2.4% faster aerodynamically**.

According to testing of Germany Cycling Magazine "Roadbike", the fastest bike they have tested (Cannondale SystemSix), and a Bike that Tour Magazine has tested to be just a little faster than the SL7 (the Canyon Aeroad was tested at 209W vs the 210W of the SL7), is 7 Watts in the frame alone (at 45kph, again, I doubt this is comparable to 45kph outdoors.) 69 Watts for the Cannondale, 76 Watts for the Canyon. That seems to be in line with the results of Tour.

*Where to save aero drag?*

The general rule goes, over 80% of aerodynamic drag is created by the rider, so a better position, a skin suit and such will likely yield the best results. Also, savings made at the front of the bike will be to much greater effect, than those on the rear. Aero front wheel with slim enough tire, an aero cockpit, a slim head tube etc. are likely the greatest aero of savings.

On this forum, I once so numbers posted that are allegedly from Cervelo. They quantify the potential savings that can be made in certain areas of a road bike. I can't varify their legitimacy. But here they are anyway:

**Handlebar: 30% Frontwheel: 16% Frame: 16% Fork: 9% Bottles: 9% Drivetrain: 9%, Rear wheel: 5% Brakes 5% Seatpost: 1%**

Regardless of how accurate these numbers are, I believe them in that regard, that they take into account, that a pedaling rider obstructs the air flow, so that things like the seatpost offer minimal saving potential. Also, bottles are taken into account, which I think is often neglected in the design of aero frames (my eyes say it's neglected on the SL7 for example).

**Conclusion of Weight v Aero.**

There will be an incredible amount of inaccuracy and lack of understanding of physics in all this. That's why I don't want this to be:" here are the facts, take them as they are!" But much rather: "this is my understanding and what I found, let's have a chat if you like."

From the data I have gathered, the advantage that the fastest all out aero bike (Cannondale SytsemSix/Cervelo S5, Factor One) has in terms of aero drag over the lightest aero bikes (SW SL7), is slightly larger (2.4%), than the weight/ rolling resistance advantage (under 2% in a best case scenario of the entire system) the lighter bike has. So even at speeds slow enough, that aerodynamics only make up 50% of total resistance (usually quoted as "over 6% gradient", under 18kph), the aero advantage will outweigh the weight advantage. Even at these small margins.

**Does any of this matter?**

Not really. The differences in Watts between aero climbing bikes and aero non-climbing bikes should be around 1% of total power output. Not really anything to get crazy about. I mean, we aren't comparing ultra-WW climbing bikes with zero focus on aero vs the most aero built with no focus on weight, but we are comparing super-light very aero with very-light super aero.

The real conclusion probably should be, you can't really go wrong with either of the bikes. Buy what feels best for you, what you get along with, what is most fun, and what gets you out riding the most.

However, what I have also learnt for myself is, that a light-weight aero bike is NOT the fastest everywhere, as companies like to claim, but the all out aerobike is likely still the fastest option pretty much everywhere. In the case of Specialized, the "one bike to rule them all" is likely called S-Works Venge.

Edit: The reason this is so focused on the SL7 is NOT to talk bad about that bike. Quite the opposite, actually. The SL7 is probably the "most aero 6.8kg bike" there is, and there is quite a lot of data that exists on it already. Could be applied to any other lightweight aero bike (or however you want to call it), with slight alterations on data.