Aerodynamics vs. Weight: What is the Tipping Level for Professional and Beginner Cyclists?

Over the 21 levels of the Tour de France, riders of the Professional Peloton might journey quite a few completely different fashions from their body sponsor. They may change between the model’s devoted aero street bike, time-trial bike, and light-weight climbing bike, making a call based mostly on what that stage will carry on the day; elevation profile, wind situations, mileage, terrain, and so on., all must be fastidiously thought of to make sure the rider is optimally geared up. The staff seems to be to regulate any issue that’s controllable.

We not too long ago acquired a thought-provoking query from a reader on this very matter, a query that was the inspiration for this function. Not being aerodynamics specialists or engineers ourselves, we now have recruited experience from a number of the high street body producers whose bikes will be seen all through any Professional Peloton. Specialised, Trek, Cervélo, and Cannondale all carry some fascinating and insightful dialogue on the subject, starting from the succinct and simply digestible to the outcomes of some fairly revealing digital simulations.

The Reader’s Query

“The gradient at which it’s stated that weight begins to trump aero is usually round 8% for professional riders. Clearly, the slower one can journey and climb should have an effect on this transition level in gradient. How does this gradient transition cut back because the out there energy output reduces for these of us who aren’t execs?”

The reply to that query is, it relies upon, as we are going to quickly discover out from our staff of consultants. The discussions provide some perception into bike selection within the Professional Peloton but in addition ship some golden nuggets of recommendation as to what sort of bike we beginner cyclists would go finest on.

Our contributors are:

• Marcel Keyser, an engineer with the Specialised Human Efficiency Staff
• The Cervélo Engineering Division
• Trek Senior Aerodynamicist, John Davis
• Dr. Nathan Barry, Cannondale engineer and aerodynamicist

Dr. Nathan Barry, Cannondale Engineer, and Aerodynamicist

The straightforward reply is that the much less energy to weight a rider has, the slower they climb, and the decrease the gradient of the tipping level between aerodynamic and weight financial savings. However that solely scratches the floor of this phenomenon. To totally tackle this query, it’s necessary to return to the elemental science that underpins this idea to ensure there are not any misconceptions. The assertion of 1 particular gradient the place weight trumps aerodynamics is an oversimplification from some arbitrary particular set of parameters.

The underlying mechanics of biking, described by the biking energy equation, present that aerodynamics is a perform of velocity, with no dependence on gradient or mass. Conversely, climbing energy is a powerful perform of mass, gradient, and velocity. Taking the acute edge instances; on a flat street (0% gradient) a rider expends 0 W of climbing energy. At a continuing velocity, mass has a minimal impression on the facility required, thus most of a rider’s energy is spent overcoming aerodynamic drag. Because the street will get steeper, the rider should expend climbing energy, leaving much less energy to beat drag, subsequently their velocity drops. So at some very steep gradients, a rider could have very low velocity such that aerodynamic resistance turns into minimal and nearly all of the rider’s energy is spent overcoming the gradient.

It stands to motive then, there can be some cross-over level alongside this gradient spectrum at which the rider expends an equal quantity of energy overcoming each drag and climbing energy. Nonetheless, this nonetheless isn’t the complete image. The query of curiosity isn’t concerning the magnitude of energy, however fairly variations in efficiency. What we’re actually all for is how adjustments in drag or mass have an effect on efficiency. A easy instance is likely to be a selection of wheels. A shallow wheelset is likely to be lighter however sacrifices aerodynamic effectivity in comparison with a well-engineered deeper rim. Clearly, on a flat street the deeper, low-drag wheelset is preferable. The query turns into, how steep does the street must be earlier than the light-weight wheels are sooner than the heavier however lower-drag wheels? Typically that is known as the tipping level.

What turns into clear from this instance is that the gradient at which the tipping level happens will rely on the relative distinction in drag and mass. A giant drag financial savings with a small weight penalty could have a a lot greater tipping level than a change that has a small drag financial savings however massive weight financial savings. The tipping level can be a perform of velocity. Even when climbing, a rider is pushing by the air so there’s all the time a element of aerodynamic drag. Whereas it’s lowered at decrease speeds whereas climbing, it by no means drops to zero, as long as you’re shifting. As famous within the query assertion, that is the rationale why the tipping level is greater for professionals. As a consequence of their greater energy (and energy to weight), they climb sooner, and at a given gradient, they’re touring sooner than an beginner and so have greater aerodynamic resistance to beat. Aerodynamic financial savings have a bigger affect on efficiency.

A second idea to layer on high of that is that these variations in rider efficiency are associated to the system as a complete, not simply the element, or the bike. Contemplate a wheelset improve that saves 100 g of weight — ignoring any aerodynamic variations for now. For a high-performance wheelset within the vary of 1,600 g, a 100g discount is a giant saving (6.3%). For an entire bike of ~7.5 kg that may be a 1.3% weight discount. Nonetheless, biking efficiency is set by the full system mass — together with the rider.

Sometimes, the bike is simply about 10% of the full system weight. For a 75kg rider on this bike, the 100g weight financial savings is simply a 0.1% discount in system mass. As such, the climbing efficiency enchancment from such a change can be proportionally small. The identical is true of variations in aerodynamic drag. Adjustments in drag must be thought of as a perform of the entire system. Nonetheless, a ten% discount in system mass is unattainable when contemplating solely the bike, this isn’t out of the realm of risk when you think about a motorcycle with no aerodynamic optimisation versus one devoted to decreasing drag.

How does rider energy have an effect on the gradient transition? There isn’t any one reply to this; it comes right down to these three key variables: change in drag, change in mass, and rider energy to weight. For a given configuration (a given distinction in drag and mass) you may mannequin a curve for tipping level versus rider energy to weight. It will present how your efficiency with a given setup compares to that of a professional who may need considerably greater energy to weight. However that may solely apply to that particular gear configuration. As quickly as you modify the relative variations in drag and mass, the mannequin will now not be related. It’s laborious to simplify this since variations can range broadly. Between wheelsets, frames, cockpits, helmets, and clothes, there will be massive variations in each drag and mass.

To attempt to present some perception to those that don’t wish to construct all this right into a spreadsheet, we will body a few of these ideas in a much less advanced manner utilizing a quite simple case examine. Contemplate what occurs whenever you add 1 kg of lifeless mass to your bike, i.e. with no profit from lowered drag. Within the context of high-performance street bikes, it is a massive change in bike weight. Definitely, one thing {that a} rider will really feel once they maintain or decide up the bike. Most street riders would take into account this to have a dramatic impression on efficiency. If we take a case examine for an beginner rider: a 75kg rider, 7.5kg bike, 7% gradient climb, 15km/h street velocity. Climbing at this velocity and slope would require ~275 W. An quantity sustainable for a match beginner rider of this dimension (3.7 W/kg). The addition of the 1kg lifeless weight would add solely 3W resistance beneath these situations. This isn’t nothing, however it’s just a bit over a 1% enhance in resistance. For a rider who isn’t competing at a excessive degree, this distinction will not be going to make or break their journey. In truth, this equates to lower than 0.5-second time loss over a 5km climb. If we cut back that efficiency to 2.5 W/kg the facility distinction is 2 W and the time distinction is 0.5 seconds to the identical vital figures. Mainly, much less energy out there reduces street speeds, which reduces the magnitude of energy differential however will increase the time saved — because the rider’s complete time over the phase is proportionally longer.

Weight will not be as vital as most riders in all probability consider. Even with no aerodynamic enchancment, a 1 kg distinction has a comparatively small impression on street velocity. Conversely, in the event you begin with a setup that has comparatively excessive drag, a 1kg mass distinction might be spent on an enormous quantity of aerodynamic optimisation. For a similar rider and energy output you would see at the very least 20W energy financial savings on a flat street when evaluating a traditional spherical tube body with low-profile wheels to a contemporary race bike. So maybe a less complicated query to handle this phenomenon is how a lot ought to a rider care about weight or aerodynamics? Whereas it’s a lot simpler to really feel and measure the load of a motorcycle, for nearly all riders and situations, aerodynamic optimisation could have a a lot larger impression in your on-road velocity than lowering the load of the bike.

If we revisit the wheel comparability from earlier, we will take into account one particular gear selection and study the calculation of the tipping level for our 75kg rider at 275 W. Take two wheelsets: the primary is light-weight at 1,300 g however has a shallow rim and excessive drag. The second makes use of a deep aerodynamic rim with a weight of 1,500 g and a drag discount of 0.010 m². This represents a practical determine based mostly on wind tunnel take a look at outcomes for a pair of wheels on an entire bike.

The facility distribution as a perform of gradient for this rider is plotted under. This mannequin successfully has the rider sustaining a continuing energy output and redistributes that energy based mostly on the necessities at every gradient. From expertise, we all know that the rise in gradient is coupled with a lower in velocity. At 0% the rider is shifting in a short time (~38 km/h), in comparison with very slowly (~11 km/h) at 10%. This plot highlights how the rider’s energy distribution adjustments because the street turns into steeper. With rising gradient, there’s extra energy required to beat the elevation achieve, leaving much less energy out there to take care of the velocity seen on the flat. As velocity drops, so does the aerodynamic energy requirement.

For the 2 wheels, we will calculate these energy distributions for every configuration. Evaluating the 2 units we will derive the tipping level to happen at a gradient of 6%. Keep in mind that that is the inflection level. At any gradient as much as 6%, the heavier, lower-drag wheelset is quicker. Solely as soon as the gradient exceeds 6% is there any benefit for the lighter wheelset. Notice, within the above graph, that at a gradient of 6% considerably extra of the rider’s energy is distributed to climbing in comparison with air resistance, and but the efficiency of the 2 configurations is equal at this gradient. This comes right down to the relative impression of these variations on the efficiency steadiness. As described earlier, altering rider energy, drag financial savings, or weight financial savings will change the worth of the tipping level.

The determine above reveals the facility distinction based mostly on a set velocity (as calculated for the primary configuration with the heavier, decrease drag setup). On this case, constructive values point out the extra energy required by the lighter wheelset, adverse values point out the place the lighter wheelset is saving energy over the heavier wheelset. The horizontal intercept at 6% signifies the tipping level. Notice that the magnitude of energy distinction between the 2 configurations will not be equal. While the lighter wheelset is quicker above 6% the facility saved is an order of magnitude smaller than the financial savings seen with the low drag wheelset on low gradients.

Cervélo Engineering Division

Aerodynamic impression grows non-linearly as velocity will increase. Professional riders produce extra watts and weigh much less, which leads to a sooner common velocity — which additionally means absolutely the aerodynamic impression is bigger.

Comparatively talking, on flat floor, an beginner rider will profit extra from an aerodynamic body than the professional rider as a result of they produce much less energy and sometimes weigh extra, and the aerodynamic discount in drag is a larger proportion of their complete energy output.

As soon as the street goes uphill, although, weight turns into the figuring out issue, and speeds drop. A professional rider will nonetheless produce extra energy and weigh lower than an beginner rider, so they are going to preserve a better velocity, which maintains aerodynamic efficiency. An beginner rider will weigh extra and produce much less energy, so their velocity will drop extra rapidly than a professional, additionally lowering the aerodynamic impression of the bike.

Assuming bike weights are the identical, together with the assumptions acknowledged above concerning energy output and rider weight, the load versus aero tipping level for the common rider is probably going round a 4-5% gradient versus roughly 8% for the professional rider.

John Davis, Trek Senior Aerodynamicist

The reader’s 8% tipping level was correct a number of years in the past for the trade-off between typical aero and light-weight bikes at professional speeds. Nonetheless, with most race bikes receiving aero therapy these days, that tipping level has been lowered.

Needless to say on a loop course, you need to descend what you climb. Factoring in descending, the Emonda versus Madone tipping level for an out-and-back climb and descent adjustments from ~3% to ~6% for the 150W rider (this simplistic calculation neglects braking losses).

Now, nearly all race bikes obtain some aerodynamic therapy, which has lowered the tipping level from the ~8% degree the reader requested about. This instance compares our 2018 Emonda versus our 2021 Emonda, however the identical development is usually seen throughout the trade.

Different elements that come into play:

• I assumed no wind; headwinds and most crosswinds will push the tipping level in favor of the aero bike whereas tailwinds will push the tipping level in favor of the climbing bike
• Rising rider weight will push the tipping level in favor of the climbing bike and lowering weight will push the tipping level in favor of the aero bike (roughly by 0.2% grade for each 10 kg on this instance)

Marcel Keyser, Engineer With the Specialised Human Efficiency Staff

For the simulation, we created three “digital” bikes — weight, CdA (coefficient of aerodynamic drag), and a constant rolling resistance — and ran them in quite a few situations. We used our Tarmac SL7, the Aethos, after which a “pure” aero bike based mostly on what we all know from many devoted aero street platforms from a wide range of producers. 

The rider has 70 kg + 1 kg of drugs. Rolling resistance is much like a pleasant Turbo tire.

Conclusions

A. Normal

1. The stronger a rider is, the steeper the street must be to hit the break-even between aero and weight. 
2. Stronger riders work extra in opposition to aero as a result of greater common speeds.

B. Tarmac SL7 vs. Aethos (Light-weight)

1. Climbs must be steeper than roughly 5% to see the benefit of the decrease weight.
2. It’s fascinating to see how completely different rider speeds play a giant function in when the Aethos versus the Tarmac selection delivers a bonus.

Primarily based on 3 W/kg, 4.5 W/kg, and 6 W/kg, representing beginner, robust beginner, and professional riders, respectively — we all know it begins to matter past 5% grade for all riders, however the big-time gaps don’t happen till steeper slopes, so we calculated the time delta (change) over an hour of driving a ten% slope on a Tarmac versus an Aethos for every of those riders. Primarily, how a lot sooner the Aethos covers the gap {that a} Tarmac SL7 does in an hour. You’ll see that, and watts saved, within the under abstract graphs.

• At 3 W/kg, the Aethos is 25 seconds forward after an hour over the Tarmac SL7 on a ten% slope. 
• At 4.5 W/kg, the Aethos is nineteen seconds forward after an hour over the Tarmac SL7 on a ten% slope.  
• At 6 W/kg the Aethos is 14 seconds forward after an hour over the Tarmac SL7 on a ten% slope.  

It’s necessary to notice that with at present’s races, the aero benefit over a whole day and the rarity of any race with sustained 10% slopes, mixed with the necessity to add ballast to make an Aethos UCI authorized, make the Tarmac SL7 the plain race selection for our riders.

We did the Tarmac SL7 versus a “Basic” aero street bike for enjoyable too. The outcomes had been fairly unbelievable. The Tarmac SL7 offers up nearly nothing on the flats and features 4X that point again on a climb.

C. Tarmac vs. “Basic” Aero Bike

1. There may be nearly no drawback on FLAT street.
2. The delta energy plot reveals approx. 1 W drawback on flat and a couple of W benefit on climbs for the Tarmac. This feels like 50%/100%.
3. BUT! Take a look at the TIME financial savings per hour bar graphs within the abstract plots above:

• The Tarmac loses 5 seconds in 1 hour on the FLAT and wins 20-25 seconds in 1 hour on a CLIMB versus the “Basic” Aero Bike.
• This delta is large, validating the unbelievable mixture of light-weight and aero that the Tarmac delivers.

Thanks once more to Specialised Bicycles, Trek Bikes, Cervélo, and Cannondale for contributing to this function.