Understanding weight and how to lose some (and a free tool!)

January 12, 2021April 9, 2023 florian 0 Comments lightweight, weight

Just coming from a ride where the topic of weight was heavily discussed, I decided to make this write-up about cycling and weight a summary of sorts. I also created a helpful excel chart with a bang-for-buck formula for anyone aiming to build a lightweight bike, which you can download via the link at the bottom of the page.

Why does weight matter?
1) Rolling resistance
2) Climbing (and any road incline)
3) Acceleration
4) Lifting the bike up a flight of stairs

1) Rolling resistance:
Weight, CRR (coefficient of rolling resistance, which depends on your tire/tube/inflation choice), and speed all matter in rolling resistance. The impact of weight is linear, so your rolling resistance decreases equally as your weight decreases by a certain percentage.
So what does that mean in real life?
On a flat road with no wind, at 12 km/h, both rolling resistance and aerodynamic drag are equal. At 30km/h, rolling resistance is about 25W for both tires and 85KG total rider/bike system weight (this compares to around 150W to overcome aerodynamic drag with a total power requirement of 180W and around 5W drivetrain friction loss).
In the case of the cyclist with a system weight of 85kg (person+bike), saving 850g (so 1% of the weight) equates to 1% less rolling resistance, or 0.25W saved. We are going to use that same 850g savings for the calculations on climbing (watts saved) and acceleration (time saved).

2) Climbing:
By climbing, I refer to any road incline. The steeper the hill, the more weight comes into the equation. Also, in climbing, the impact of weight is linear. A specific percentage decrease in weight means the same percentage decrease in power needed to overcome gravitational forces, more correctly called slope resistance.
Here is the calculations for 30 km/h and a rider system weight of 85Kg and 1% incline:
Using Fs = g* sin (arctan (slope)) * M, in our case this is
9.8m/s2*sin(arctan(0.01))*85kg = 8.34N of slope resistance. (F = Force, M = mass).
Power to overcome slope resistance at 30km/h is 8.34N*8.33m/s = 69W.
So losing the same 850g in weight as in the above example of rolling resistance equates to 0.69W saved.
At 15 km/h and 10% incline, so a proper hill, Fs = 82.89, and the power required to overcome that resistance is 82.89N*4.17m/s = 345W.
Losing 850g in weight resistance equates to 3.45W saved.

A note here in regards to climbing:
The uphill effort of a cyclist is sometimes measured in W/kg, as opposed to absolute weight (kg) e watts. In this case, it typically refers to the cyclist’s weight only, not the system weight, which would include the bike, which is what matters when it comes to slope resistance.
Why does that differentiation matter:
Sometimes you might see that a heavier rider is putting out far more watts uphill than a lighter rider when going up a hill at the same speed, but when expressed in W/kg, the lighter rider is outputting more. Why don’t they output the same w/kg for the same speed? It is because the lighter rider has to carry proportionally more dead weight (the bicycle) than the heavier rider. Imagine a rider weighing 60kg who is on a 6Kg bike; for every kg in bodyweight, he has to carry an extra 100g in bicycle (=dead) weight. An 80kg rider on the same bike has to carry only 75g in bicycle weight. This is where the extra watts are needed for the lighter rider and hence more w/kg of bodyweight. W/kg of system weight, on the other hand, would be the same.

3) Acceleration:
Weight might matter in the case of sprinting to a finish line because less weight should result in quicker acceleration. The effects of acceleration are relatively straightforward to calculate, with F=M*a. This leads to the direct proportional relationship acceleration a = F/M, meaning that 1% in weight reduction equals 1% increased acceleration, and speed at time t is v=a*t. Using the calculator from analyticcyling.com, our rider with 85kg system weight would need around 46.5 sec to sprint 500m on a flat road, no wind, if averaging 450W with 4s at max power of 650W.
850g in weight-saving save 0.1 sec (or around 1W).

What weighs how much?

See below graphic of typical weights, based on a bike build of 6.8kg. The wheelset is typically by far the heaviest part of your bike, followed by frame and crankset! A side note: After finishing the illustration, I realized I forgot to mention something easy to overlook, that is, the weight of the compressed air inside the tires or tubes. For a 25mm road clincher tire at typical pressure, the compressed air weighs 7g per wheel! So if you calculate your bike weight by adding all component weights, then weigh your bike with pumped-up tires, and come up 14g heavier, you now know why!

What to watch out for when saving weight:
1) Cost
2) Safety
3) Performance
4) Practicality
5) Looks

1) Cost:
Lighter parts are often more expensive. There are vast differences in how much a specific savings in weight can cost you. It is an excellent idea to express that as bang-for-buck, or cost/g.
Lighter brakes might save you 40g but cost an extra USD 200, so in this example, every gram saved costs you USD 5.00.
In comparison, switching from butyl inner tubes to TPU-type tubes like the Tubolitos saves around 120g at an extra cost of USD 40, or USD 0.33 per gram saved, hence a better bang-for-buck. The weight calculator linked below automatically calculates the bang-for-buck for any planned upgrades.

2) Safety:
Lighter parts are often (but not always) more prone to breaking. Examples are lightweight rims and lightweight saddles. While all products sold to consumers do generally fulfill minimum requirements, sometimes they come with a limit on rider weight, and chances are higher that something goes wrong when used in extreme situations (for example, extended braking with rim brakes during long descents)

3) Performance:
Lighter weight often means less material (or material density), which can lead to more flex. Examples are lightweight frames, wheels, handlebars. While flex can also mean more comfort, when great power is applied, flex results in less responsiveness, potential loss of power, and undesired effects such as brake rub.
Even on minor parts such as computer mounts, flex due to weight savings can impact, in this case, by resulting in more vibration of the bike computer and hence decreased readability.

4) Practicality:
Not all lightweight parts are as practical as heavier counterparts: Example here are lightweight carbon handlebars that might not allow the mounting of TT bar extensions due to the risk of breaking.

5) Looks – they matter.

Weight savings that extend beyond the bike:

Everyone understands that losing some body fat is the most effective way to reduce system weight, and it is for sure the cheapest. After all, a bike typically represents only 10% of the rider’s weight.
But there are many more areas where weight can be saved:
1) Clothing: bib shorts, socks, helmets, gloves, jerseys, sunglasses shoes can weigh a combined 1.5 kg. Much savings can be achieved here. It does not turn your bike into a bragging-rights-worthy sub-five machine, but those grams matter equally when cycling uphill.
2) Accessories, such as mini-tool, computer, and tire levers, have significant differences in weight as well.
3) Since we might be willing to spend hundreds of dollars for a few grams of savings, you might also want to know that shaving head and face can save you 25 g (depending on existing style) for almost free. Then there is Chamois butter, sunscreen. Not heavy, but also not without weight!

For all that plan to get the weight of their bike or system down, I have made a nice calculator in the form of an excel spreadsheet that is free to download here.

And below, just for fun and because sometimes component weight is compared to the weight of a bike bottle,here is the opposite: The representation of the weight of all parts if they were to be fitted into a typical 680ml bike bottle!