How does weight actually matter?
 

I read a lot about lighter frames, tires, even seat posts etc. What confuses me is that difference between a superlight and a super heavy bicycle is under 10 kilograms.

Since light bikes are very expensive and I've never had a chance to ride one for a long time, I'm curious to hear from experienced people: does it really matter, how big difference does it make for everyday riding?

I weigh some 75 kgs and often carry around 10 kgs of stuff (water, food, clothes, books etc). Does a 5 - 9 kg lighter bike really make some noticeable difference? Does it go a lot faster up hills, handle better, or is weight important for racing only?

Having switched from a mountain bike to a hybrid, I feel that speed and light ride is more about thinner tires and aerodynamics, but how does weight come into that equation?

Weight and bikes is a fairly hotly debated subject, and it's not always the scale and the personal experience go hand in hand with what you'd expect regular physics to tell you.
My theory is that a fairly small change in weight can influence the feel of the bike quite a lot, and us humans are suckers for picking up and reacting to things like that.
One take is that a lighter bike is likely to be made of better parts that are put together with more care. This means that the rider will be hit by a broadside of positive reinforcements. Rider knows the bike to be better, it may well feel nicer, better parts and better assembly does mean it'll ride a tad easier. In that setting, it's real easy to exagerrate the improvement that reduced weight alone offered. If the bike feels nicer, people tend to ride harder, and go faster.

Weight should matter most when climbing and during rides with lots of speed changes, and should matter the least on long consistent slogs across the flats.

F=ma, Work=mgh, Power = work/time

It largely a physics problem. The less weight that a vehicle carries, the less energy is required to move it done the road. Or, conversely, the less weight a vehicle carries, the same amount of energy is going to propel it done the road faster...to a point. You do have to take all the forces that work against the bike and rider and those forces tend to increase exponentially with speed but, yes, weight does make a difference.

That said, there are trade-offs. The weight of the system is important to consider. For your example, you have to consider the weight of the rider (doesn't change all that much), the weight of the bike (can vary quite a bit) and the weight of 'other'. If you are going to have to carry 10 kg of other stuff, dropping a few kg from the bike probably isn't going to make that much difference. It may even cause other problems. 10kg of unsuspended weight (the rider has its own suspension) on a very lightweight bike is going to make the frame do things its not made to do, like flex and twist under load. A heavier frame is usually built for dealing with a heavier load so it doesn't flex and twist as much. Flexing and twisting can take away from the energy you have to propel yourself down the road.

You have the perfect test bed to do your own experiments. Take the bike that you currently are riding with all the stuff you ride with. Ride it over a course of known distance that you lay out. To do it properly, ride two different course. Do one in an urban setting with lots of stops and starts and one on an open road. Time the ride and then repeat it without the load. To control the variables as much a possible, do it at the same time of day under similar weather conditions. Do only one ride per day so that your energy levels are the same. Pump your tires to the same pressure before each ride too.

If you want to go way over the top(from 4 rides to 12), you could do the rides in triplicate so that you take the variables like wind, weather, rider condition, etc into account. Further randomize the runs so that you don't do all the loaded urban runs, then all the loaded rural runs, and so on.

If you wanted to go way, way over the top, add in a second bike that is significantly lighter than the original bike but has the same rider configuration. Now you are up to 18 rides.

I predict that you will find that weight makes a difference. What you will probably be amazed at is that the difference in speed is small. A light bike is faster but it's not stupendously faster. You'll probably gain 1 to 5 kilometers per hours but not a whole lot more. You can spend a whole lot of money to get a bike that is a whole lot lighter but you only go a little faster. The returns diminish pretty quickly. If you are competing at a very high level, the investment can be worth the returns. If you are an average joe, your return on investment is much lower.

I switch between a 24.5 lb mountain bike and a 30 lb mountain bike all the time and I gotta say that lighter = funner.

The light bike is a little skittish on rough descents, but even that's pretty fun, though.

I just swapped out the fairly light Suntour Cyclone/Mavic 192 rear wheel on my single speed for a super heavy generic MTB wheel. Bunnyhops require a different motion and a little more effort now. That Cyclone/Mavic wheel didn't have to be asked to leave the ground.

I rarely pay more for lightness but if I can find lightness at the same price I go for it.

Slaninar, The last paragraph of dabac's post sums it up nicely. Wheel and tire weight matter the most as there are two factors involved, static weight and rotational weight. It simply takes more energy to spool up a heavier tire and wheel combo and more energy to slow them down. A heavier tire and wheelset can be a benefit on an uninterrupted (no traffic signals) ride on fairly flat terrain as there's some flywheel effect.

Handling is more effected by wheelbase, frame geometry and fork offset than by weight, IME. My crit bike weighs slightly more than a very similar road bike I had, yet is noticeably crisper in tight and fast turns. My touring bike with a long wheelbase and relaxed geometry just wasn't meant for crisp, point and shoot type riding.

I can feel the added weight of the touring bike when climbing some of the short, but steep hills in my area and it isn't much of a difference even if using the same gear inches as I would on the road bikes.

In summary, I've pared my road fleet down to just the crit bike and the touring bike. There is an ~8 lb. difference in weight and the weight isn't a factor in my weekend recreational rides.

Brad

top speed won't change much, but acceleration and responsiveness is better.

My case-in-point. I ride two very different bikes depending on the weather;

My good weather bike weighs 18 pounds (8.2kg) and has 700-23 slick tires at 110 psi
My rain bike weighs 35 pounds (15.6 kg) and has 700-32 treaded tires at 85 psi.

On level ground at constant speed the effort to ride the heavier bike is slightly higher due to the tire difference. On hills, the heavier bike is night-and-day more effort to ride and my times and average speed for equivalent rides are significantly worse on it.

So, yes, weight matters but mine is a fairly extreme example as the heavier bike is nearly twice as heavy as the lighter one and adds 10% to the entire rider/bicycle system.

Where weigh watching gets extreme is those who spend great amounts of money to shave a few grams and agonize over even slight weight differences. The physics don't support the cost but that can be a hobby all in itself.

The exception is very serious racers for whom both psychologically and physically lower weight is desireable as they are competing with near equals and any advantage is worth while. In fact the UCI, the governing body for professional racing, has set a lower weight limit for bicycles in an attempt to head off a very expensive weight war. It didn't work but that's a topic for another thread.

Every bike ride starts at zero miles per mile. It's very easy to feel the difference between a light bike and a heavy bike over the first 100 or so feet. Once you get up to speed I don't think the difference in feeling is as noticeable.

"How does weight actually matter?"

This ought to be good!

http://desmond.imageshack.us/Himg141...jpg&res=medium

Personally, it matters very little to me. My healthy weight is 210, and I'm 60 pounds over that. I prefer 15-20 year old mountain bike frames and wheels with lots of spokes and bombproof rooms even for commuting. I usually have fenders and an array of lights on my bikes, plus at least a rear rack, at least one heavy lock, my tool roll, and whatever I am taking to/from school most days. Therefore I don't really see any point in spending money on anything merely because it is lightweight (all other things being equal).

Yes......The more weight you have,the easier it is to go fast downhill......:)

Yep, I recall the case of a very light pro rider in the Tour de France years ago. He was a great climber but not heavy enough to descend with the others. His team manager slipped him a water bottle full of lead shot at the top of one of the major climbs to help him keep up on the descent.

:roflmao2::roflmao2::roflmao2:

Some googling turned up this: http://en.wikipedia.org/wiki/Jean_Robic

IIRC, it was in 1878 when the hollow backbone (made from tubing) frames came out, replacing the solid steel frames and shedding a significant amount of weight. Cyclists have been voting with their wallets for lighter weight ever since.

+1. I've ridden relatively heavy bikes, and I've ridden relatively light bikes. Unless the bike has a special purpose where some extra weight actually helps (like a bike designed for loaded touring or commuting), I'll choose a light bike nearly every time.

It's funny that this thread has come up right now, we've been slow at our shop the last couple of days, and we've been going around putting various wheels on various bikes and weighing them to compare the weight difference based on the different wheels, tires, and cassettes. It's pretty amazing how much weight you can drop off a bike with relatively light wheels if the bike had relatively heavy wheels on it before.

I've been calculating weight (and cost) for a lightweight wheel build I'm going to do soon for my new carbon framed bike. Fun stuff at the bike shop when things are a little slow.

It makes more of a difference if you're a fit and strong rider than if you're not.

They're much easier to get in and out of a car trunk where you're lifting at odd angles and more pleasant to carry up stairs to an apartment or office where there's no elevator.

Speed increases on steep enough hills (where most of your power is going into overcoming gravity instead of aerodynamics) are inversely proportional to total weight.

With 75kg of rider only, loosing 5kg going from a heavy old bike to a 9kg (entry level) contemporary road bike will make you up to 6% faster on the steepest grades. Making some assumptions about aerodynamics at 250 Watts you'd gain 5% going from 15.2 km/h to 15.9 km/h on a 6% grade. Dropping another 2.2kg to the 6.8kg minimum allowed for UCI sanctioned races would increase the gain to 2% from an average road bike or 8% from the heavy setup on steep enough grades or boost speed to 16.3 kilometers/hour on a 6% grade which is a 2% gain from the normal weight bike and 7% from the heavy one.

On flat ground dropping the full 7kg of bike weight might increase your speed 0.4% from 37.66 km/h to 37.84 at 250W.

Such differences aren't significant in terms of performance (cyclists have a hard time gauging effort within 10%) if you're not an otherwise competitive cyclist racing up a hill where it may make a difference in your placing.

Feel can be really different though - 6.8kg of pannier and luggage is very noticeable out of the saddle for me.

Actually it makes a bigger difference for less fit riders because at lower speeds the energy used to overcome aerodynamic drag is a smaller fraction of the total compared to higher speeds meaning reductions in rolling resistance and getting your weight up hill are a larger share of the total.

Weight only matters as much as YOU want it to matter.

The impact of rotating mass doubles as you move from the center of the axle (where it's not rotating at all) out to the tire surface although it's not enough to be significant for reasonable variations in component weights.

There's about a 150g difference between a heavy rim (DT585 or Velocity Deep V) and a light but durable rim (Kinlin XR-270/300, Mavic Open Pro although some would argue that those aren't durable enough). There's another 180g separating a heavy tire (32mm wire bead Gatorskin) from a light one (23mm GP4000S). That's 660g for a pair of wheels.

As an upper bound you can treat all that mass as being where the rubber meets the road. With a 75kg rider + 9kg bike combination having that 660g as rotating mass instead of elsewhere on the bike means the rider needs to provide less than 0.8% more kinetic energy to reach a given speed. With an extra 660g in the rims and tires bringing the bike weight to 9.66 kg the rider needs 1.6% more kinetic energy to reach a given speed.

That's just within the accuracy limit of bicycle power meters and the effect on time to accelerate (especially in a sprint) is a lot less because most of your power is going into overcoming aerodynamic drag.

Lighter tires can be easier to accelerate but the physical effect comes from the reduced rolling resistance that goes with having a thinner, more flexible carcass to deform as you roll down the road. The psychological impact from what things sound like changes peoples' perception too.

I was running 1000 gram department store MTB tires on one of my MTBs and recently bought a bunch of 490 gram IRC Mythos tires. The difference in stop and go riding is astounding when you shave a bit over a pound off of each tire.

I don't think it makes much difference if the rider weigths 250lbs to begin with.......

I reckon it would be a drop in the bucket. I try to keep my GVW down to 175-180 unless I'm hauling stuff.

My lighter bikes go faster because they are also more aerodynamic and have lighter wheels and tyres and when I am climbing hills on my road bike I am only moving 165 pounds up that grade whereas when I am riding my extrabike or some of my commuter / utility bikes I am probably looking at moving as much as 200 pounds up those grades and do that on heavier wheels and tyres.

I am one of those guys who could use a bottle full of lead shot for descents.

Unless you are 5% body fat and are getting paid big bucks a light weight bike won't make a difference. Even then the difference is small and not that important because the competition also has lightweight bikes.