Weight difference
 

Would an average rider riding 50-75 miles a week in semi hilly terrain see a noticeable difference going from a 34 lb bike to say an 18-20 lb bike?

Losing 14-16#? I'm going to say that would be noticeable. You could try strapping 14# to the bike for a week, then taking it off to see what it might feel like.

Yes

Thanks!

Yes absolutely !!

More than you think. The lighter bike is likely to have much lighter wheels which physics says requires less effort to move from a stop plus the components like bearings and hubs are going to be better so the frictional losses from these components are less. I sometimes ride with kids who own cheap bikes. They can be pedaling down a hill and I coasting alongside and I will pass them up even when the rider+bike weight is closely matched. It is the combination of narrower tires and better wheels that makes that possible.

Yes, Newtons law..
Newton's laws of motion - Wikipedia, the free encyclopedia
It is simple

1) Remember it is total weight that matters (rider+bike+crap). So, depending on how much you weigh, the effect may not be huge. 12 lbs may only be a 5% effect and that 5% is only for very steep hills, not flats and it's a smaller effect on rolling hills.

2) Wheel weight makes almost no difference for the wheels commonly used.

3) What does make the biggest difference at lower speeds is better rolling tires (this is what many people think lighter wheels buys them is really this).

4) At higher speeds, aerodynamics makes the biggest difference (a good rider in an 70 lbs velomobile will be significantly faster than on an 18lbs bike on rolling hills). Likewise, having a drop bar bike is likely to have an advantage over a flat bar bike.

I agree with cplager but there's also this:

Acceleration is where low weight benefits the physics most. Every ride starts at 0 MPH so you'll notice a 15 pound weight difference in the first 1/4 mile that you ride the bike.

15 lb is a huge difference.

On the flat, the light bike will accelerate faster and with less effort (from a stop, or speeding up while on the move) and will be easily flicked from side to side (e.g. to avoid a pothole) and unweighted/hopped (e.g. if going over that pothole). Climbing hills, its hard to imagine anyone who wouldn't notice hauling the weight of two gallon jugs full of water, and that is about how much weight we're talking.

Also very important: the type and quality of bike that is 18-20 lbs versus the type and quality that is 34 lb. Unless the 34 lb bike is a 20 lb bike weighed down with 14 lb of stuff, which raises other issues!

I agree with everyone on here. The heavy bike will start out slower, but once you get it going, you could go as fast as someone lighter. But there can be a huge difference in tires as well. If you're rolling on knobbies, it'll take more effort to maintain speed. I'm sure going up hills you'll feel a difference, how much? IDK I guess it depends on you. I say that if you feel good about the bike you're riding, then perhaps that will translate into you pedaling harder and going faster. I know everytime I change something on my bike, I think, WOW, my bike is so much better and I ride faster....it's all psychological I'm sure, LOL.

I swapped out my stock boat anchor tires to some Scwhalbe Marathon Supreme in the same size and for me, it seems like I can maintain speed easier as well as accelerate quicker. I think that small drops in weight and you won't notice a difference. I put my bike on a diet and it lost close to 4 lbs. Could I tell a difference? On climbs it seems like it's easier, but I can't tell if that's because I'm getting in better shape or if it was the bike. Perhaps both?

Load a backpack with 15 pounds of stuff and walk around for a few minutes. I'm sure you'll feel a big difference when you remove that backpack.

I'll be a bit contrary on just one aspect: all the accelerations on more or less level ground. If you're riding with people with lighter bikes and you're matching their surges I think you'll fatigue more, but otherwise it's not noticeable with respect to overall effort. Yes you can feel a faster acceleration from start, but what it really amounts to with the heavy bike (once you get acclimated to it) is a longer period of acceleration at the same effort to get up to speed. Slightly longer, slightly more energy used. Over the course of a ride the difference from just this part is trivial.

Try it in a race. It's the repeated accelerations that kill you.

I meant for "riding with people with lighter bikes and you're matching their surges" to generally include racing. Sorry if I didn't make that clear. I'm sure that it would fatigue you more. Other than that, even in a time trial, I don't think that you'd find the difference significant. Not having to match an acceleration (to draft for instance) you'd just accelerate for a fraction longer, lose a tiny amount of time and have a small amount of extra effort.

Wrong. The amount of energy needed to spin your wheels is trivial. Common sense can quickly prove this to you: hold the back wheel off the ground, spin it up to super speed, and then hit the brake - stopping will be instant.

It's the weight of the bike and rider COMBINED that matter. So 10lbs for a 200lb rider+bike will only reduce acceleration by 5% at most. In practice, a lot less - because most work will be against resistive forces even while accelerating. So this is the type of stuff that can matter in a race, but it's small by other standards.

Yes. But you won't walk slower because of the backpack, which is what's relevant here.

At moderate speeds rolling resistance will be 20-50% of resistive forces. Swapping OEM boat anchors for Supremes might HALVE rolling resistance - they're an excellent tyre that combines race tyre technology with a kevlar anti-puncture belt. This will make a much bigger difference to speed than a 14lb weight loss.

My present bike is 3 kg lighter than my previous bike.

I notice that I accelerate faster, my overall speed is greater, and I can bike longer distances easier.

Comparing apples with apples, you can throw a lighter apple further with less effort.

Do we really need to have such a long thread on this?

Your lighter bike probably has faster tires. So not an apples to apples comparison.

And you can throw a heavier apple further than a light one (momentum versus air drag).

Anything else you wanna get wrong today? :)

Only if your throwing arm can launch the heavier apple with the same velocity as the lighter. Otherwise, you're back to apples and oranges.

In addition to the points cplager made, I have to tell you that - unless you are EXTREMELY weak or are hurling Mutant Fruit - you'll throw a heavier apple further. (It will have a smaller surface area relative to mass and therefore air resistance will have a less rapid effect - this is why you can throw a coconut further than a raspberry.)

Also: you don't "notice" that you are accelerating faster. What you mean is that you THINK you are accelerating faster. There's a decent article by Keith Bontrager on subjective impressions on speed and acceleration in testing bicycles and motorbikes; Bontrager's experience is that even experienced professionals usually got this wrong.

Oops - cplager had already got that one. My mistake!

As for you, Nermal... Time to put the bike away and spend some time with the kettlebells.

Wrong. I noticed a world of difference swapping tires on my bikes. In one case I went from 1.95" Crossroads to 2" Marathon Supremes,in another I went from 45mm Borough XC's to 42mm Contact Lites;both were much more pleasant to ride in stop-and-go traffic,and were easier to climb up steep hills with the much lighter tires.

Huh? First,you just proved yourself wrong. You actually had to put effort into spinning that wheel up,while hitting the brake required almost none. Second,what does this have to do with riding the bike? In that example,there's only the momentum of the wheel spinning,when riding the bike you have the weight of the bike+rider,plus gravity if going down hill to deal with.