Speed of 25 lb. bike vs. 18 lb. bike Help Settle a Fight!!
 

Hello,

Please help settle a ongoing heated debate between a me and my friend.

Here's the deal.

We ride a 40 miles on a bike trail twice a week together.
We avg. 21 mph the whole way breaking our own wind.

The deal is this...

I ride a 25 lb. aluminum bike with sora componants, stock, cheap rims.
He rides a 18 lb. titanuim Airborne Zepplin with Mavic Krysiums SSCs and Dura Ace componants.

In a couple of weeks, I'll be upgrading also to a the exact same 18 lb. Ti Zepplin with Krysiums rims and Dura Ace.

The arguemnent is in regards to how much faster I will be on my new bike expending the same amount of energy.

My friends contention is that the new bike will ONLY make me 1/4 to a 1/2 a mph faster.
I disagree with him, I believe I'd be faster than that.

How much do you think the new bike with make me faster?

Thanks very much!

Lance from Chicago

I think your buddy right on this one! Although a lighter bike will make you faster, it will not be considerably faster. You may FEEL faster due to the lighter weight, but once you're rolling the weight is less significant.

Acceleration will significantly improve.

The only way to really compare the two is to do a Solo Time Trial in the same conditions. Same headwind, similar Temps...etc.

If you're taking turns pulling, your average speed shouldn't change much!

L8R

Bike weight matters mostly during acceleration and climbing. If you're riding a flat route, your upgrade probably won't improve your speed by a whole lot. Hard to say how much. I'd guess 1-1.5mph at most. (That would be an immense difference to a professional racer, who are the people who really need to care about this stuff.)

Much of that improvement will come from lighter wheels and tires. Some of it will come from a combination of many other little things: the more vertically compliant frame may cause a bit less fatigue; the reduced weight will help on climbs; the 9-speed drivetrain may help you optimize your cadence a little better.

And there's also the feeling that when you ride a Zep, you just have to go faster.

Enjoy your new bike!

RichC
(Airborne Carpe Diem owner)

How many hills on your ride? How often do you stop and start?

Weight makes the most difference in climbing and accelerating, and much less of a difference at a steady speed (an object in motion tends to stay in motion). On the flats at 21 mph much - even most - of your effort is spent overcoming wind resistance, and not keeping an extra 7 lbs. moving forward.

On a generally flat ride with few, or no, stops the dropped weight will make you a little faster, but not as much as you think, so your friend may be right.

The more climbing and accelerations (sprints, traffic signals, stop signs), the more the lost weight will help.

Years ago when I upgraded from a 31 lb. x-mart road bike to a bike in the 23 lb. range I was generally disappointed in my overall speed. I climbed much better, but on the flats I rode at virtually the same mph.

Your overall performance and "feel" will be so much better that it will not matter to you that it is not that much faster on the flat.

What I would like to know is how you both break wind avg. 21mph, how many farts per mile is that?

At least you guys are breaking your "own" wind,, not each others.;)

oooook....i think its time to hit back........:crash:

Loook at it this way... If you weigh, say 160 lb... the totals for bike and rider will be:

Old bike: 185 lb
New Bike: 178 lb

That represents a 3.8% weight reduction. I'm not sure that this will significantly improve your speed. What it WILL do is reduce the amount of power that you have to apply to overcome your inertia; this means that you will very likely notice improved acceleration, but once you get up to speed on the flats, I don't think you'll notice much difference at all.

The real advantage in the new bike will not be the overall weight savings, but overall greater stiffness, for better energy transfer, thanks to newer and stiffer frame materials, and a reduction of rotating weight in the lighter wheels that should aid acceleration and improve [somewhat] climbing.

Okay here’s a quasi-scientific answer. I happen to have the needed equations sitting in a little Excel sheet.

First, on the theory. As others have already said, the weight matters with acceleration and climbing. It factors into rolling resistance, but that becomes a negligible part of the total resistance once you reach any appreciable speed. Acceleration is too hard to calculate, so we’ll ignore it. We also have to make assumptions about your frontal area, but since only your weight changes, it won’t matter. (I am assuming 0.4 meters squared). For the rolling resistance, I also assume a very low frictional coefficient (.004), one often cited for very good road tires. I also ignore the 3-5% efficiency lost from resistances in the drive train (grease, bearings, etc.). My equations are set for metric, so I’ll convert where necessary, and I round most numbers.

So, we’ll say you weigh 160 lbs. making the old bike 185 lbs (84.1 kg) and the new one 178 (80.1 kg). If your 40-mile (64 km) course were entirely flat, at 21 mph (33.8 kmh) on the old bike you’d be putting out 233 Watts (202 against Air; 31 against rolling resistance; 0 against gravity from the slope). With the same number of Watts, you would then be able to go 33.9 kmh—about 21.06 mph. A massive increase! Your buddy is right here—provided it is a flat road.

But, now let’s say you guys do an 8 km (5 mile) climb with a 5% grade. You pump out the same 233 Watts. On your old bike, you would go up the climb a bit faster than 16.8 kmh (10.4 mph)—requiring about 28 min, 34 seconds. On the new bike you could climb it at 17.4 (10.8 mph)—requiring only 27:35. You save nearly 1 full minute. You are going to toast your pal, who is conditioned for a lighter bike—i.e. he is a bit weaker than you are. By the way, the Watt distribution for the resistances on the climb is interesting: 28 for Air; 15 for rolling resistance; and 190 for gravity from the slope (for the new bike).

But, you have to come back down the climb. On the old bike, you’d be descending at about 53.4 kmh (33.2mph); on the new bike 52.6 kmh (32.7mph). You’d cover the descent in 8:59 on the old bike and 9:08 on the new bike, losing 9 seconds because of the lost weight. Yes, indeed, heavier cyclists do descend faster. But, your overall savings for this ten-mile climb and descent was 51 seconds. Not bad for a 37 min ride. By the way, for the new bike the Watt distribution on the descent would be: 761 for the Air; 46 for rolling resistance; and –572 for gravity (i.e. gravity gives you the bonus you need to overcome the air resistance).

I’ll leave it for you to decide whether you or your buddy has won the argument.

Cheers,
Jamie

prediction: you will get your new bike and ride more often because it is your new bike..... and get faster that way.
i counter that acceleration is not just a function of weight. power transfer can also be found in the drivetrain, wheelset, etc

now a real experiment would be putting your buddies wheelset/tires on your old bike and checking out rolling resistance, etc.

at any rate.....enjoy trying to find out which is faster.

It depends, if you pump up the tires all the way, you will go faster

You'll want to cane your mate's arse on the new bike (just to prove him wrong of course!) and will probably work much harder to effect a bigger difference.

jmlee,
:thumbup: I want software to make those cool calculations! :thumbup:

First, on the theory. As others have already said, the weight matters with acceleration and climbing. It factors into rolling resistance, but that becomes a negligible part of the total resistance once you reach any appreciable speed. Acceleration is too hard to calculate, so we’ll ignore it. We also have to make assumptions about your frontal area, but since only your weight changes, it won’t matter. (I am assuming 0.4 meters squared). For the rolling resistance, I also assume a very low frictional coefficient (.004), one often cited for very good road tires. I also ignore the 3-5% efficiency lost from resistances in the drive train (grease, bearings, etc.). My equations are set for metric, so I’ll convert where necessary, and I round most numbers.

The previous email contained a quote, I mistakenly did not give credit to jmlee...

Russell. (sorry)

No need to apologize.

You may download my wattage calculator sheet at http://www.uni-bonn.de/~jmlee/. It's at the bottom of the page, below all the cadence stuff.

Note that it is an Excel worksheet, so you have to have Excel to use it (sorry about that). But, my equations are there, so you should be able to implement your own version in a software of your choosing. Please read the instructions. Have fun.

Cheers,
Jamie

If you want to see the effect of weight (and exclude all the other variables like wheels), then get some old waternbottles filled with lead or steel bolts.
Using your new bike, ride a 1/2hr circuit at a constant power (ie constant heartrate), with and without the extra weight.
If you do 3 circuits for each weight you may be able to detect an difference in the average time it takes.


Dont ride as fast possible, or at constant speed, but at constant POWER.

Some good worksheets at

http://www.analyticcycling.com/

Just about ANYTHING you would want to calculate about performance

I have not read all the replies but I am sure that they say some what of the same thing. You will not increase your speed all that much. The lighter bike will however, accelerate faster, climb easier, be able to flick through tight turns better and help you ride longer using less energey which in turn will give you more energy to pedal harder at the end of your ride. Since you are used to riding a heavier bike the lighter bike will make it easier to spin the pedals.
Congrats on the new bike
Slainte:beer:

Some interesting discussions and l pretty much agree with the consensus. Weight matters only for acceleration and climbing.

But to mangle a quote 90% of cycling is half mental.

I have seen people go from an OK bike to a state of the art bike and not notice any effect. Heck, I had a friend do this and he seemed to get WORSE! But I have seen other people get a nice little performance boost. I think in these cases the people like the responsiveness of the bike and the faster acceleration and that can have a noticeable effect on average speed if there is much starting and stopping or if you are in a group that likes launching attacks to drop people.

Yup. It's pretty hard to disagree with physics--one can try, but physics will always win.

But, Pat raises an interesting point. I recall Andy Hamstead saying something like "it's the rider that matters, not the bike."

The simple fact is that the quality of our bikes generally says more about our spending habits than our riding ability. A geared-up, enthusiastic rider can make major progress on a lower end bike. The person who started this thread was moving from a 25 lb bike to an 18 lb bike. He had been keeping up just fine with his buddy who was already on an 18 lb bike (I hope he challenged him to that hill climb I hypothesized).

Cheers,
Jamie

OOOHH...you're in Chicago?? I want to see that Zeppelin! Get your friend to reply to the thread about Airbornes in this forum (you too once you get yours) ;)

What time do you guys ride? I will camp out by the beach, waiting for you to speed by. I can get a good look at your bikes, and judge who's the fastest :D