Weight versus Performance Questions

[chainstay]

These aren't power training questions, but I am posing these questions here since there appear to be some trainers and racers at this forum who might know some actual equations to give me some rough answers. If someone has a better forum for me to pose the questions, please let me know.

When I resumed cycling training a few years ago, I was pretty heavy, so I didn't sweat bicycle weight much when I purchased a new bicycle. Now, several years later, I am 20 kgs lighter than my peak weight when I purchased that bike, and I currently weigh about 69 kgs with about 8% body fat. And I am not really interested in continuing to get smaller and lighter. So now that I have lightened up my body about as much as I want to, I am eyeing my bike and wondering just how much difference (roughly) putting the bike on a diet,or having a lighter bike can make to overall performance? I have participated in a little entry level type racing recently, so it is a question I am interested in.

Let's start with a couple of examples. I recently completed the 206 mile (331.5 kms) Lotoja race, which has about 7,600 feet of ascent, and 5,800 feet of descent, in about 10 hours, 12 minutes. I weigh 69 kgs. My bike with cages, no tools, no water bottles, weighs 8.45 kgs. If that same bike were able to keep exactly the same earodynamic profile, but magically became 7.25 kgs, is there any kind of formula to roughly calculate what kind of time I might be able to save in the 206 mile (331.5 km) race above? Another example: there is a 12 mile climb (19.3 kms) near my house that takes me about 65 minutes to get to the top. Same question---how much time would shaving 1.2 kgs off the bike save me on that 19.3 km climb? And does one give back a lot of the time savings a person gains racing uphill when he has to turn around and race back downhill with less weight?

Are there significant differences in expected time savings between taking rotational wheel weight off versus taking frame and component weight off versus taking weight body weight off?

Sorry to pepper the forum with questions, but I am wondering whether dropping bike weight would make any meaningful difference to an entry level, intermittant, sometimes racer, like myself? I know that bike manufacturers would like me to think so and pony up big dough to save 2 or 3 pounds, but I am skeptical unless I can see some actual, numbers that impress me to back it up. And I full well realize that what may be significant time savings to some higher level racers on here, may be totally insignificant time savings for me. "Significant time savings" is in the eye of the beholder.

Thanks for any answers regarding these questions.

 

[strader]

Probably not as much as you think:
http://www.analyticcycling.com/
http://www.kreuzotter.de/english/espeed.htm

I punched some numbers into the kreuzotter calculator for my 22 lb cross bike. By dropping 2lbs from the bike I would pick up 0.1 mph on a 6% grade. In contrast, by increasing my FTP by 10% (30 watts) I would pick up approximately 1.2 mph on that same grade.

 

[chainstay]

Probably not as much as you think:
http://www.analyticcycling.com/
http://www.kreuzotter.de/english/espeed.htm

I punched some numbers into the kreuzotter calculator for my 22 lb cross bike. By dropping 2lbs from the bike I would pick up 0.1 mph on a 6% grade. In contrast, by increasing my FTP by 10% (30 watts) I would pick up approximately 1.2 mph on that same grade.

Thanks for those links!! That Kreuzotter calculator is great to play around with. Yes, the speed increase for dropping 1.2 kgs on my bike looks very insignifcant, especially for someone like me who undoubtedly has a lot of room to make significant gains in FTP. This is exactly the kind of information I was looking for. I will check out that analytic cycling link as well. Thanks again.

 

[millzebub]

Sorry to jump in here, but I input what appears to be the same data at both sites and came up with different wattages. Maybe I am omitting something but when I put a 4% grade with the same weight and height at sea level, I get two different wattages. At 12.5mph on the kreuzotter page it says I have 315 watts, and analytic says 271. Does anyone know why there is that much of a difference?

Probably not as much as you think:
http://www.analyticcycling.com/
http://www.kreuzotter.de/english/espeed.htm

I punched some numbers into the kreuzotter calculator for my 22 lb cross bike. By dropping 2lbs from the bike I would pick up 0.1 mph on a 6% grade. In contrast, by increasing my FTP by 10% (30 watts) I would pick up approximately 1.2 mph on that same grade.

 

[strader]

Both sites are using a best estimate of other factors: frontal surface area of rider/bike, aerodynamics of rider/wheels/bike, tire rolling resistance, coefficient of rolling friction for road surface, air density relative to altitude/temp/humidity, and a multitude of other factors.
Where the calculators are useful is when comparing performance differences when changing a single variable (i.e. bike weight) and leaving everything else equal. I went through the same decision when purchasing my last mtn bike. According to the calculators buying the $3k more expensive, and 1.5kg lighter bike, would reduce my 2 hour race time by about 30seconds on a good day. I decided to go with a mid range $2k bike and save for a Powertap.

 

[asgelle]

Both sites are using a best estimate of other factors: frontal surface area of rider/bike, aerodynamics of rider/wheels/bike, tire rolling resistance, coefficient of rolling friction for road surface, air density relative to altitude/temp/humidity, and a multitude of other factors.

A clarification. Analyticcycling does not assume values for any of the parameters. They supply default values, but users are free to supply their own, more accurate values.

 

[chainstay]

I have been fooling around with that Kruezotter caculator. For trying to determine what changes a 1 kg drop in weight would make for that 206 mile race above that includes three mountain passes, I had to make a lot of assumptions for the inputs, which hopefully are roughly correct and plausible. Now I am thinking that shaving 1 kg off my equipment could at best save me somewhere up to three minutes. This would entail inreasing my overall average speed from 20.2 mph to 20.3 mph. I notice on this Kruezotter that when I start shaving weight off my bike nothing happens to my overall time for quite a while as I lower the weight, even at 2.2 lb savings there is nothing, but when I hit 2.5 lbs in weight savings, I suddenly get a little over three minutes in time savings. That is why I said "up to three minutes" in time savings.

While up to three minutes sounds like a lot, at my lower racing level even a full three minutes doesn't gain me much standing on a 206 mile race, and I have to agree with Strader's comments above, and conclude that I would be better off putting the money towards a power tap and good training programs to try to move up my sustainable watts ouptput. What is more, I can relatively inexpensively take 350 grams off my current bike with a rear wheel warranty replacement upgraded to next level, cassette upgrade to DA and a saddle upgrade. I still ride one of those heavy old Trans Am Max saddles. The cost per gram to get futher weight savings, which realistically means purchasing a new bike, isn't worth it to me at the moment. I think I will probably continue on with original plan to ride this bike for 20,000 or so miles and improve my sustainable watts output before looking at a new one.

Thanks for pointing out those calculators to me though. I still need to familiarize myself with the ones at analytics cyling.