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MountainBikeReview.com's Forum Archives - Save some Weight
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Half a pound off your bike, or 1 watt off your tire? Choose (5 posts)
|  Half a pound off your bike, or 1 watt off your tire? Choose | Chester
Oct 20, 2003 12:55 PM | | Suppose someone asked you if you would rather be able to drop half a pound of weight off your bike (227 grams) or lose 1 watt of rolling resistance off of just one of your tires.
I suppose everyone goes for the half pound of weight, right?
Well I just ran some numbers on the analyticcycling site and came up with interesting results.
Using the following imput
250 watts
72 kg (bike+ rider) I'm skinny
.03 grade
3200 meters climb (two miles)
.012 surface resistance (fire trail?)
OK...First I added 18 watts of power to see what that would do versus 250 watts of power. (I realize this is not percisely the same as lowering the watts on your tire but its close enough for our purposes)
Adding 18 watts increased my speed per second such that I was able to drop my time from 7 minutes 50 seconds to 7 min 27.5 seconds.
Thats a drop of about 22.5 seconds.
Then we go to another calculator and see how much weight that same person (defined above) would have to drop in weight to achieve the same drop in time. The figure I arrived at to drop 22.35 seconds would be almost exactly 6 kg or about 13.2 pounds (211 ounces)
So what we have is a situation where 18 watts gives you the same performance boost as 211 ounces....or....1 watt equals about 11.7 oz.
Thus losing half a pound...8 ounces will not inprove your time in this situation as much as dropping 1 watt off just one of your tire's rolling resistance.
Of course when you are talking about a set of tires and dropping several watts per tire, then you are talking about pounds and pounds of weight loss to achieve an equal improvement in performance...."all other traction matters being equal"
So there you have one perspective on common sense assumptions versus the actual (theoretical) reality.....(can you say that?)
Real world situtations will always be a bit different but for the most part the formulas do indeed reflect real world results, such that when the indications are so clear and overwhelming you must listen to them rather than following long held beliefs to the contrary. |
| in my case I'd choose | heatstroke
Oct 20, 2003 8:10 PM | | the weight.
I cant sustain 250 watts, I am around 185watts on average.
anyway my numbers 90 kgs
90kg flat ground, 185 watts (38.2 watts rolling resistance), speed = 35.4kph
Reduce the rolling resistance by 1 watt (by changing the coeff. of friction), my speed increases by 0.1 kph.
Add 2 degree of grade, with the same power, my speed drops to 23.6 kph. Reduce 0.5kg ( about 1 lb) and my speed climbs to 23.7kph.
If I reduce rolling resistance by 1 watt instead, then my speed is also up to 23.7 kph.
Anyway, it might be better to do your calculations assuming a percentage change in rolling coefficient of friction. Because with different grades & fixed power, you'd have to manipulate the coeff differently at the different grades to come up with the 1 watt drop ( because your speed changes)
i.e at 2 degree, I would have to change the rolling coeff by 5% to save 1 watt, but at 5 degree I would have tp change it by more than 10% to save a watt. |
| Look I understand it can get complicated, but the point is | Chester
Oct 20, 2003 8:54 PM | | It was a little hard to follow exactly all the changes you were doing compared to my example. I do agree that when you begin to change several factors things get a bit more complicated....BUT the point I was trying to make is that even tiny changes in watt reduction of your tires is far more important than many rider can imagine.
Perhaps 1 watt is not equal to 11 ounces of weight loss but it is "worth" far more in performance than most people realize COMPARED to efforts at weight reduction.
I was using a almost absurd "micro" example of reducing your entire rolling resistance by only 1 watt. That was to make an extreme example.
Whether it was equivilent to 11 ounce or 8 ounces is something we can debate depending on the circumstance but it is still far more effective to drop rolling resistance than it is to drop weight.
Certainly in terms of dollars. Of course if you have already maximized your tire situation then you have to look elsewhere.
I would be willing to look at your numbers more but I didn't quite understand what you were saying...such as
"90kg flat ground, 185 watts (38.2 watts rolling resistance), speed = 35.4kph"
What do you mean by 185 watts (38.2 watts rolling resistance)? How did you calculate your "rolling resistance" to come up with 38.2 watts? Formula? or something on analyticcycling? |
| I used a program called : | heatstroke
Oct 21, 2003 9:29 AM | | powercalc - excellent piece of software. All numbers I crunched were default except for gradient, weight and coeff of rolling resistance.
Basically I had to tweak the coeff. each time I changed the grade & weight to make sure I had 1 watt reduction in rolling resistance.
I agree with you that 1 watt is significant, and I am sure that tyre choice can make a huge impact. I just read on one of the posts that switching to tubeless (stans) can save 13watts rolling resistance ? I would like to find out where that number came from.
It would be nice to caluculate some real numbers based on real MTB tyres and then calculate your theoretical speed based on a fixed power input. If I do then I'll post something. I'd be interested to see some numbers if you crunch thme.
here os the link :
http://www.machinehead-software.co.uk/cyclists_power_calculator.html |
| Thats a interesting program.Thanks....and here are some numbers | Chester
Oct 21, 2003 10:22 AM | | Thats a very interesting program you have linked me to. I've been using the analyticcycling.com programs which can be used without downloading. I'm not a math wiz but I do understand that when you begin changing imputs lots of other things happen. Sometimes when I am adding or subtracting watts and climbing degrees, I realize that its not the precise method to be exact, but many times it comes fairly close to what the scientifically correct method would actually give, and after all these are just reasonably close approximations of all the real world factors anyway....down to and including the loose gravel or mud on the road on a given day.
Soon perhaps one of these cyclocomputer companies will offer a large handlebar mounted screen that will include all the detail seen in the "screen shot" of that program you are using.....then I will be able to ride down the road reading hundreds of changing numbers in real time....at least until I crash from not paying attention to that tree.
In case you have not already seen the follow data,
Here are some previous posts with the real mtn bike tire results. I'm not certain this is what you are looking for. Hope you can read a bit of German to understand the BAR (psi) used as well as the speed and load factors. Of course each of these tests must be taken independently as the conditions from one set of test results may be different than another....so it is all relative to the group of tires being tested in a given situation.
The first site I give you will also include a test of a tire using the Eclipse tubless system which should be fairly similar to Stans.
The only problem is that I can't be exactly certain which of the two listed Racing Ralphs they are using the Elipse system on. So I can't tell if a set would give you 6.8 watts less or 10.4 watts less. Nino who posts here uses the figure of 9 watts less per set. While I suppose Stans and Eclipse systems are different, I would have thought the watts reduction would be fairly close. I've never seen anyone mention that 13 watt figure. I assume it was for set of tires.
nino "I just had a big laugh when going through MBA..." 7/27/03 1:28am
Here is some other data from another post with some different tires.
http://forums13.consumerreview.com/crforum?13@59.guNwa573tAI^21@.ef6a2fc/0
There are some more sets of results available is you don't find a tire you were looking for. |
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