Bike Geometry/Weight Distribution: Origin of the 45/55 rule???- Mtbr.com

# Thread: Bike Geometry/Weight Distribution: Origin of the 45/55 rule???

1. ## Bike Geometry/Weight Distribution: Origin of the 45/55 rule???

So I've been interested about bike geometry with particular interest in weight distribution lately but I've only explored internet resources thus far. I have read, that generally speaking, having 45% rider weight on the front wheel and 55% rider weight on the real wheel is optimal in terms of efficiency, traction, and fung shuei, oh and weight distribution. But I have not been able to find any study or origin from which that rule came despite it's common acceptance. Anyone know, have ideas, or recommended reading? Thanks.

Happy Trails

Taylor

2. Can't say I've seen much about this rule, and haven't spent time thinking about it. I move my weight around on the bike, and tend to think about it in terms of balance between the wheels, and being able to move that point front and back fairly regularly as needed, depending on terrain. I suppose in a static pedaling position on the flats it might be fair to say it's around 45/55, though. I don't know how I could test it without an industrial scale I could trackstand in a relatively neutral position...

3. ## Two Scales

We used to measure the weight distribution of motorcycles using two scales. One uner each wheel.

4. Thanks guys, I guess the thing is that the 45/55 rule is pretty much the industry standard for building bikes both mtb and road. So pretty much I think it has to do with where the frame builder/designer places the seat, handlebar, and pedals with relation to the two axles or tires. And I think it's based on if the average person was sitting on the bike on flat ground sitting on the saddle and whatnot on a flat surface.

5. And why you need to experiment with your stem length/rise, hand and seat position to achieve your personal feng shui, something the designer can't do for you unless you're that ideal average person a particular bike is designed for.

6. Actually I'm not that interested in tricking out my own ride or anything I just wonder if you're Lance Armstrong cranking up a hill that's ten miles long at 9000 ft and a 8% grade, the geometry of the bike could play a very significant part in efficiency when a race or climb comes down to seconds, it could work the same way going downhill as well.

7. ## 45 / 55 ???

Thats odd; the rule I've allways heard about was 30 / 70%.
I can even tell you, that when I called Manitou back in Feb 06, when I was setting up a new San Andreas, ( I didn't want to try half the springs on the planet ) I talked to an engineer named Ed that asked me my weight and the leverage ratio of the frame. He then said that about 70% of my weight was about right on the rear wheel, then did the calculations based on the 25 to 30% sag that I wanted with an appropriat amount of preload ( 3mm ). Manitou sent me a spring that was dead on in spring rate for my riding style and weight, so the guy had to know what he was talking about.
Could this 45 / 55% rule be for road bikes?

Later, Eric

8. [QUOTE=Ericmopar]Thats odd; the rule I've allways heard about was 30 / 70%./QUOTE]

FWIW, I always go with 35/65 for rear spring sag calculations. Those are the most common values I've come across in spring rate calculators.

9. Yes,i would guess that the mentioned 45/55 rule would apply to road or xc race given the trend of todays frame designs being bias towards all-mtn which puts the majority of the riders weight on the rear.
I'm fairly strethced out on an xc bike and did a caculation with two scales and came up with 60/40 and thats about all the pressure i would want on my hands.
Interesting topic though.

10. There is not a single mass-produced bicycle company that pays the slightest attention to weight distribution.

If they did, chainstay lengths would increase for larger frame sizes just as top tubes do. As it is, taller riders have their weight farther back than shorter riders, because no matter how long the top tube gets, the chainstays stay the same length.

This is because bicycle geometry is driven by fashion and uneducated pronouncements from bicycle magazine writers, not by physics.

11. Originally Posted by tmabiker
So I've been interested about bike geometry with particular interest in weight distribution lately but I've only explored internet resources thus far. I have read, that generally speaking, having 45% rider weight on the front wheel and 55% rider weight on the real wheel is optimal in terms of efficiency, traction, and fung shuei, oh and weight distribution. But I have not been able to find any study or origin from which that rule came despite it's common acceptance. Anyone know, have ideas, or recommended reading? Thanks.

Happy Trails

Taylor
As with other "rules" of bike fit and geometry I think they were developed by surveying how experienced and successful riders sit on their bike then figuring out how to make a "rule" out of it.

In other words: The experienced riders find the positioning that works for them, then later someone else tries to explain why it works.

12. You know I've also wondered about that - how come the TT gets longer, but the chainstay stays the same length no matter the size. Always thought that was weird and wrong and as someone who rides a Xl frame I wouldn't mind a longer chainstay.

Originally Posted by El Caballo
There is not a single mass-produced bicycle company that pays the slightest attention to weight distribution.

If they did, chainstay lengths would increase for larger frame sizes just as top tubes do. As it is, taller riders have their weight farther back than shorter riders, because no matter how long the top tube gets, the chainstays stay the same length.

This is because bicycle geometry is driven by fashion and uneducated pronouncements from bicycle magazine writers, not by physics.

13. I don't see how it could be anything more than a generalization/starting point in proper bike set-up. Everyone is built differently and has different preferences toward bike positioning. The numbers to me would suggest a more aggressive position like a racer would use.

14. Originally Posted by LyNx
You know I've also wondered about that - how come the TT gets longer, but the chainstay stays the same length no matter the size. Always thought that was weird and wrong and as someone who rides a Xl frame I wouldn't mind a longer chainstay.
I'm convinced this is one reason why a lot of taller riders like 29ers...it forces frame makers, kicking and screaming, to add an extra 31.8mm of chainstay length. Although many of them are bending seat tubes to try and get around that...or just stuffing the tire so far back that you can't run decent rubber in the rear because it hits the front derailleur. The "chainstays must be as short as possible, even for tall riders" myth is incredibly tenacious.

If you want a hardtail and have \$700, you can go custom. Spicer or Clockworlk will both do a steel geared frame for that price.

15. ## Amen Brother...

Originally Posted by El Caballo
There is not a single mass-produced bicycle company that pays the slightest attention to weight distribution.

If they did, chainstay lengths would increase for larger frame sizes just as top tubes do. As it is, taller riders have their weight farther back than shorter riders, because no matter how long the top tube gets, the chainstays stay the same length.

This is because bicycle geometry is driven by fashion and uneducated pronouncements from bicycle magazine writers, not by physics.
I've tried to tell people that the larger frame sizes should have slightly longer chainstay lengths, but they don't understand the concept of Center of Gravity.
The best demonstation I've been able to come up with, is to show the bike in a 30 deg. nose up attitude and show them how the taller seat height ( used by taller riders ), in relation to the bottom bracket, plus the larger rider's butt overhanging the back of the seat, puts the rider's weight close to or even behind the rear axle, which makes the bike want to "loop out" on climbs a lot easier.
You can watch smaller men and women on the same kind of frame, on the same percent grade and notice that their bodies are well forward of the rear axle path, becuase the seat is lower for shorter persons. ( Remember to visualise the bike at a 30deg. angle.) This puts a lot more weight on the front wheel. Its one of the reasons smaller riders can climb so much more efficiantly. Its not just that they are lighter, its also because their Center of Gravity is in a much better spot. I think it might also be the reason, that all of the riders I've meet, that like 29ers, are taller. Those bikes usually have longer chainstays to accomodate the larger wheel size.

In addition to El Caballo's reasons, I believe that the chainstay length is the same for all frame sizes, to make production cheaper and simpler. That way, they don't have to waste time, making sure that unappropriate seat and chainstays parts aren't attatched to the wrong main frame.

Later, Eric

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