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I recently ran into a rider on a bike so light I dare not even mention it here.
You would suggest he wasn't telling the truth and that even my "lift/feel" picking up the bike was totally unreliable.
While not a "every day" bike for training, this bike was out, with other riders on a 20-40 mile ride over roads with imperfections.

But it got me to wondering. I see the road bikes in the gallery and the frames listed.
BTW I have a good friend with a newish Scott road bike, all dura ace etc which comes in about 15.0 pounds flat so I am familiar with the feel of picking up a light bike.

So anyway, this bike I saw and lifted is made with a custom made aluminum frame.
All the tubes carefully selected for a ride who only weighs about 133 to 135.
Those Scott (and others) frames have to be built in such a way that some 240 pound guy might jump on for a ride.

So how light could you make a frame for a "real" bike (not just for show) if you only had to support a 135 pound rider?

The Scotts seem to be listed at about 905g etc. and there is one Giant prototype listed at 804 grams. But those are both carbon.
I just saw this bike so briefly at the top of a climb that I didn't have more than 30 seconds to give it a lift before the guy and his group pedaled off. I couldn't tell you one more thing about the parts etc... Can't even remember the name of the custom frame builder.

I will only say this. As the rider went to hold my bike, and me his, I grabbed it by the stem/handlebar and it almost floated off the ground.
A weird feeling like you get when you go to lift a milk carton that you think still has a quart in it and it really only has 2 ounces......where it kind of just flys up off the table before your mind/muscles adjust.
 

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Remember that if you only drop 30% off your frame, your bike will lose just 6% overall.
Litespeed's Ghisallo has been at 750g and lower already. But somewhere you're going to cross the line of what makes sense. I bet it's possible to make a 500g frame that even holds up, but with current technology it would not make you faster than a 600g frame, au contraire!
My ca. 1250g L TCR feels really slow on me. My ca. 1550g got stolen, and I got a new one, which had been lightened significantly. The was little time between bikes, but I've never been in the lead of a road race ever since, just trying to keep up. For 3 or 4 years, I've been refusing to ride that TCR anymore, my twice as heavy steel Surly frame is way way more capable, and I feel faster. I perform the same as on the TCR, but now on a SS bike that is heavier than the 2x9spd super roadie.

Picture yourself a graph. Lines for the effect of frame weight on performance (climbing/accelerating) and efficiency (stiffness). Somewhere a lighter frame will be just impracticle, and waste of expenses. For each rider different, of course.
My own expereince is that the heavier the frame, the more capable the bike. My 2720g steel hardtail has the best acceleration on any bike I encountered. It has my back, and I feel it.
 

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Custom frames

I know that several custom frame builders will build you a carbon road frame with different diameter tubes. Serotta, for example, has the Meivici carbon frame that they can make for you with different stiffness levels for the seattube, downtube, toptube, and headtube. Using the least stiff (and therefore smallest diameter) carbon tubes available, you could make a bike that would be EXTREMELY litghtweight and ride quite smooth. For a 135lb guy, I'm sure it still wouldn't be stiff as a brick, though. However, I'm sure it would be the smoothest riding bike available! Think of all that flex, not just in the chainstays and seatstays, but in the entire mainframe of the bike!

https://www.serotta.com/pages/meivici.html



Now, is it worth it to make a bike this light? I'm not sure. Serotta gives the least stiff carbon tubes for their bikes a stiffness rating of 4.5 out of 10.5. That means that the small diameter tubes that would be used on such a frame would not even be HALF as stiff as the carbon tubes on their stiffest frame. That's quite a big difference (assuming that Serrota rates the stiffness on a linear scale).

Still, I would think that Serrota could get a custom frame down in the 650g range for a 135lb or less rider. As stated above, you could then safely run a <1000g tubular wheelset as well, like the Extralite Ultralimb SPN at 865g! Those wheels only cost 2049 US dollars! (actually quite amazing for a wheeset that light, BTW).

Completing this bike with a full complement of weight weenine daily riding components, I could see this being a fully ridable, raceable 11lb bike.

Now at 165lbs and a FT power output of 345 watts, there's no way I'd ever ride this bike. But for the ultralight rider who wants a superlight bike, this could be the ticket. Although I'm sure a frame that's under 700g sacrifices climbing and sprinting capability due to it's lack of stiffness, as stated above.
 
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