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Discussion Starter · #1 ·
I have been trying to get my head around the reviews of bikes. In particular, I am confused when the writer explains that the frame is super stiff when you jam on the pedal, thanks to oversized bottom bracket and headtube, but in the next sentence explain how the frame will flex due to thin seat stays and shape of the chainstays.

It must have to do with how pedal forces impact the front triangle and not the rear, while bumps predominantly impact the rear. However, I am having trouble picturing this. Could someone kindly enlighten me? I would think that if you crush down on the pedals, the rear would have to give if it is designed to flex.

Thanks

Pete
 

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It's mostly marketing BS but if you watch the angle your pedal stroke moves, or would move, the frame then imagine how it would move from hitting a bump on the bottom of the tire they are forces coming in from different angles so theoretically different flex/stiff is possible.
 

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It's because people have a very stiff frame, but have lower tire pressure. This gives a bike a very stiff feel when accelerating, but smooths out the bumps due to lower tire pressure.

That and the fact that people just spent several thousand dollars on a new bike and it's perfect...better than any bike they have ever ridden. Basically justifying their purchase, even though the ride isn't much different than a previous bike, but the fit may be a whole lot better this time.
 

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Think of the bottom bracket axle as a bar with a lever attached (crank arm) that your leg moves side to side while pedaling. If you beef up the Bottom bracket area of the frame, the crank arm axle housing if you will, then that bar shouldn't sway back and forth as much. This involves the chainstays also since that whole contraption works together, being that the rear wheel with the drive train is attached to them via the chain. So you make this lowest part of the bike really strong to prevent forces from pedaling input.

Next, if you look at many carbon fiber bikes nowadays, the seat stays are crazy thin. Since your butt supports most of your weight on the bike, you have to adjust the frame to allow shock absorption for your rear, for comfort. So the thinner seat stays are supposed to provide this, along with carbon fiber manipulation along the seat tube and the rest of the frame.

All in all however, we're talking really small improvements here. Pros will notice and deserve the improvements since they are in peak physical condition, but they also get the stuff for free. If you are not a pro however, in my opinion, any bike that fits well and makes you happy is fast enough. Improving the engine always trumps bike tech. Otherwise it's like throwing a carbon fiber spoiler on a stock 4 door Honda Civic. May look cool, but it does nothing for the performance compared to a better engine.
 

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Discussion Starter · #5 · (Edited)
mitmoned said:
Think of the bottom bracket axle as a bar with a lever attached (crank arm) that your leg moves side to side while pedaling. If you beef up the Bottom bracket area of the frame, the crank arm axle housing if you will, then that bar shouldn't sway back and forth as much. This involves the chainstays also since that whole contraption works together, being that the rear wheel with the drive train is attached to them via the chain. So you make this lowest part of the bike really strong to prevent forces from pedaling input.

Next, if you look at many carbon fiber bikes nowadays, the seat stays are crazy thin. Since your butt supports most of your weight on the bike, you have to adjust the frame to allow shock absorption for your rear, for comfort. So the thinner seat stays are supposed to provide this, along with carbon fiber manipulation along the seat tube and the rest of the frame.

All in all however, we're talking really small improvements here. Pros will notice and deserve the improvements since they are in peak physical condition, but they also get the stuff for free. If you are not a pro however, in my opinion, any bike that fits well and makes you happy is fast enough. Improving the engine always trumps bike tech. Otherwise it's like throwing a carbon fiber spoiler on a stock 4 door Honda Civic. May look cool, but it does nothing for the performance compared to a better engine.

That's a great explanation, thanks. I have been riding a Giant OCR for three years now and I find it relatively comfortable. My longer ride are approximately 65 to 70 miles. I did my first group ride last week and had an absolute great time. We averaged almost 21mph, which is fast for old body. It was fun to pull a bit and then sit back and recover behind four or five guys. My bike is stock, except for the seat and I certainly did not feel I had to work any harder due to my bike. I'm sure it was one of the heavier ones of the bunch, but a couple pounds cant make much of a difference. Regardless, it started me thinking about the benefits that the folks had around me, since some of the had some pretty nice bikes.

I'm guessing that my frame is relatively stiff. I run tire pressures of 95 lbs (I weigh 163lbs) and it certainly helps a lot with ride comfort.

So, now that the frame issue has been resolved, let's talk cranks. I'm sure I have a low end square taper crankset. Does square taper have more flex in it than the external bottom brackets that most bikes come with these days?
 

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peterk said:
So, now that the frame issue has been resolved, let's talk cranks. I'm sure I have a low end square taper crankset. Does square taper have more flex in it than the external bottom brackets that most bikes come with these days?
Meh, again, we're talking minor improvements here. If you went from your current bike setup to a new CF bike with BB30 and all the new improvements, you'd feel a difference. If you slap an external BB and crank on your bike, your body will respond more to the change in q-factor (which is the distance from the crank to the frame center). If the newer cranks are "narrower" it will feel stiffer to your legs compared to a wider stance on the pedals.
 

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Discussion Starter · #7 ·
Thanks. I go through this once each year. I come to the forum for a reality check and end up saving thousands of dollars. My bike is down to costing me 17.5 cents per mile :)

Pete
 

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I built a bamboo bike that theoretically should be quite cushioned through the seat stays since I used more flexible tubes there. I made no effort to inordinately beef up the bottom bracket nor are the chainstays remarkably stiff.
I made it so the rear wheel practically brushes the seat tube. When traveling over the roughest roads the rear wheel hasn't shown even a hint of so much as brushing the seat tube as any flexion occurs.
The concept of a bike being stiff and yet flexy is probably based on such minimal frame movement as to be almost total bs.
Having said that the bike is still forgiving but I think that's based on total frame distribution of vibration rather than having some sort of shocks effect.
 
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