I have been enjoying a CAAD7 recently, and it certainly lives up to it's reputation. It is incredibly stiff and responsive...in a good way.

I have quite a few bikes, and in all common frame materials, but this CAAD7 from 2005 is absolutely the funnest frameset I have ridden to date.

Anyway, here is the question. Even though it is incredibly stiff, it doesn't "beat me up". I am a big rider at 197 pounds, and very few aluminum frames are uncomfortable for me. I mentioned this to my father (old school engineer) and he said it is because I am a heavy guy.

Any road input has to displace a greater weight (my body) so I have a built in dampening effect of sorts, where some 150 pound scrawntoid gets beat to a pulp because any input has less mass to move about. In a nutshell, the lighter rider should be more concerned with frame stiffness. Hmmm.

Any thoughts on this theory?

There may be a small difference but it's likely overshadowed by the tires, tire pressure, frame type, riding position, road surface etc.

In general it's easier for lighter riders to unweight themselves off the bike and absorb bigger bumps with their legs. The CAAD bikes have a reputation for being comfortable bikes for everyone.

I'm one of those scrawny guys in the 150's. Can you run tire pressures in the 70's without pinch flatting?

I agree with your theory somewhat, based on my observation from the other end of the scale. At 135 I've never felt any frame flex, and don't see how the tiny bit a certain frame may "give", even with a larger rider on board, could make any difference in comfort. It's all about the tires and tire pressure.

"Frame compliance" and other such terms are just a way to sell more bikes.

Sure - weight and power matter a bunch as does the type of riding you do.
I got a Raleigh 753 Team the year it won the TdF when I was a moderately fit 200#, 6'2". It was a noodle, but so great to ride - it was my primary bike. The bike I raced on at the time was about 3# heavier Columbus SP. I could not really bend that thing. Now - older and heavier I ride a Cannondale Six13. The bike impresses me. I buy all this cool stuff for my son, but the Six13 works.

I had a Cannondale 3.0 frame in the early 90's and hated it. Broke two of them and the ride was incredibly harsh. It had a wheelbase similar to my track bike and high bottom bracket. Great crit. bike, terrible bike to train on.

I definitely agree with the theory. After losing around 20 pounds one Winter/Spring, most of the stuff I didn't ride during the time suddenly felt stiffer when I dusted it off for Summer..

By the way, I had an old CAAD8 (I think 2006 or 2007?), and I loved it. I got it REALLY cheap of Ebay, coz it had a small dent in the top tube.

The dent eventually turned into a crack after a couple of years. I had it welded, but it only lasted a couple more months before cracking even worse, right next to the weld. I knew a successful repair was a big long-shot, but I thought it was worth a try, because I liked it so much

What you might be surprised to know is that I couldn't be more surprised at how much comfort my Supersix EVO provides. Same goes for the 2015 Specialized Tarmacs I demo'd earlier last year. Having ridden anywhere between 190 and 205lbs, I'm definitely a heavier rider and I would suspect that influences the frame's ability to flex as designed, but it definitely does it.

I've said this before.....I don't think there even needs to be a Synapse or Roubaix. The "comfort" versions offer (to me) little more than a higher bar position that could just as easily be achieved on a Tarmac or EVO with steerer/stem/bars because the EVO & Tarmac already have such a cushy ride, it often STILL surprises me sometimes.

Along these same lines......The Six13, Supersix, EVO, CAAD10, 12, etc. etc......The design hasn't changed much in the last several years on C'dale's top bikes. The Six13 looks almost identical to even the latest EVO's from HT, TT, DT, seat stays ,chainstays, etc. etc. Small improvements here & there, I'm sure, but they still look basically the same. (whereas a Tarmac & EVO look nothing alike). So your observations are certainly right on..........again....at least for us heavier riders.

I had an old Cannondale 2.8 that was amazingly stiff. It also had the comfort of a rock. Tires, pressure, wheels, etc. didn't make a difference - it was uncomfortable. I used to see them a lot in crits because of their stiffness and generally low resale prices. New Cannondales are night and day different.

That an "amazingly stiff" frame would ride like a rock makes sense. But not that tires and tire pressure "didn't make a difference". It seems strange to me that someone who can't feel a difference in tires/pressure would be able to feel a difference in frames. Surely tires make a greater difference.

Shocked that I never got an answer.

Want some popcorn..?

Light on the butter please. Trying to get to race weight.

How many aluminum frame bikes have you built up and ridden, with exact swap of parts from one frame to the other? I have done it multiple times, and absolutely experienced significant differences among aluminum frames. Wheels and tires absolutely make a difference, just as frame absolutely makes a difference, and seat post absolutely makes a difference.

With 20 fewer pounds, there was less of you to flex the bike.

I always thought stiffer frames were for heavier or stronger riders. Heavier riders benefit as the bike doesn't flex when they expect it not to, and strong riders benefit from the power transfer, maybe.

+1

I also had a 3.0 bike a long time ago. I call it "the brick". Was the only frame that claimed that you could stand on the seatstays and not bend them. I'll believe it. You must have had a pretty spectacular crash to break this one...

I'm 225 and ride a Caad10 because it feels stiff and responsive compared to lower end carbon bikes in the same price range. I'd have to spend twice as much to get a decent riding carbon frame that wouldn't be a noodle under my weight.

Sorry. I wasn't clear. Tires and pressure didn't really make it comfortable. It still ride like a rock but I could tell a little improvement - 25's were the largest it could take though.

Correct. Lay a 3.0 or earlier frame flat on the ground, and you can put your whole body weight on the dropout. I think they ran an ad of a guy standing on the dropout.

I wonder if my son's back would be less sore if he wasn't on my old Cannondale .... hmmmm.

With my Cinelli Xperience, I had to change to 23mm wide rims (Velocity A23's), 25mm tires, and a 27.2mm carbon seatpost (from stock aluminum 31) to make it tolerable for a 50 mile ride. I have other aluminum bikes that are all day comfortable on 19mm rimes, 23mm tires, and aluminum posts. So I absolutely got what you meant.

I had a CAAD5 (I'm 208#) and it beat the crap out of me on broken asphalt. I got a Gunnar with the same parts, wheels, tires, even seat and post and there is a big difference in comfort.
Now I also have a Seven Axiom Race and it is almost as harsh as the Cannondale. Beautiful ride on a nice mountain road, however.

Keep in mind that stiffness typically refers to lateral BB flex, and frame comfort is usually determined by vertical compliance. Hence bikes like the Roubiax/Domane, which can be both stiff and comfortable.

And yes, the stiffness of the bike frame needs to be matched to the rider's power & weight. A bike optimized for a 140 lbs rider is going to feel like a wet noodle to 250 lbs rider. The only real way to avoid that is things like full suspension bikes, where the suspension is tunable.