cormacf

My favorite (wheel-on) trainer bike was an aluminum track bike (a Giant Omnium). The lack of gear choice, the straight chainline--there was nothing to do but grunt and pedal. I actually did a couple of Zwift centuries on that thing. But the thing I REALLY loved about it was that it was so noticeably stiff. I don't put out a lot of watts, objectively (I don't think I ever got far past 800ish, which is VERY far from people who are actually good at cycling), but it always felt like everything I put into the pedals made it straight to the wheel. It would have been terrible for riding along cobblestones, but for a trainer or a track, that bike was fantastic for the money.

I've long since donated that bike (I moved away from a velodrome and the Juniors team there needed bikes), but I think the time has come to get another trainer bike I might take out on the track (in a velodrome near my new place). It got me wondering where all that stiffness came from, though.

Possibilities:

• Aluminum frame
• Crankset (SRAM Omnium) -- it's not the BB, because that's a standard BSA
• Shortish chainstays (400mm, so not as short as more aggressive track frames, but a bit shorter than road bikes)
• Straight aluminum fork

The fork seems like it might be the biggest contributor, because when I've put my old ('77 Super Le Tour 12.2) or retro (Rivendell Romulus) bikes on the trainer, it feels like the front end is on a spring.

Kierin racers put out gobs of watts on steel lugged frames, though, so I imagine those are plenty stiff.

Thanks!

Trakhak

Keirin are raced on bikes that are certified to be identical by the NKS (Japan Keirin Association) according to rules that were formulated decades ago. That's why they're still steel.

Since the frames are all identical, the frame material is unimportant. Switching to another material would be costly for the racers or sponsors, would require rewriting of the standards, and would be pointless, since the riders would again be on matching bikes.

That said, for your use case, aluminum frames (and forks) are great for torsional stiffness, which is what you want for sprinting. Add the short wheelbase of a sprint (or even pursuit) track frame, and you're in business. (The short wheelbase is an important factor in the frame's stiffness that's often overlooked.)

There are a few fairly inexpensive aluminum track frames and bikes around, although not as many as there were after track bikes became fashionable but before steel versions became even more fashionable. AliExpress has incredibly cheap aluminum track frames available, for instance.

cormacf

Thanks. Would the difference in stiffness be noticeable to a mere mortal like myself, though? I ask because if I'm going to build up a track bike, I could spend a little extra to buy an NJS frame with a cool paint job if it's not going to feel like a noodle. Like, if the answer is "There's a difference that's meaningful, but only if you've > 200 pounds or pushing 1500 watts," then gimme the lugs. If not, I'll probably just do the sensible thing and get a Dolan or Jamis Soik (or a used TK2).

Andrew R Stewart 

Perception can be rather different from measurable qualities. The geometry and rider positioning are two huge factors which influence the rider's ability to apply power and to maneuver the bike. Many riders think a quick handling bike is also faster, a bike that moves off line easier will also travel a longer distance than a bike which is more directionally stable. But so much of human performance is motivationally based...

Having said that most current Al frames are designed to be measurably stiffer, compared to steel, as to not suffer from fatigue limits too soon. Forks and especially the steerer is a big contributor to overall stiffness but so few people mention this. Perhaps because few builders, brands, make their own. Then there's the wheels and that true track wheels often have better spoke bracing angles which make the wheels feel stiffer laterally. Andy

cormacf

Thank you!

zandoval 

I was talking to a guy not long ago. He had gone from an old Motobecane to a new Bikesdirect Carbon. It was a gift from his kids. Any way, he said he did not have too much problems adjusting to it. He said the hardest thing was getting used to the Brifters. Oh... and that he needed to paint it light blue.

What? I asked him.

Yep... He said the dam thing was so stiff it had to be on permanent Viagra...

unterhausen

From what I have seen on bikeforums, you do not want one of those keirin bikes used. I think the biggest improvement they would get from carbon is aero. I don't think stiffness is particularly important property of a bike, as long as the front derailleur doesn't shift itself.

Andrew R Stewart 

I would have said "as long as the rear der won't auto shift". Back when Al frames were being played with and the traditional tube diameters were still used the Alan and Vitus frames could be so flexible as to tug on the shift cables with each pedal stroke's side forces. With friction shifters the lever would bit by bit migrate to a position that wanted the chain to begin shifting. I would install a short length of cable between the der's anchor bolt and the adjusting barrel/cable stop on the der, the frame and lever were no longer "in the system". I would place the chain in a mid placed cog the rider could deal with and tell them to test ride for that auto shifting. My shop was at the base of a short but steep hill and a good place to test for chain skip. My customers would return reporting zero auto shifting or chain skip. The next part of the service stop was the hard one, how to get the rider to swap to parts that didn't either help cause the problem or have a feature that precludes the problem from happening (as in indexed shifters that hold their position with far greater ability that any friction lever can.

Someone smarter than I am said something like "a frame only needs to be as stiff as needed to keep the other parts in their right relationships and allows the rider to balance and steer". It has been my view that factors of a bike have that can be measured and thus have a specific number attached to that aspect will be used by the marketing departments as a benefit, much along "the bigger the better". I do find it interesting that nearly parallel to the stiffening up of frames, wheels, handlebars/stems is the number of suspending and vibration damping attempts that are so cool right now. As though the super stiff bits are too stiff for the riders to suffer with. Andy

Calsun

When nearly all the bike frames were made from Reynolds 531 steel tubing the variations in frame stiffness was in the chain stays and seat stays securing the rear wheel in position. Changes in materials as well as tubing profiles has resulted often in stiffer frames overall but there is always the trade-off with frame weight.

I think of what may be absorbing the power from my pushing on the pedals and result in less energy driving the bike forward. On some bikes I would feel the initial flex or when climbing a very steep grade and could hear the crank arms creaking.

Andrew R Stewart 

Framers don't "absorb" pedaling power, I had thought that by now most would understand a frame is a spring with essentially 100% return of the bending forces (pedaling pressure). Now the timing of this return to the system energy can easily not be when it benefits the rider. This is called "not planning"

When I was taking Eisentraut's frame building class way back in 1979 he was asked about frame stiffness. He's the one who mentioned that frames need to be stiff enough to support the rider and keep the other parts located properly and in a good enough alignment to safely steer and balance the bike. He also asked if we ever felt our frames warming up from all that energy they absorbed (well, not in those words but that was the meaning). Of course we didn't. Andy

TC1

Even if the frames were absorbing that energy, they wouldn't be warm to the touch -- due to the cooling effect of the airflow. Even if you were pedaling at 400 watts, most of that is going into development down the road, deforming the tires, and pushing air out of the way, which leaves only a small percentage theoretically heating the frame, and that wouldn't be close to enough to warm a moving frame with such a large surface area to mass ratio.

The point he was making isn't wrong, but the evidence used in support was not accurate.

Andrew R Stewart 

And I agree. My point was the understanding of frame stiffness was understood by some long before the current hype. Even if their way of explaining it wasn't the best. Andy

duanedr 

I took that to mean that the energy absorbed and released is so small that ambient environment was enough to overcome it. I have done some crude testing on this and I believe there is a LOT more flex in tires, wheels, pedals, cranks and stems and frames don't flex very much at all in real world conditions. HT's flex more than rear ends which are very triangulated.

EDIT: Not sure why HT ended up there. I meant front triangles.

cormacf

In this case, there would be no derailleurs, and it would only be used on a track, so ride quality over bumps wouldn't be an issue.

rando_couche

SACRILEDGE!!!!