Titanium frame flex
 

One of my riding buddies has a Litespeed Ghisallo. Last weekend I was on his wheel and noticed that he gets a big deflection in his rear wheel with each pedal stroke. I think his form is not great and he is definitely stronger in his right leg, but even so, I would guess he is getting 3-4mm of flex every stroke, enough that he needs to open up his rear caliper to keep from rubbing. I thought maybe his frame was cracked so I checked it as best I could when we stopped. It seems to be okay, but pushing on the crank while holding the bike shows a lot of frame flex. This frame is highly rated and respected, and I am sure it gives a nice ride, but how efficient can it be to flex the frame on every stroke? My Felt F1 is a lot stiffer and still rides well, but I wonder if the Ghisallo frame design is not good or maybe this is just the nature of full titanium frames?

All our frames flex to some extent. It isn't if they flex but how well they accept and then return the energy back into the system after storing it during the flex that counts. It is that sort of flexing that makes good steel and Ti frames so nice to ride. Yes they flex but they give back what they take and provide a more kindly ride quality while doing so.

The Ghisallo was Litespeed's lightest frame and was as light or lighter than most carbon frames. I believe Litespeed advertised it as less than 800 grams. To get it that light, they had to sacrifice somewhere and the result was a quite flexy frame, particularly in the larger sizes.

A smooth, high cadence pedal stroke will overcome (or actually prevent) the flex but apparently your buddy isn't in that catagory. As BCRider notes, he isn't actually losing any energy from the flex so other than appearances, it isn't a real problem.

The frame flex unflexes on a dead part of the pedal-stroke when you're not exerting as much force. This typically is around the bottom of the stroke before the downstroke on the other side. The unflexing opposes the downstroke-force on the other side and gives you a little more force to push the pedal against. It's kinda like the end of the wrist-flick in squash.

Titanium is very elastic compared to steel, closer to CF. It springs back smartly and returns all the energy. If it didn't, the frame would get hotter and hotter as you ride and melt.

I'd like to add that I hate it when that happens.

It makes sense that in the best case, the frame would return energy that can transfer to the crank, but I honestly can't see how that will happen. The frame is flexing laterally on the chain side down stroke and then I presume springing back as soon as the crank is at the bottom. My sense is that this will translate to heat in the frame (though not quiet the melting point of titanium) and is purely lost energy.

The flex is why people buy Ti frames its more forgiving than steel and Aluminum,
and won't corrode.
as said the goal was light weight, add in a bit more wall thickness and reshape
the tubes and that flex can be reduced.

Yeah, when a metal frame flexes you lose very very little of the energy as there is no damping. Does the energy get back into the pedal stroke? Maybe, maybe not... but it really does not seem to slow anyone don too much. Back in the '70s and '80s a company called Vitus made narrow-tube aluminum bikes and they flexed like crazy, but were still very fast while climbing - even though climbing is often at the low-rpm situation that result in more frame flex.
All that being said, I'll bet a whole lot of the flex you were noticing was actually in the wheel and not the frame... spoked wheels flex much much more than any frame when subjected to a side load.

Do this test:

1. Lean a bike next to a wall with the handbar on the wall

2. hold the brake lever down to stop it from rolling forwards

3. step on the pedal near the crankarm and push it down and sideways, notice how the frame flexes 2-3cm sideways?

4. gently let up on the pedal, can you feel the frame pushing back against your feet?


When you're pedaling the bike, that frame unflexing pushes against your foot and spins the crank. Time after time, tests are done on this frame-flex phenomenon (it appears to be done every couple decades). The results over and over again indicate that frame-flex robs little or no energy from the total energy output. No one has been able to measure quantitatively and to a significant amount any speed differences between a flexy versus super-stiff frame.

^ A lot of the flex<->return you see in that test is contributed by the tires and wheels, isn't it Danno?

Titanium is much less stiff than steel, much less stiff than carbon fiber, but about 60% stiffer than aluminum. Bike frames are designed based on stiffness and weight, and never get close to the elastic limit of the frame. That said you can build a stiff or a flexible frame out of steel, carbon, titanium, or aluminum. It is just easier to build a light and stiff frame out of carbon as you can vary the "tube" shapes and junctions to provide the ride that you want

Try it and see. The BB moves laterally while the wheels stay in place. You can easily see it when following a strong seated sprinter. The wheels stay close to being in the same plane, but the BB moves much more than that. Ti frames have 50-100% more strength for the same weight as steel, but also 50-100% more flexibility as well (depending upon 3D geometry). When you make it lighter to be the same strength as a steel bike, you have a lighter frame, but end up with 100-200% more flex.

I was talking about elastic deflection and how the material springs back, not how much force it takes to bend it in the 1st place. Ti has less internal damping (hysteresis) than steel or aluminium. If you take equal-strength samples of steel, aluminium and CF and flex them back & forth, the resonance frequency of Ti and CF sample will be higher than steel or aluminium; they are more "springy". Ti and CF will also take more cycles close to their yield-limits than either steel or alloy as well. Of course this will vary somewhat depending upon whether you have the 3-2.5 or 6-4 alloy, but the generalization applies for both.

I'm not so sure this is even frame flex, it could be the wheel deflecting from the torque being applied to it.

When the rider flexes the frame the whole system acts as a spring right from the tires to the wheels into the forks and frame and even the seat post and handlebars. Some of the parts, such as the tires and tubes, will have more damping (the hysterisis that LarDasse mentioned a couple of posts up) and some parts less. This energy will return into the system or be converted to heat as already mentioned. And since we don't see the frame and wheels glowing red hot it stands to reason that much of this stored energy goes back into pushing the bike forward. There won't be an easily resolved vector path of forces that show the side flex to wheel drive because the combination of forces through the wheels, cranks, fork, frame and rider's body are a convoluted mess. But the final resolution of all the pulling and tugging will be something that aids the rider. If it didn't then we would be choosing our frames, cranks, bars and wheels based far more on how rigid they are. But as we've seen here that is not the case. Flexy Fliers can do as well in racing as Rigid Racks.

Exactly, there are pros and cons to all properties and rigidity/stiffness is not a high-priority in performance. If it was, we'd all be riding rebar-reinforced concrete frames; absolutely zero flex!!!

In a sprint though, I do prefer stiffer frames since they make keeping the bike under control and aimed where you want it easier. Someone had posted a front-on photo of sprinters where it showed the two wheels being out-of-plane by a huge amount, something like 5-10 degrees and the front one was aimed off to the side by 20 degrees or more!!!

I had a Vitus 979 and Peugeot Comete way back in the '80s. These were alloy frames modeled after traditional steel ones with 1" top-tubes and 1-1/8" seat & down-tube. Boy, were those suckers whippy! Of course there was the inevitable debate on stiffness and Laurent Fignon did back-to-back time-trials on a Vitus compared to a stiffer steel bike. Zero difference in speed, zip. Of course the +1 lb savings really helps in the hills .

I remember seeing that pic or one like it. It must have been during that last dash for the gold sprint. The looks in the riders' faces was all about explosive effort and the frames were torqued so hard that the front and rear wheels were a good 5 to 7 degrees out of vertical alignment. And remember the wheels? The front wheel on one bike was so distorted that it looked like it would taco if there were one more ounce of side thrust on it.

That shot was sure fire poster material for any red blooded cyclist.... :D