Stormy Archer

Anyone heard of a DIY way to stiffen the rear triangle of a frame? I was thinking you could use steel cable like a slingshot frame (except across the rear triangle like a mixte) or carbon tow.

My related question is: what parts of a frame flex to cause reduced power transfer to the pedals? Does stiffening a rear triangle with another pair of tubes (like a mixte) have more impact on power transfer or ride quality?

I know that pretty much everything about a frame contributes to the stiffness, I'm talking more about specific cases when the bike is noodly enough to feel the rear wheel steer or see it move when pedaling hard.

AnkleWork

None. Frames are elastic enough to efficiently release the tiny energy stored during flex.

Andrew R Stewart 

Frame stiffness as an indicator of efficiency or "power transfer" (whatever that means) is highly manipulated by those who want to sell you something. Andy

HillRider

Unless your frame is so flexible that you experience "ghost shifts" or brake rubbing under pedaling forces the stiffness is fine. The "stiffer is better" concept has been badly oversold.

Crankycrank

Sometimes you can add a bridge between the chainstays right behind where they attach to the BB shell to eliminate some flex. What is your frame, steel, CF, Aluminum? IMO though the best answer is to get another frame that feels right to you since farking around with what you have could make things worse. Might be a good idea to also post in the "Framebuilders" section.

Andrew R Stewart 

I'm not sure how much stiffness a missing bridged frame would gain if a bridge was added, at the usual location. Years ago Bicycling magazine had a series of articles involving what was then considered leading edge engineering, a finite stress analysis computer modeling, of bike frames. They used a few different arrangements of frames including with and without chainstay bridges. Their findings were that the CS bridge didn't change the results. My seat of my pants experience bears this out.

I attribute the focus on stiffness as a selling quality began about the same time. Cannondale had just begun to go to the bank on Klein's patented use of oversized tubes to increase frame stiffness. Those of us who rode small sizes already knew the truth, overly stiff frames increased rider fatigue and this led to a slower end of ride. Andy.

Crankycrank

^^^I have read from at least 3 frame builders that they claim a bridge can help in some situations but I am not a frame builder and you probably have more experience than me on this. Would definitely agree that overly stiff is not fun on longer rides.

Gresp15C

Unless the frame is heating up from flexing, you're not appreciably losing power.

3alarmer 

...you can experiment with running a very long seat post in hopes that further increasing the stiffness of the seat tube portion will change the feel and flex.
If you have a set of bottle bosses built in on the seat post, this kind of limits how far you can go with this experiment.

Homebrew01

Maybe your rear wheel is flexing. Loose spokes, or not enough spokes .

Andrew R Stewart 

Interesting, this is what Al Eisentraut told us back in 1979 during one of his building classes. Andy

Brofessor

More Stiff==>less energy absorbed in the frame==>better power transfer to the road (ceteris paribus) ==> Speed
Less Stiff==>more energy absorbed in the frame==>better dampening of road harshness (ceteris paribus) ==> Comfort ==> Speed

We have a bit of a paradox here....

zebede

I am convinced that an experienced frame builder can change the level of flex per design. My primary example is an 1980s Trek 720 touring bike. It was an "OK" ride when unloaded, but when you put loaded panniers on it became luxurious pleasure to ride. Another example was a 80s Gitane with Super Vitus. Many people called it "flexy" , and I think for "masher" it was, but for a "spinner" like myself it was a comfortable road bike. You could stay in the saddle longer and it wouldn't beat you up as bad as the compared to the currently popular stiff bikes.

So what type of frame do you have? What kind of peddler are you? Maybe the bike is not right for you?

raria

Im a steel guy but ended up buying a CAAD12 with almost the same geo as my steel bikes and exact same grupo including casette.

CAAD is faster on any ride longer than 20 miles. So what could it be if its not better power transfer.

Ferrouscious 

weight, aerodynamics, placebo effect, perceived benefits unconsciously making you work harder and therefore faster, tires...

JohnDThompson 

+1 this. The relationship that a stiffer frame was a better frame became popular with Klein's use of oversize diameter aluminum tubing back in the 70s. But the oversize diameter was as much to provide sufficient weld joint area to ensure against failure as it was to increase stiffness.

JohnDThompson 

A light rider may find an overly stiff frame to be uncomfortable to ride long distance, and difficult to control on rough surfaces.

wphamilton

Regarding the first question, steel cables on the rear stays could increase the rigidity but you'd be adding considerable compressive stress that would likely be beyond what the frame was designed for. That's my intuition, not any specific calculations or design experience, but I'd be very reluctant to try it.

On the derivative question "why?" it's clear that energy IS lost in flex, and it is not "returned" to forward motion by the frame springing back. While most of that energy would be converted into heating the frame, it does NOT follow that you'd feel the frame getting hotter or otherwise note an obvious heating, for the same reasons that our bodies don't cook while riding. So I can't agree with responses that dismiss your concern completely, although it probably is true that differences are very minor.

I think that frame designers are the more appropriate group to ask because where and how they beef up stiffness is likely where you'd need to address in any DIY efforts. That is beyond my specific knowledge but from general reading on the subject, after the tubes are selected joints are the main areas of concern. In that case, it seems to me that adding more tubes to the rear triangle will be disappointing, having limited impact.

fietsbob

if you thinka tube is not stiff enough?
wrap with soggy carbon fiber sheet already wet with epoxy..
let it cook.





....

Gresp15C

Right. The energy has to go somewhere. Either it comes back out as mechanical energy, or it's dissipated as heat. Granted, one other possibility is that too much frame flexing (or too little) makes your body less effective at converting your strength into forward motion of the bike.

McBTC

how about filling the tubes with spray foam for added support and stiffness? ...a web search shows that spray foam is, said to expand up to 60-times its unmixed volume and is the result of combining isocyanate and polyol resin.

Andrew R Stewart 

How about if you do this test and report back to us? Andy

cny-bikeman

Your assumption seems to be that if the rear triangle does not flex the pressure you apply to the pedals will result in more efficient forward motion. But pedals are off-center to the bike, so it seems to me that some counter force would have to resist that off-center force. I believe there are only two possibilities - the frame or your muscles. Only one of those potentially has elastic properties, so that the energy can in large part be recovered. If a frame is completely stiff then either the entire bike will sway from side to side under hard pedaling force or you will use energy trying to prevent that.

raria

I was starting to think I was the only sane person here!

AnkleWork

Really? How much energy is clearly lost? Got a source?