# Frame flex , whats acceptable,



## ade ward (Jun 23, 2009)

On the 29er short chainstay thread I showed my e stay 29er and it's stiffness was the subject of some conversation

Flex is subjective how do we measure and define it whats acceptable and whats not


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## febikes (Jan 28, 2011)

You can measure frame flex using weights placed on the end of a long bar and inserted into the a frame that is held in a strong fixture.
http://www.sheldonbrown.com/rinard/rinard_frametest.html


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## ade ward (Jun 23, 2009)

i did build a rig , in 1999 when i worked for a company called ATB sales in the uk 

it had three stations on it front wheel back wheel and bb , any two stations could be fixed and a torque applied to the third point the deflections could be measured with dial gauges

at the time we measured a number of hard tails , and compared them to the single pivot suspension bikes we were designing at the time ,, unsurprisily the FS bikes were all more flexible even with a solid bar replacing the shock but , none of the testers ever complained about the lack of stiffness when riding,,

but with a hardtail does the rider expect a certain level of stiffness, and with a FS he expects a certain level of compliance which can mask a lower level of stiffness 

i would have assumed that one of the requirments is to keep the wheels in the same plane and or a stiff bb area


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## Walt (Jan 23, 2004)

*Completely subjective, at least to a point*

If the bike isn't throwing the chain, what's "too much" flex is pretty much up to the rider. At this point in my life, for example, I've ridden so many different bikes that I get used to whatever flex exists in about 2 minutes, then forget about it. People get very hung up on it, but I'm not sure that there's any meaningful way to even talk about it unless we all had some objective standard for how much a given frame flexes in a given direction for a certain load. And that's assuming all the components are flexing/moving the same amount between bikes...

So the bottom line, to me, is that "too flexy" or "too stiff" or whatever are pretty much all up to the personal preferences/feelings of the rider, unless the frame is so flexy that there are problems making the drivetrain work right.

-Walt


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## pvd (Jan 4, 2006)

I design my hardtails to be very flexy. Without a fair bit of flex, high speed performance suffers considerably. The flex is for keeping traction while in corners.

I do design my chainstays to be stiffer in scale compared to the rest of the bike just to keep the bb and wheel working as a unit.


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## Schmitty (Sep 7, 2008)

It's very complicated. Like Walt says, if components start not working due to flex, that's bad.

The whole 'bb flex' deal has been overblown to a certain degree. Scott Nichol had a funny quote about guys who 'test' bb flex by standing next to the bike and reefing on the crank.. I'll see if I can find it.

Bicycle Quarterly also investigates 'flex'. Some of the methods and conclusions are out there, but there's some good stuff in there.

The bottom line is be open about it.. check your assumptions at the door cuz it's complex and you may be surprised. Take keeping the wheels in plane for example. No frame/fork does this while being ridden. Take a look at some of the sprint pics in pro road racing.. downright alarming how out pf plane the wheels can get. I think we'd all agree that the frame should hold them in plane when not being loaded, and under smaller loads.

-Schmitty-


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## ade ward (Jun 23, 2009)

currently i am sitting on the fence about using an asymetrical chainstay setup on my e stay bike,, 

but the problem i have is that the frame would be stronger one side than the other ,, this could give you different feel for the bike depending on which way the corner goes ..


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## Schmitty (Sep 7, 2008)

ade ward said:


> currently i am sitting on the fence about using an asymetrical chainstay setup on my e stay bike,,
> 
> but the problem i have is that the frame would be stronger one side than the other ,, this could give you different feel for the bike depending on which way the corner goes ..


Yeah but keep in mind, with a rear wheel bolted in there it's one unit. So lets say the non elevated stay is 'stiffer' laterally. It's stiffer both left and right. So the _sum_ of the lateral stiffness *may* be compromised both directions, but not in one more than the other. I would just beef up the diameters on the elevated side.

-Schmitty-


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## Andy Gray (Apr 18, 2011)

This is a fun thread. My first bamboo bike had 27mm down and top tubes with 14-16mm chain and seat stays. If you "hold it to the side and reef on the crank" it would move like 6 inches and the front and back would taco inwards. It was nearly unridable but I could putt around a little. My wife called it the "clown bike" (she's very supportive).

I redid the front triangle with a 35mm downtube and a seattube that had more interior woodiness which made it significantly stiffer. If I ride sitting down and put my feet in the right places it doesn't flex while pedaling. You still can't stand up and crank it out up hill, but I take the kids to the park with it, go the store, etc. If you do stand and press the stays flex enough that the chainring rubs the stay which is a very unsettling noise/feeling when you don't expect it.

Andy


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## ade ward (Jun 23, 2009)

reading PVD comment i went to the workshop and did the "hold it to the side and reef on the crank"
and had a close look where the movement was ,,

interestly it seemed the most of the movement was in the rear triangle , not in the front triangle


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## Vlad (Feb 7, 2004)

pvd said:


> I design my hardtails to be very flexy. Without a fair bit of flex, high speed performance suffers considerably. The flex is for keeping traction while in corners.
> 
> I do design my chainstays to be stiffer in scale compared to the rest of the bike just to keep the bb and wheel working as a unit.


What's your take on the old Slingshot bikes?


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## Yogii (Jun 5, 2008)

> interestly it seemed the most of the movement was in the rear triangle , not in the front triangle


Yea, surprising that 4 skinny tubes are more flexible than 3 stout tubes....


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## pvd (Jan 4, 2006)

@Vlad - Slingshots are just the worst junk in the world. Complete lack of structure or handling is not what I'm getting at.

@Ade - Are you sure it was rear end flex entirely? You are pushing at the rear of the bike. If the rear is flexing entirely you should be seeing the wheel move into the stays. Also, the wheel will be flexing too.


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## ade ward (Jun 23, 2009)

pvd;[email protected] - Are you sure it was rear end flex entirely? You are pushing at the rear of the bike. If the rear is flexing entirely you should be seeing the wheel move into the stays. Also said:


> i will have to look into it more but the wheel does move relative to the chainstay ,
> 
> i need to get out the DTI and take some measurments,
> 
> ...


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## MDEnvEngr (Mar 11, 2004)

pvd said:


> @Vlad - Slingshots are just the worst junk in the world. Complete lack of structure or handling is not what I'm getting at.


I'm no sligshots-are-the-greatest-thing-ever dude for sure. But, way back in 1993ish we rented one for a week in Moab. The bike felt "normal". It didn't feel extra flexy, or bend up under braking. I was suprised. The other bike we rented that trip was a nice Bontrager Racelite or something. My wife and I swapped them back and forth. I really couldn't tell the difference.

This was back in the ages of mag21's, skinny tires, straight bars, long stems....

I didn't feel the supposed "energy return" or whatever the slingshot fans go on about though.

Way back then we were trying to figure out the suspension vs rigid thing around here...the slingshot seemed like it might have been a good compromise. FWIW, almost 20 years of riding later I still haven't figured out the rigid vs suspension thing (for me).

B


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## mickuk (Jul 6, 2007)

Hi Ade

If you are still set up singlespeed, then squeeze the upper / lower runs of the chain together with your hand (holding just behind the chainring). On my Bianchi singlespeed (still no problems after 8 years constant use) this gives fairly substantial sideways flex of the bb / chainstays / wheel area (enough to make me look for cracks on a regular basis).

I built up my E-stay 29er raw last weekend (no back brake as still sorting a disc compatable rear wheel). Doing the same test produced almost no sideways flex but some visible movement of the bb shell rearwards relative to the tyre. I couldn't detect anything flexy or unusual when I tried a quick ride with some repeated steep up / down - certainly no issues with the chain flapping or derailling. 

For some real measurements I was planning on fitting the bike with a pair of 28mm tyres which when inflated would wedge nicely in the steel bedplate slots at work. Then also a clamp to stop the back wheel rotating and a horizontal balljointed tie bar coming in sideways to the top of the steerer tube to hold everything upright / simulate the resultant force from hauling against the bars / cranks. Was then planning to measure flex when applying load to the crank (2-3 o'clock position).

I think at the end of the day all we care about is keeping the chain on and not having the frame break. I was toying with fitting a strain gauge to the chainstay but having seen the movement of the bb I'm not certain where the most highly stressed area would be (chainstay, down tube or seat tube). So I stopped worrying and sent it for powder coat instead! We've got a GOM displacement measuring camera at work and I know some people that do stress analysis of entire components using a speckle paint finish but think this is starting to go a bit far for my after hours bicycle project 

Now got the bike back from powder and fully built pending a rear wheel - pictures finally on here later this week!


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## ade ward (Jun 23, 2009)

HI Mick
well done for getting it done,,looking forward to the pics,, 

the chain squeezing thing is a great idea ,, the usefull thing is a can do this with a clamp to a known distance to get a repeatable load 

then i can get my dti and measure movement from chainstay /wheel bb wheel and bb to chainstay 

should give me some idea ,,, 

i have seen some rearward movement on the bb when standing on the pedal with the front wheel up against the wall


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## LyNx (Oct 26, 2004)

I'll agree that frame flex and what is too much is personal preference, but if it was me and the frame would not let both wheels track on the same plane when riding down off camber sections, then I'd consider that too flexy in a bad way - I had such a bike and it was horrible.


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## ade ward (Jun 23, 2009)

just did a quick and dirty test ( chain was oily) 
so as Mick suggested i grabbed both runs of the chain and squeezed them together,,as hard as i could

placed fingers between rim and chainstay and could feel no movement at all here 

there was only some rearward movement of the bb 

I'll break out the dti later in the week and try to put some numbers to this


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## laffeaux (Jan 4, 2004)

Vlad said:


> What's your take on the old Slingshot bikes?


Funny you should bring that one up, as I was going to mention it. I owned a Slingshot for a couple of years, although it was never my primary bike. When going into turns hard, I could feel the bike twisting and the rear taking a much different path than I expected. This was a really odd sensation for the first few turns, but then it "was what it was" I got used to it and if felt normal. If it were the only bike that I rode, I likely would not have noticed it at all, but because I switched between the SlingShot and a "standard" hardtail the oddity of the SlingShot was noticeable when I made the transition.

I think that the SlingShot speaks a lot to what's "acceptable" flex. The flex in the SlingShot was very noticeable, but was it "acceptable"? Had I only owned the SlingShot, yes, it would have been acceptable. But since I owned more than one bike and I didn't like adjusting how I rode each time I took out a different bike, no, it wasn't acceptable. What that means is that there's way to determine what's too much or too little flex - different frames flex differently under different situations, and it's up to the individual rider to determine if it's too much or too little.


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## D.F.L. (Jan 3, 2004)

Look at the best handling bikes and you will find good torsional stiffness. If flex was cool, you'd want a DW-link Slingshot with worn bearings.


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## ade ward (Jun 23, 2009)

interesting artical in velo news http://www.velonews-digital.com/velonews/201104/?pg=67#pg67

torsional stiffness test v's rider feeling

ok it road bikes,, but the physics is the same ?


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## manual63 (Nov 5, 2006)

Interesting question. I don't like side flex in the bottom bracket area. Too many frames flex too much there. I do a lot of standing when going up short climbs. When standing I am pulling up and the handlebar while pushing down on the pedal on the same side.....this twists the frame and if the bottom bracket area flexes a lot, you lose a lot of power...icky! My f-bom frames will have ovalized tubing to reduce flex in the bottom bracket area.


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