Component flex

asdfhjkl

This is probably a stupid question but I'd feel more stupid not knowing the answer. What does "flex" feel like?

I keep seeing people talk about frames, forks, wheels, components being flexier or stiffer than others. Is there anything to this or are people just talking out of their arse? Why does stiffer appear to be considered better? Is being flexy always a bad thing? Does it actually have any noticeable impact on riding?

My bike is almost certainly flexy but I can't say I've ever noticed anything... well... flexing!

Chunkers1980

Flex can good or bad - it's really finding a medium you like.

No flex (stiff) leads to a very harsh ride in the frame and fork but tracks to exactly what you want.

Flex can be good in a frame for comfort but bad in the crank for example as it wastes valuable turning force.

paul.skibum

If you dont notice it dont worry about it - I can feel it some times and in some situations. Frame and wheels particularly in situations where they are out of their comfort zone - so my steel hardtail with budget wheels in the bike park or large gnarlier terrain exposes some of its weakness - cranked over in a bermed corner I can definitely feel the flex.

Cant say it is generally a defining characteristic of why I can or cant ride something.

Ryan Jones

It's a bit of a black art for frame designers to find a happy medium. On one hand you want something that tracks well, is responsive and doesn't waste your pedalling effort so you make it stiffer, however if it's too stiff there's a lack of compliance which makes itself known on longer rides or in wet conditions where the lack of deflection results in every little rock or root bumping you off line, and you'll get trail buzz even on relatively smooth sections.

My nukeproof mega tr is as stiff as a stiff thing compared to my cannondale rz, great when pressing on downhil as it feels planted and tight, but wet technical climbs/flats were noticeably more tricky.

supersonic

With regards to comfort, I find tyres, seatpost, bars and saddle make much more difference than the frame (hardtail) as far as comfort is concerned.

Some frames however do have compliance built into the seat mast, and very few have managed some vertical deflection in the stays.

rockmonkeysc

Ryan Jones wrote:

My nukeproof mega tr is as stiff as a stiff thing compared to my cannondale rz, great when pressing on downhil

Just wait until your rear suspension bearings wear, then it's like having a hinge in the middle of your frame :shock:
Some flex is ok but flexy forks lead to poor tracking, making it difficult to hold a line. Flexy frames cause similar problems. Flexy cranks are in-efficient and surprisingly noticable.

jairaj

I don't notice the forks or frame flexing as such but the flexibility usually has a side effect such as poor tracking which is usually noticeable as you ride. Its also one of those things that you don't really notice or just learn to live with until you try something stiffer.

Also flexible and compliant aren't always the same thing. My Boardman hardtail frame has quite a bit of flex in it but my old Reynold 631 tube Saracen Zen frame was more comfortable. I guess the Zen has the cliqued description of vertically compliant yet laterally stiff whereas the Boardman is just plain flexible in all directions?

dusk

Ducati switched their moto gp bike from a carbon frame to a steel one as the carbon didn't flex enough, I'd say flex is only bad if you start to notice it.

I had a full susser (qr axles) that felt fine, I put some 20mm axle forks on and it made the rear end feel awful like it was loose or had a puncture, turns out it was just the difference in flex between a qr rear wheel and 20mm front

Monty Dog

It give bike review writers and bike shop assistants something to talk about - a well as the materials-science bollox most of them spout. In the real world, most just get on with riding their bikes and don't really care to notice - from someone who enjoys running big fat tyres at low pressures, component flex doesn't really come into it.

marcus'73

You only need to ride a fork with a thru-axle after having ridden a QR fork to understand flex and you won't want to ever ride a QR fork again! The improvement in the precision of where you can place the wheel and how it will continue to track the line you pick against the deflection and stuttering of a QR fork is monumental and the braking is much more linear. I was out a mtb holiday last year and the guide was running one of the hire bikes with a QR Recon and his front wheel was dropping out due to the fork twisting through the rock gardens - not nice at all.
That said I run a steel hardtail and it's ability to take a bit of flex is welcome when you have no rear bounce, my last bike had an alloy frame which snapped due to metal fatigue - flex isn't always bad!

supersonic

You only need to ride a fork with a thru-axle after having ridden a QR fork to understand flex and you won't want to ever ride a QR fork again!

Really does depend on the fork. Fox forks are only 15% torsionally stiffer in 15mm format compared to their QR counterparts. A well engineered QR fork can be stiffer still, in fact I've seen a few tests where QR forks have come out on top.

rockmonkeysc

supersonic wrote:

You only need to ride a fork with a thru-axle after having ridden a QR fork to understand flex and you won't want to ever ride a QR fork again!

Really does depend on the fork. Fox forks are only 15% torsionally stiffer in 15mm format compared to their QR counterparts. A well engineered QR fork can be stiffer still, in fact I've seen a few tests where QR forks have come out on top.

Depends on the hub as well.