Fork flex, how much is normal?

[Nick Bull]

How much flex is "normal" in a fork?  See attached video ...

Back in March, I had to do a panic stop on my Soma Grand Randonneur, and the fork flexed back so much that it was rattling against the under-the-downtube water bottle.  At the next control, five miles down the road, I took a careful look: Holding the front brake tight, with high downward pressure on the front wheel so it wouldn't slip, I could rock the bike back and forth enough to flex the back of the front wheel back and forth in a range or about 3/4".  All agreed that it seemed like too much fork flex, possibly indicative of a steering-tube-weld or fork legs gone bad, so I had to abandon a 300km brevet.  I disassembled the bike and found no problems, so I took it to an LBS to have them double-check it and adjust the headset.  They also found no problems but suggested replacing cable and pads, which I did.  Then I didn't have time to get it back into "production" so I rode the rest of the brevet series on my Gunnar with a Waterford-build low-trail fork.

After finally finishing the brevet series, I did a couple 20-mile commutes on the Soma, decided it seemed OK, and rode a 200km on it.  By 3/4 of the way through, noticeable play had developed in the headset, and I tightened the headset, having brought headset wrenches for just that purpose!  But in the process of adjusting the headset, I noticed that there is still what seems like an unnerving amount of flex in the fork.  I finished the brevet, figuring the fork has probably been like that for the 8000 miles I've owned it, but I just never noticed it before.

So I took a little movie of the fork flex in (first) the Gunnar and (second) the Soma.  I'd say the Soma has a little more flex than the Gunnar, but not a whole lot more.

What do you all think?  Should I be concerned?  Or does this amount of flex seem normal?  Or do I need to make a better movie so you can tell? 

Nick

[Brad]

I can't unlock the movie at the moment without some download wizardry, but based on your description the place I would look is not the welds in the legs of the fork, but the steerer tube itself inside.
The thing that gets my attention is that your headset came loose during a ride.  That's weird.
You might have fixed cups that weren't seated are compressing while you ride.  I have experienced that.
But you might also have a defect somewhere else.  At one time it was pretty standard for pro race mechanics to put a piece of broom stick inside steerer tubes just in case.

You are going to get significant flex over things like milled pavement rumble strips, but on ordinary paved roads it shouldn't be moving much.  Of course I do  not have an under downtube bottle.
I am basing this on being a big human (think typical Tandem mass) and riding a bike set up with one of the forks that was made for the Kogswell PR with the most rake option.
So pretty much the same fork Soma spec'd and probably built at the same factory.

[Jeff Bertolet]

It doesn't seem urgently worrisome. Maybe some more commuting miles before another brevet. Emailing SOMA would be worth it. I know there was some past discussion from Evan B of Soma about some beta testers finding the GR fork too flexible and others not flexible enough.

As far as the headset, adjusting threaded headsets is fast becoming a lost art at bike shops with lots of employee turnover, that combined with the steerer mounted cable hanger means a loose headset could just be a loose headset.

I would be interested to know what kind of inspection/testing the LBS did. Specifically if they held the steerer tube in a vise (with wood or other protective material) and flexed the legs looking for movement in the crown area.

If you really want to get to the bottom of it, this would be a good candidate for a dye penetration test. Maybe a machine shop would be able to perform something like this. 

On Thursday, July 7, 2016 at 9:46:41 PM UTC-4, Nick Bull wrote:

[Nick Bull]

Thanks, Jeff.  I don't know what kind of testing the LBS did.  But I took the fork out of the frame and put the steering tube in a vice and then I held the vice and the table to which it is mounted in place while my wife put her weight on the fork legs.  We did this with the fork tips pointing up and then again with them pointing down.  Sideways pressure on the legs seemed like it risks cold-setting.  I could not detect any play at all between the crown and the steerer.

[Will Vautrain]

I have a 55cm GR and have also noticed a flexy feeling when braking hard (I don't carry a water bottle under the down tube). I've checked the headset many times on such occasions, and have never found it to be loose. I have generally chalked it up to the somewhat long quill stem, and generally lower stiffness of threaded vs threadless systems, with which I have more experience. But maybe the GR fork is just flexy.

[Evan Baird]

Probably worth taking it apart to see if there is any visible cracking or bulging going on. Depending on what headset you have that could be a contributing factor. The newer complete bikes come with IRD roller drive headsets that can be run a bit tighter and seem to be less prone to jiggling and gnome related issues.

[David Cummings]

Neither can I.

[Nick Bull]

Not sure why that is.  Even when I am logged out of Gmail/Google I can still go to the 650B group and download the video and watch it on my PC.  So it doesn't seem like it's a privacy-setting related issue.  What happens if you download the video and run it?

[Nick Bull]

Hi, Evan,

As I mention above, it's already been taken apart both by me and the LBS and neither of us can see any visible cracking or bulging.

Headset is a Miche headset bought from Mike Kone.  Crown race appears to be solidly seated.  That was one of the things I had the LBS check.

Nick

[Evan Baird]

Sorry if the gnome reference was too oscure for y'all. I really though everybody already know about this blog.

https://quarterlybicycle.wordpress.com/2016/05/06/oscillating-gnomes-found-to-be-the-cause-of-planing/

No, I have nothing to do with this, and no I don't know who the author is. If I did I'd give them the biggest high five though. Sew lol.

[Jeffrey Kane]

Evan ... genius. Watch yourself though ... wouldn't want to get banned from two lists!

[Evan Baird]

¯\_(ツ)_/¯ There's always twitter

[satanas]

And how long is a piece of string???

The answer is "it depends" - on both what sort of fork it is, and also on who you ask.

FWIW, I have a V1 GR and my fork *never* flexed noticeably, no matter what; if yours is a later version maybe they listened to some of the gripes about v1. If there's nothing wrong with your fork and it actually flexes(!), then IMHO that's a significant improvement. However, I'd inspect the joints between the steerer and fork crown, and between the crown and fork blades very carefully, but please note that lightweight forks (ie, ALAN, Columbus SL, Toei, etc) *will* flex noticeably under hard braking and over large bumps. IMHO, it's probably impossible for anyone to tell if yours is behaving normally without close inspection or a test ride.

The video is inconclusive - it's very hard to see where the flex is taking place, and it'd be useful to see the fork crown area. And the other fork seems to flex quite a bit too - how much load is being applied?!? It looks to me as if the GR blades aren't flexing much, but maybe that's the angle. If all the movement is taking place near the crown then I'd be worried.

Later,

Stephen

[Mark Guglielmana]

After viewing your videos, and reading the other responses, I would think beefier fork blades may be better for you. With the tires you're running, most of the shock absorption is handled by air. 

Thanks for posting those videos, I'm going to go and do the same on my quiver of bikes and see how much different they are. 

On Thursday, July 7, 2016 at 6:46:41 PM UTC-7, Nick Bull wrote:

[Steve Palincsar]

On 07/08/2016 10:04 PM, Mark Guglielmana wrote:

After viewing your videos, and reading the other responses, I would think beefier fork blades may be better for you. With the tires you're running, most of the shock absorption is handled by air. 

Maybe it's just me, but it didn't look to me as though what I saw that could possibly be "flex" in Nick's video was coming from the fork blades at all.  It wasn't much, but it looked to me almost as though it was all happening at the headset.

[Eric Keller]

On Fri, Jul 8, 2016 at 10:07 PM, Steve Palincsar <pali...@his.com> wrote:
> Maybe it's just me, but it didn't look to me as though what I saw that could
> possibly be "flex" in Nick's video was coming from the fork blades at all.
> It wasn't much, but it looked to me almost as though it was all happening at
> the headset.

agreed, that looks like rigid body motion to me.

[Nick Favicchio]

I too own a v1 soma gr and the fork NEVER flexed. Very stiff fork. Put a few thousand miles on it and rode it loaded for touring, including a trip that was entirely front loaded. Fork was a rock.

Fwiw

[Mark Guglielmana]

Nick,

To test the headset theory, crank it down and "lock it up". Overtighten. Repeat your video. See if it changes the amount of flex. 

[James Roedl]

Nick,

To my eye the Gunner flexes more.  It should a rigid MTB that's the idea.  The Soma looked like normal flex.  I have had the same scares, did the research, built a couple frames and now appreciate the flex.  Flex is good.  It's the best kind of suspension or compliance.  It absorbs shock without absorbing a measurable amount of rider energy.My Surly Krampus fork may have more flex than your Soma and it is very beefy.  The issue is more the clearance being tight.  I bet if you could use a bottle cage with adjustable holes you could get it not to hit.

Best,
James

On Thursday, July 7, 2016 at 8:46:41 PM UTC-5, Nick Bull wrote:

[Jeff Bertolet]

I think the most telltale sign would be a creaking noise. Most cracks would make noise when flexing like that. Without any noise, I would not be too concerned.

[Brad]

You can make a DIY headset press pretty simply.  A couple of fender washers and a long hex bolt and two wrenches.  One variant uses bronze bushings.
Another thought.  Make sure that the threads on the fork go far enough down that the adjustable cup on top can actually completely tighten against the balls and the race.
If you do not have access to a shop with fork threader you can switch out the top cup with one from a threadless headset of the same specs and then use the locknut and an extra thin spacer to compress it.  I read that Jobst Brandt use to file the threads off the adjustable cup and do this to reduce the thinning effect that threading has on the steerer tube.

[WMdeR]

Dear Jeffrey,

Off topic: 

Evan brings a lot of insight and insider knowledge to the discussion, but his tone and online personal demeanor went over the line set by the BOB community on a couple of occasions. On one of those, he was called out on-list.

Evan ended up getting moderated (i.e. somebody like me has to see his post and approve it), not banned, over on the BOB list. It happens, and it is ordinarily a temporary thing. 

We ban spammers, and, to the best of my knowledge, we have never outright banned an active and contributing poster. 

Best Regards,

Will

William M. deRosset

one of the moderators over at the bob list

[Steve Palincsar]

On 07/09/2016 01:13 PM, WMdeR wrote:

Dear Jeffrey,

Off topic: 

Evan brings a lot of insight and insider knowledge to the discussion, but his tone and online personal demeanor went over the line set by the BOB community on a couple of occasions. On one of those, he was called out on-list.

Evan ended up getting moderated (i.e. somebody like me has to see his post and approve it), not banned, over on the BOB list. It happens, and it is ordinarily a temporary thing. 

We ban spammers, and, to the best of my knowledge, we have never outright banned an active and contributing poster.

Not so far as I know, either.  We have had a couple of occasions where people banned themselves, one just moments ahead of the Official Axe (a troll who joined the iBOB list to go on at length on the superiority of Cannondale aluminum race bikes over any other bicycle ever made).

[Mark Guglielmana]

Sounds like the same guy who did the same at the bikeforums.net C&V site. His postings were Tolstoy-esque in length.

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Mark Guglielmana

[Steve Palincsar]

It could easily be.  Details have blurred, but I do recall finding him doing the same thing elsewhere.  

[Evan Baird]

Speaking of which, have y'all ridden that new Cannondale? It has a SIGNIFICANT amount of flex, if you know what I mean.

[Fred Blasdel]

On Sat, Jul 9, 2016 at 1:04 PM, Steve Palincsar <pali...@his.com> wrote:

(a troll who joined the iBOB list to go on at length on the superiority of Cannondale aluminum race bikes over any other bicycle ever made)

That's not trolling it's the gospel truth! A CAAD10 will plane better than any steel mass production bike you've been able to buy since the demise of Lemond and they max out the clearance of short reach brakes. Aluminum is not harsh and steel is no realer than any other material.

Not necessarily better than any other bicycle for any purpose, but certainly the platonic form of a sporting bicycle

[Evan Baird]

This one looks especially supple. Subtle. Soupple?

[Steve Palincsar]

and that's why you've exchanged your Elephant for a CAAD10, right?