xifias

hello to everyone.
i bought a second hand road bike,and i have some suspicion over the fork,it looks a bit bent back.previous owner told he had a very minor crash.

i want to find some ways to diagnose the exact problem (if it is,which blade is bent,to which side,in what extent,etc)

one clue is that when i put the wheel in place,it seats with an uneven gap between the blades,as if the one forkend is longer than the other.in other words the tire isnt centered below the brake bolt.the wheel is properly dished.tried both ways.

is this the case,or its just a poorly made fork with uneven forkends?

is some way to trace out the fault? some string method?

frame material alu
fork steel.

thank you

dabac

String is good. Pull fork from frame. Run a string straight across steerer tube and through both dropouts. See if the string runs symmetrical on both sides. This ought to show sideways deflection fairly well. Also lay fork on a flat surface to check that fork crown is parallell with dropouts.
An even bend backwards can be harder to detect, due to design features of crown and curve of fork legs.

Airburst

It's hard to tell which it is from where the rim ends up if it's only a minor misalignment of the wheel. If it was severe enough to make adjusting the brake difficult or cause tyre/fork blace contact I'd definitely be inclined to blame crash damage.

Another thing to check for would be paint rippling, cracking or flaking off somewhere on the fork - paint tends to do that on metal that's been bent, the metal can deform plastically (i.e. bend) but the paint can't, so it cracks.

The string thing dabac suggests will reveal more.

What's the bike?

Andrew R Stewart 

The easiest way to gage the fork is to remove it from the bike and place it, via the sreerer, on a relatively flat surface. A vee block is perfect for this. Then one can compare each blade/drop out to the surface, the crown and the steerer.

For the second time in a week I'll provide a link to a post I made a year or so ago about checking fork alignment with simple tools at home. Andy.

https://www.bikeforums.net/bicycle-me...ard-right.html

xifias

frame is a willier triestina evasion with 1" headset.fork is propably not the original one.the bike was built from a local bike store with nos parts,nitto stem,3TTT bars,shimano exage cranks and sachs aris RD.pretty weird setup.

Though there are some dents,ripples to the higher part of the fork blades near the brake hole but i suspect these are manufacturing fault in the welding process of the blades.badly the fork is painted nickel plated though and i think its more tolerating to distortion than usual paint does.

ive read briefly your guide and i have some questions.lets say the axle is finally parallel to the fork crown.fork blades straight etc.
how do we check the correct angle of the blades compared to the steerer?this is the main effect of a front crash and my concern.

fietsbob

Putting a Known straight tube that fits snugly inside the steerer and having it pass By the fork tips offers a reference point

Though Butting in the bottom of 1" steerers wont help that be easy .

the Flat table, V block and flat block under the fork hold that fixed

then Lay out an accurate base line on the table that is the same as the centerline of the steerer tube..

height gages use the flat base as a Zero point.. to get the axle CL to the base distance.

you should be able to measure the distance variations on either side of the CL to the fork tip face.


(perhaps buying thick Plywood and making a way to Rigidly fix your fork steerer holding V block will help your Improved accuracy in measuring ..

a DIY alignment table ..

cny-bikeman

Which angle? do you mean the rake (distance the fork dropouts are forward of the imaginary line drawn through the head tube)? Or are you asking how to determine if the blades are equidistant laterally from the head tube?

xifias

equidistance of the blades is solved with the wheel example above.im talking about the straightness of the forkblades in comparison to the head tube,just like it was before the crash.

fietsbob says an interesting approach.

FBinNY 

A curved blade fork pushed back as from a crash is very easy to diagnose.

Stand next to the bike with the wheel pointing forward. Bend over and sight down the steerer/headtube and out the the blades. The blades will clearly be in line with the sight line until about half way down where they curve forward. If you can't see the error, then the fork os fine.

Straight blade forks are harder to diagnose this way, but they rarely bend in the blades, so you're only looking for a bent steerer which will manifest by a crown race that isn't concentric to the lower cup. Rotate the fork and look for the crown race to appear to emerge or recede into the cup. (it shouldn't). You'll also usually (but not always) have a headset that binds a bit when turned to either or both sides.

Andrew R Stewart 

Agree with Francis above WRT looking for bent blades. But I'll add that what really counts is the straightness/alignment of the fork, not whether the rake is less but otherwise the fork is straight. A fork that is offset to one side will affect the tracking (no handed riding) of a bike (assuming the rest of the system/bike isn't also off). But a twisted or bent back fork need not be offset to one side. So my biggest goal when aligning forks is to get the wheel aligned with the steerer. Next is having the axle parallel to the crown, but this isn't truly needed for a straight fork. Last is the blades' upper sections (the portions that are not raked) being in line with the steerer.

I find that these two last aspects is very easy to see, and therefore to dwell on. But not all that important to a straight steering bike. It's the first aspect, the wheel/rim being inline with the steerer, that's most important but the hardest for most to see. Andy.

FBinNY 

What we'e all forgetting is that the one definitive factor isn't how to diagnose a fork. Before trying to determine how fork is bent, if even If it's bent, we need to know if there is a problem in the first place. The right answer is to ride the bike and see if it tracks straight or pulls.

I offered advice on how to quickly determine if a fork is pushed back, but didn't go farther because I cross bridges one at a time if and when I come to them. If the bike rides OK, then everything else is academic. If it pulls to either side, then we have something actionable, and how it pulls is the starting place for diagnosis.

BTW- there are many ways or approaches to checking a fork for squarness, starting with how the wheel sits and how the brake seems aligned to that wheel. Those will point you in a direction, and then the fork can be checked in detail by a number of equally viable methods, using tools and stuff at hand. IMO- it's not about tools but understanding the principles of measurement.

xifias

the first days i bought the bike and rode it,was pulling left strongly.
then changed the wheels,put a fulcrum 5s and the effect got very weeker but still exists.

i have to mention that the wheel in the fork sits unevenly and gets closer to the right blade,IF i dont center it by hand before closing the Quick Release.

although its steel,im a bit afraid to put it on a vice and start bend it.i surelly dont want to snap suddendly when im on 50km/h with cars passing by!

FBinNY 

The steel used in fork blades is very ductile and will tolerate a decent amount of cold working (bending) with no increased risk of failure. So you can get over your fear of bending.

OTOH, knowing where and how much to bend calls for some measuring skill and understanding. You're not referencing the fork itself as much as theoretical lines and planes that exist in the space the fork defines.

For example, a wheel may sit nearer to one blade at the top because the entire fork is offset to one side, or because one tip is higher so the wheel sits at an angle. I'm not saying it can't be done at home --- it can --- but it calls for patience and the ability to see the whole picture.

xifias

i see.indeed it may not be very sophisticated of a repair but it's a critical one,that calls for some expertise to deal with it.
ill try for a start to see what its going wrong,following the paradigms above and then decide.

if i put another one, ie. carbon,will worth it?
i rode a friends alu-carbon bike (frame-fork) and seemed very light front-end.was it of the lightness or the different leverage of the system (44cm bars instead 40 on mine)

FBinNY 

Here's a quick diagnostic you can try without removing the fork.

Use bungee or rubberbands to tie the front wheel to the down tube and keep the fork straight. The tie a loop on one end of a string, and slip it under the QR. Bring th string up and around the top tube behind the headset, and down the other side. Tension the string and trap it under the QR which will hold it there.

Stand in front of the bike and look at how the fork sits within the string's triangle. Does it look asymmetrical? Measure various points from a frame reference to the string. It should match on both sides everywhere. One telling reference is the brake center bolt. If the fork is symmetrical, and the front wheel centers between the blades, the bike should track. If not, check the frame for alignment via a similar string method, rear axle to head tube.

This is a coarse diagnostic because it can miss subtle issues, but is very good at showing that a fork is off, and give a rough sense of where and how it's off.

xifias

thank you FBinNY.ill try it that also.

i already did the string method you described to trace out any deforming on the frame.looks straight,also put a ruler on the tubes and seem straight.

as one described above,looking the bike from sides,indeed the fork seems a bit bent back.