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After a low speed collision with the front of a car in which a cars bumper met the side of my fork (I was in a track stand-esque stance) my fork was bent slightly so I replaced it with a (inexpensive but good enough) fork from amazon that very closely resembled the dimensions of my original fork. The length from the bottom of the head stock to the wheel may be slightly longer on the new one.

Since replacing it, my bike has become difficult to ride without handle bars. I know that the forks geometry basically determines this, but I think the change is slight enough it wouldn't have made it unstable.

Could there be another reason for causing the instability or is it likely that the new fork is different enough geometry to cause this issue. Things I've thought of so far:

  • Front wheel is untrue (checked and it's pretty good).
  • Front wheel is out if dish (didn't think about this until now so I'm not sure but it's hard for me to check since I don't have a truing stand).
  • Rear wheel out of true or out of dish (possible because I had to replace some spokes).

My bike is a 1980s Raleigh Capri (which is a steel frame) with some minor mods and generally well kept.

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  • Did you use a new head set? Or did you either re-use the old headset's race from your old fork? Has your fork a threadless or threaded steerer?
    – gschenk
    Commented Apr 29, 2018 at 20:52
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    You do not need a truing stand to check if a wheel is true. Put your bike in a work stand, prop it onto something, or turn it upside down. Turn your wheel and observe it compared to the gap next to the brake shoes. As a very general remark: if your brakes don't rub because of an untrue wheel, it is not badly enough warped to cause instability when riding.
    – gschenk
    Commented Apr 29, 2018 at 20:55
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    Have you checked if your frame is warped? Are seat tube and head tube in the same plane as the rest of the diamond frame? Does your aft wheel track properly your forward wheel? You can do this roughly and quickly on wet ground: look at the prints when you push it straight.
    – gschenk
    Commented Apr 29, 2018 at 20:59
  • A relatively minor change in the fork rake/trail can cause major changes to steering stability. Of course, it's also possible that your frame was bent in the collision.
    – Daniel R Hicks
    Commented Apr 29, 2018 at 21:14
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    So i took apart my head set and realized that i had my bearings upside down. After fixing it it is much better and the steering is a bit easier (dont know how i missed it lol).
    – user74671
    Commented May 5, 2018 at 4:18

1 Answer 1

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Bicycle dynamics

A bicycle may only be ridden because of the peculiar steering geometry. The centre of the contact patch of the tyre is behind the point where the steering axis intersects with the ground. The distance between these points is called trail. In order to visualise it you may have a look at this figure from the bicycle dynamics article on Wikipedia:

Wikipedia image that displays the geometry of a bicycle that determines its steering[

The trail is determined by a number of constraints: head angle, wheel diameter, fork rake, fork length.

In general: If trail increases, the bike becomes more stable on straights, if trail decreases the bike becomes less stable on straights.

How will your new fork change your bike's geometry?

A longer fork with the same rake will slightly slacken the head angle, as the bike is tilted a little. This increases trail.

However, it will also reduce trail as the ratio of rake/length becomes smaller. this is not correct

If the fork has more rake, it will very directly reduce trail.

Headset

A lose or misaligned headset will cause an unstable ride. You may test your headset as follows, if you have a rigid fork, i.e. no shock absorbers.

Stand beside your bike hold the bars, and pull the forward brake levers. Your brakes should lock the forward wheel. Then rock the bike fore and aft. Do you feel any give, any instability? Push down on the handlebars and force the bike fore and aft. Turn the wheel sideways and repeat.

Anecdote: I had an unstable bikes for years. I found only out when stripping it for parts that I confused the top and bottom ball cages of the headset bearings. The lower one led to a slightly too tight fit, the upper one was slightly too large. I never could really tighten that headset properly.

The original poster mentioned in the comments that their problem was caused by a similar mistake: "[I] realized that i had my bearings upside down."

Warped frame, damaged head tube, cracked frame

If a collision was strong enough to render your fork unusable it might have damaged the bike's frame. Some points you might want to check:

  • are there any cracks at the head tube? Did paint flake off recently?

  • Are head tube and seat tube parallel. Are head, seat, top, and bottom tube in one plane?

  • do you hear any creaking noises when cycling?

Bad tracking

Bad dish of your rear wheel and a warped frame may cause bad tracking. That is, when going straight the rear wheel does not follow the track of the front wheel exactly. When cycling the symptom is usually not so much instability, but more a tendency of the bike to steer to one side. You might still ride hands-off, but have to nudge the bike by shifting your centre of mass to one side.

You may easily check gross tracking problems by pushing your bike through a muddy puddle. If the tracks are straight, parallel, but do not coincide you have a tracking problem. If they are not straight, or not parallel, you have to push more straight (not easy at all, bikes are not meant to go straight!).

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  • Thanks for the answer, lots of good information. Can you clarify about the effects of a longer fork on trail and rake? would it increase or decrease trail? Additionally, I did reuse the bearing and bearing traces (is that what thats called?) from the original. This was my first time assembling the head set so I may have screwed up.
    – user74671
    Commented Apr 30, 2018 at 2:29
  • @user74671 you are right not to understand that part. I think I fell for a fallacy or confused something. Let's look into it later.
    – gschenk
    Commented Apr 30, 2018 at 15:51
  • Although not the issue in this case, if the center of mass of the handlebars, fork, and front tire gets moved back, that will also reduce stability. I've seen this issue on a road bike where the drop down handlebars were replaced with swept back handlebars.
    – rcgldr
    Commented Apr 30, 2018 at 17:28

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