Mrs Mick F is running a charity auction at the end of the month down at the Rising Sun Inn, and the bike will be one of the items on offer. It's not worth a great deal, but every little helps. At least the bike will be tested, checked over, and had a proof-ride from me.
A few weeks ago whilst doing Bikeability Training in school, one girl with a new bike had her forks reversed. " We bought it last Saturday in Halfords " was the answer to our question about it. Unfortunately not an uncommon experience. Always recommend using LBS.
Some boffins have produced a mathematical model of bike stability called jbike6. It runs in Matlab and can be downloaded for free.
Two important outputs which it gives are the weave speed and the capsize speed. The weave speed is the speed at which the oscillations become smaller and smaller when the bike is perturbed. Below this speed, the bike will wobble more and more after being perturbed, until it falls over. Above the capsize speed, the bike will slowly topple after being perturbed. Between the weave speed and the capsize speed the bike is self-stable.
All this is based on a rider who is firmly attached to the bike, makes no attempt to balance and has no arms. The bike is completely rigid, has infinitely narrow tyres and no friction in the headset.
The effect of radically increasing trial by reversing the forks is to increase both the weave speed and the capsize speed, as well as the total stability. This means that the bike is more stable, but only if you are going fast enough. Mick F was probably not going fast enough to benefit from this stability, so at the speeds he was going at the bike was indeed less stable.
Sorry Stephen, that's not correct. I'm having difficulty explaining.
Trail is only half the issue.
Thought experiment.
Imagine your bike held rigid on a bike stand with the front wheel off, forks dead ahead. Look downwards. The pair of dropouts will be in line equidistant either side of the stem. Turn the 'bars left a bit.
How did the dropouts move? Did they also move left? They should have done if the forks were correct.
Make the forks wrong. Do the same experiment. Turn the 'bars left a bit.
How did the dropouts move? They would move RIGHT despite having more trail.
I know what you're getting at, but I don't think the position of the dropouts matters much. What matters is the position of the contact patch. In both of the cases you describe, the contact patch will move to the right when you steer left. With the forks reversed it will move further right (more trail).
In jbike6, I ran the default bike (some kind of Schwinn cruiser), then I adjusted the trail such that the fork was reversed (it had a negative offset). The maths takes care of the rest!
I can post the outputs of the program here if you are interested. I think it is quite interesting! If you give me the necessary measurements I could run it for this specific bike.
It's easy to get focussed on the shape of the forks, which is actually irrelevant. Straight forks, raked forks, forks with a forward then backward bend like the Pinarello Dogma, all will steer the same if the trail, steering angle and contact patch size are the same. (Fork shape might affect other aspects of handling of course.)
Ok, how do you explain the difference in handling riding along, and the "oversteer" when moving slowly round a tight bend? No doubt the oversteer was there all the time too.
All completely went when the forks were correct.
I'm convinced it's due to the phenomenon I've described. Not difficult to ride, just weird, though one could get used to it.
