Simple question; is there a formula to derive the offset (or rake) of forks from a given head angle? Or visa versa?
eg 72deg = 1.75" 71deg = 2.125" etc
How do I calculate fork offset?
Re: How do I calculate fork offset?
Do you mean trail? i.e. the castor effect of the point of contact of the wheel being behind the steering axis of rotation.
Calculation would only be possible if the fork were straight and in line with the head tube/steerer. Otherwise the curve or offset is totally independent of the head angle.
Calculation would only be possible if the fork were straight and in line with the head tube/steerer. Otherwise the curve or offset is totally independent of the head angle.
Re: How do I calculate fork offset?
fork offset should be varied at any given head angle to give a desired trail value. There is a table here
http://johnforester.com/Articles/BicycleEng/Kvale%20Geometry.pdf
to a first approximation the bike will (over a small range of head angles) steer 'about the same' at any given trail value. High trail gives good stability in a straight line, does not change direction quite as quickly, and will follow cambers somewhat.
In extremis a slack head angle will leave the steering prone to 'flop' and a steep head angle feels a bit better when you are carrying a front load.
I've ridden bikes that steer well (in a particular situation) with head angles from ~71 to 74 degrees, and trail values that vary from ~30mm to 80mm. If in doubt, find a bike that you like the steering on, and base your choice on that.
You can measure offset fairly accurately by taking a measurement from the BB shell to the hub, and repeating the same measurement having turned the steering 180 degrees. The offset is about half the difference in the two measurements.
hth
cheers
http://johnforester.com/Articles/BicycleEng/Kvale%20Geometry.pdf
to a first approximation the bike will (over a small range of head angles) steer 'about the same' at any given trail value. High trail gives good stability in a straight line, does not change direction quite as quickly, and will follow cambers somewhat.
In extremis a slack head angle will leave the steering prone to 'flop' and a steep head angle feels a bit better when you are carrying a front load.
I've ridden bikes that steer well (in a particular situation) with head angles from ~71 to 74 degrees, and trail values that vary from ~30mm to 80mm. If in doubt, find a bike that you like the steering on, and base your choice on that.
You can measure offset fairly accurately by taking a measurement from the BB shell to the hub, and repeating the same measurement having turned the steering 180 degrees. The offset is about half the difference in the two measurements.
hth
cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Re: How do I calculate fork offset?
Thanks Brucey,
That was a very interesting article.
It seems then,that it is a case of 'I have found that this combination (of head angle/fork offset/resultant trail) works for me', rather tha 'I use the XYZ formula'. i.e. received wisdom rather than 'hard science'.
That was a very interesting article.
It seems then,that it is a case of 'I have found that this combination (of head angle/fork offset/resultant trail) works for me', rather tha 'I use the XYZ formula'. i.e. received wisdom rather than 'hard science'.
Re: How do I calculate fork offset?
there is (now) a hard science answer as well, but you need to be a well trained mathematician to really understand it, which means it is less than useful to normal folk.
Also I have yet to see a simple formula that allows you to establish a stability index that relates to how it feels to ride a bike with x-type steering geometry, so in the absence of that it might as well be another form of received wisdom.
The reality is that unless you are building your own frames you will be stuck with a particular head angle and it is just a question of choosing a fork offset that is
a) available and
b) gives you steering that you will either like (because it is a bit like a bike you have ridden previously) or that you will get used to.
Note that you can get used to almost anything.
cheers
Also I have yet to see a simple formula that allows you to establish a stability index that relates to how it feels to ride a bike with x-type steering geometry, so in the absence of that it might as well be another form of received wisdom.
The reality is that unless you are building your own frames you will be stuck with a particular head angle and it is just a question of choosing a fork offset that is
a) available and
b) gives you steering that you will either like (because it is a bit like a bike you have ridden previously) or that you will get used to.
Note that you can get used to almost anything.
cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Re: How do I calculate fork offset?
Brucey wrote:Note that you can get used to almost anything.
+1 on that.
I get used to my 60s Moultons within a few hundred yards even after not riding them for months. The 16/17" wheels with relatively large fork offset result in a trail of just about 1". I don't really like the feel of this but I do get used to it. Makes me wonder if I could reduce the offset, thereby increasing the trail, by a bit of "cold setting" of the forks. Any thoughts on this? It might need a longer reach brake...
Re: How do I calculate fork offset?
I suggest that you make up some hub offset plates and try out your new steering geometry before you go modifying forks. You might find out that it isn't as simple as making the trail the same as on a bike with bigger wheels.
One possible reason for this is that there is a range of times for the steering to autocorrect which is acceptable, and if the response of the steering falls outside this range then the steering may not be as good.
For example if you ride normally and perturb the steering, it will self correct at a certain speed. If this speed happens to correspond with a mode of oscillation of the bike (in bending or twisting) then you can end up with a bad shimmy. Increasing the restoring force (by increasing the trail) may increase the tendency to shimmy.
It is quite common for small-wheeled bikes to have a low trail figure. However the moment of inertia of the steered assembly is also very low; thus it could be argued that you don't need to a strong restoring force to make a bike with acceptable steering, and if you increase the trail you might provoke some other unintended effect.
Even on large wheeled machines, changes in wheel weight do affect the feel of the steering.
I'd be interested to hear how you get on if you do try something.
cheers
One possible reason for this is that there is a range of times for the steering to autocorrect which is acceptable, and if the response of the steering falls outside this range then the steering may not be as good.
For example if you ride normally and perturb the steering, it will self correct at a certain speed. If this speed happens to correspond with a mode of oscillation of the bike (in bending or twisting) then you can end up with a bad shimmy. Increasing the restoring force (by increasing the trail) may increase the tendency to shimmy.
It is quite common for small-wheeled bikes to have a low trail figure. However the moment of inertia of the steered assembly is also very low; thus it could be argued that you don't need to a strong restoring force to make a bike with acceptable steering, and if you increase the trail you might provoke some other unintended effect.
Even on large wheeled machines, changes in wheel weight do affect the feel of the steering.
I'd be interested to hear how you get on if you do try something.
cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Re: How do I calculate fork offset?
Great suggestion to use hub offset plates before committing to permanent changes. I'll give it some thought and knock something up when the mood takes me.
Re: How do I calculate fork offset?
Brucey wrote:I suggest that you make up some hub offset plates and try out your new steering geometry before you go modifying forks. You might find out that it isn't as simple as making the trail the same as on a bike with bigger wheels.
Well I have given this some thought and not come up with much! How does one make up offset plates that can be firmly attached to the existing front fork dropouts without fouling the front hub/spokes? Any pointers, preferably with pictures, gratefully accepted.
Re: How do I calculate fork offset?
put the adaptor plate outside the extant droputs, and secure it using a shallow-headed bolt, from the inside. Pack the hub with washers to get the OLN right for the (now offset) slots into which the wheel mounts. To prevent the adaptor from moving, it is best if there is a tang (the longer the better) on it that is braced against the fork leg (and secured using a hose clip or something). The adaptor plate can have slots in it at ~15mm centres to allow different offsets to be tried. Don't expect to be able to use the front brake with the adaptor in use. Don't expect to be able to try an offset that is very close to the original offset (unless the wheel is lower than originally fitted); the locknuts on the hub will foul with the adaptor bolt otherwise.
I suggest 4mm thickness Aluminium or 3mm steel as minimum thicknesses for the adaptor. If you are trialling large changes in offset, best to go thicker.
cheers
I suggest 4mm thickness Aluminium or 3mm steel as minimum thicknesses for the adaptor. If you are trialling large changes in offset, best to go thicker.
cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Re: How do I calculate fork offset?
I made some adaptor plates up to increase the rake on my trike conversion to help with reducing the effect of camber on the steering. used some 1/8 steel strip i had hanging around. I carefully cut and filed slots to replicate the the drop out but it would have been easier to have just drilled a hole to suit the axle. I didnt worry about the oln dimension increasing by 6mm and just accepted that it pulled the forks together a fraction when the qr was tightened, These were old forks with mudguard eyes and no lawyers lips.
To reduce the offset you would need the plates fitted rearwards and as mentioned unless you are careful this will bring the rim higher up in relation to the brake so needs some careful measuring
To reduce the offset you would need the plates fitted rearwards and as mentioned unless you are careful this will bring the rim higher up in relation to the brake so needs some careful measuring
At the last count:- Peugeot 531 pro, Dawes Discovery Tandem, Dawes Kingpin X3, Raleigh 20 stowaway X2, 1965 Moulton deluxe, Falcon K2 MTB dropped bar tourer, Rudge Bi frame folder, Longstaff trike conversion on a Giant XTC 840 
Re: How do I calculate fork offset?
Thanks for the ideas (and photo). Funny how you dig yourself into a hole and can't think your way out. I was stuck on the idea that I had to achieve the same OLD by cranking the extension plates - never thought of spacers- DOH! If I can find some small coach bolts I can probably make the square section just under the dome head lock into the existing dropouts...