Fork offset and its effect on trail numbers

[fzrbrandon]

Hi everybody

I posted this up over on the Archives but I wanted to bring up the discussion here as well:

I've been pondering quite a bit lately the issue of fork offset and it's effect on trail numbers. The information that I'm getting seems to conflict with what, to me anyway, is most logical. It is my belief that fork offset should have zero effect on trail distance (so long as the only thing that happens to be changing is, in fact, the offset between the fork tubes and the steering head). Here's what I DO know (someone please jump in to correct me if I'm wrong on any of this):

Rake = The angle between (a) the axis of the fork tubes and (b) the axis of the imaginary vertical line that runs perpendicular to the horizontal plane of the ground.

Trail = The distance between (a) the point on the ground where the axis of the fork tubes meets the horizontal plane of the ground and (b) the point where the imaginary perpendicular vertical line, which runs through the axle, meets the ground.

So... the trail number should not change because the offset between the fork tubes / steering head is completely independent of the angle of the fork tubes and the fixed position of the axle at the end of said fork tubes. If the offset increases by 5mm (for example), the WHOLE assembly will move forward by 5mm. The ONLY thing that should be effected by an offset change is the wheelbase (which increases by 5mm).

Note that I DO believe that an increase in offset will net slightly slower steering because of the increase in wheelbase but it should have NO effect on steering where the trail is concerned. That said, I would agree that offset SHOULD be considered when doing fork swaps because of it's OVERALL effect on steering performance but the discussion should be regarding wheelbase numbers, NOT trail numbers.

Now raising or lowering the rear end of the bike (or front for that matter) WILL dramatically change the trail number (because of the obvious resulting change in rake). However, that is a completely different discussion.

Am I missing something here???

Then about an hour and a half later I posted this up...

Okay...

I was out in the garage staring at the front end on my track bike and I thought of ONE way where a different offset could change the trail.

Considering that the triple clamps sit at a right angle perpendicular to the axis of the forks (i.e., up on the front side, down on the rear) and assuming that the axle is the exact same distance to the bottom triple clamp (or any other fixed reference point) on either example of offset (i.e different front ends), I can see where the RAKE would be slightly decreased due to the fact that the front end would drop ever so slightly. Since the offset is moving upwards toward the higher side of the triples (thereby raising the axle slightly), given the same wheel and same tire - the front end has to come back down so the tire can touch the ground. THIS would decrease the trail by a small amount. However, I think the offset distance by itself is ultimately WAY less of a factor than the distance between the axle and common fixed reference point will be when swapping to a completely different front end. By that I mean, the fact that front end A is way shorter than front end B is a bigger concern than the difference in offset between the two.

Please chime in as I'd like to hear what the peeps think about this...

[ragedigital]

It is my belief that fork offset should have zero effect on trail distance

Look at the image below. Increasing the offset, distance forward of the neck, will push the centerline (BLUE line) of the front wheel forward decreasing the amount of Trail.

They also have a calculator here that you can use: http://www.rbracing-rsr.com/rakeandtrail.html

[fzrbrandon]

It is my belief that fork offset should have zero effect on trail distance

Look at the image below. Increasing the offset, distance forward of the neck, will push the centerline (BLUE line) of the front wheel forward decreasing the amount of Trail.

They also have a calculator here that you can use: http://www.rbracing-rsr.com/rakeandtrail.html

Hi Darren!

Sorry, but that still doesn't make any sense. What is that red line and why does it NOT run straight down the axis of the head tube? You can see that it angles down slightly. The shorter of the two trail numbers is based on this red line (which is not parallel to the black one and it SHOULD be). Also note the description of the shorter trail length - "Trail (raked triple trees)". This, to me, implies that they ARE in fact changing the angle of the fork tubes (i.e., "raking") which, of course, will change the trail numbers. That's rake angle though, NOT offset. So... I still say that, when speaking of offset, if all that is happening is that the fork tubes are farther forward (or rearward) by however many millimeters, the trail will not change (again - assuming that the axle is the same distance from the ground). The only difference is that everything has moved forward by "x" millimeters thus increasing only the wheelbase. As I alluded to earlier, given the same fork length, the trail will change only due to the top of the forks being at a slightly higher location and therefore the front end needing to drop a bit to get the wheel back on the ground (given the same wheel / tire diameter).

Now, is it possible that newer forks have a different offset between triples of the same tree? In other words, maybe the slots that the fork tubes go though are beveled somehow which would give a different rake? I don't know, I'm just guessing at this point.

Thanks a bunch for your response! I appreciate it!

[ragedigital]

What is that red line and why does it NOT run straight down the axis of the head tube?

Don't pay attention to the RED line unless you want to involve "raked" triples.

As I alluded to earlier, given the same fork length, the trail will change only due to the top of the forks being at a slightly higher location and therefore the front end needing to drop a bit to get the wheel back on the ground

Excessive offset will drop the frame on the ground if the fork tubes are not extended. You may be able to compensate a bit by raising or lowering the back end.

The image below shows a decreased TRAIL due to a greater offset; however, the forks were extended to accomplish this.

[fzrbrandon]

Excessive offset will drop the frame on the ground if the fork tubes are not extended. You may be able to compensate a bit by raising or lowering the back end.

This is exactly what I was figuring.

The image below shows a decreased TRAIL due to a greater offset; however, the forks were extended to accomplish this.

I agree with the first part of your quote (due to the above). The second part however, is somewhat contradictory I think. By NOT extending the forks, you will definitely lose some trail due to the front end dropping slightly (and therefore decreasing the rake angle). By extending the forks out (by the right anount of course), you can maintain the previous trail number by making sure the front end doesn't have to drop at all to get back on the ground.

I think I've got a handle on it now. Thanks a bunch for your help as always!

BTW, I'll be pimpin' the FZR Online shirt on race day this weekend!

[ragedigital]

The second part however, is somewhat contradictory I think. By NOT extending the forks, you will definitely lose some trail due to the front end dropping slightly

By adding OFFSET, to the graphic I created, it caused the front wheel to raise off the ground. In order for me to bring the wheel back down, I either would have had to increase the fork length, as I did, or drop the front of the bike. Once I extended the forks, I simply extended the lines they had drawn.

BTW, I'll be pimpin' the FZR Online shirt on race day this weekend!

Sweet!

[yamaha_george]

Brandon,
got late to the party with this one BUT there is a WIKI page on suspension tuning that may help you get a handle on it better

OR

There is a WIKI link for Tony Foale's definitive chassis design and suspension sorting book.with ISBN number if you want to order it from the library