rake and trail

[spinman]

I'm thinking of extending the front fork of Billy Jack with a 1/4" thick piece of steel as in pic to offset the wheel and give me a little more extenstion. this would be welded on top of the existing drop out.

I'm just not sure if this will screw up my rake and trail.

THx

 

[TornadoDave2]

without going into a long explanation.... yes it will increase the trail, and alter the way the bike handles. It will tend to make the bike less maneuverable. Whether or not this is a bad thing is a subjective matter. Some folks like a sluggish handling bike.

 

[Wolf]

By welding these plate on to the legs you will drasticaly increase the OFFSET which will REDUCE the trail- not increase it, in fact by looking at it they could possibly create LEAD- also sometimes refered to as negative trail- where the contact patch of the tyre is in front of the virtual steering axis/tarmac interface. If this happens the steering characteristics will be totaly different to normal- to a degree which is very dependant on the frame's steering angle.

Reducing the trail will increase the steering responce and reduce stability, however if you reduce it too much and you have a large wheel- 26"- then the steering inertia will start to become a significant influence in the handling characteristics. This is why on my fast steering bikes I use a 24" front wheel which requires less offset for any given rake/trail combination than does a 26" wheel and gives lower steering inertia!

 

[spinman]

Thanks for the feedback.

Sounds like the bottom line is DON"T DO IT!

It steers great now, and I don't wanna screw with that.

Spin

 

[TornadoDave2]

Wolf.... you are right if the added extension is on the front of the fork. either way, I wouldn't recommend it

 

[Bendix]

it's fun to read about rake and trail and then sketch out a few things. since i'm no bike designer i find some of it surprisingly counterintuitive.....but interesting

 

[spinman]

there must be more to this, as Sugar bear's forks work perfectly, yet look like they would have have a great deal of wheel flop.

 

[Wolf]

The bike in the pic has a very very shallow steering angle of about 45 degrees and a large diameter front wheel. Therefore to achieve a usable steering character the fork needs a large offset! If you used this amount of offset on a bike with a smaller diameter wheel and a steering angle of say 30 degrees (motorbike measurement which is same as 60 degrees in bicycle terms due to bicycles being measured to the horizontal amd m/bikes to the vertical) then the steering would be screwed up as the geometry would tend to 'lead' as I decribed above. looking at your stock fork's offset I doubt your bike has a steering angle much less than 65 degrees to the horizontal so again you will encounter problems.

Also remember that the motorbike in the pic goes well in a straight line at highway speeds but will be an absolute nightmare in tight manouvering!

Why don't you make a pair of bolt on plates and try them out, it will be a good learning exercise for you, and also allow you to keep a sweet handling bike for riding but convert it to radical for shows etc.

 

[spinman]

another thing that complicates this concept is that my bike is a trike, meaning there's no lateral (side to side) movement to consider

 

[Wolf]

Even though Billy Jack is a trike the steering character is still dictated and defined by the geometry. You could quite happily reduce the trail to zero- I doubt that you are riding fast, a trike weighing 155lb is going to take some pedaling making the steering very light if a bit unstable down hills or on bumpy surfaces. You are still limited to how much you can increase the offset. the fork crowns look to have about 3/4" offset plus about an inch at the drop out giving about 1 3/4 to 2 inch total offset. This will give a trail in the region of 4" with the steering angle as it appears in the photos of the trike. Therefore you could add about 3 to 4 inches of offset and have a very light steering trike- again down hill will be somewhat tending to instability! But remember that by increasing offset you reduce trail which reduces the self centering forces generated at any given speed, however as you increase the offset you will also increase the unbalanced steering inertia which will become a dominant force in the feel of the steering and hence overall handling.

If you go for too much offset and end up with lead then the trike will not want to turn because in order for the bars to turn the fork you will be lifting the headstock vertically upwards. Now the trike weighs about 155, you weigh for the sake of augument 160 with about 25-30% of the total weight on the front wheel meaning to turn you must lift a weight of around 80-90lbs by turning the bars. And then when you start to straighten up the steering will want to snap back to the straight ahead position- it wont feel nice!

It seems that you really like the idea so I suggest you make a set of bolt on plates with a series of axel slots giving incrementally increasing offset to allow you to experiment before making any final changes. looking at you fabrication skills this will be straightforward for you to do.

 

[spinman]

Thanks for the elaboration.

I'm gonna leave well enough alone. It's much of a chance to screw things up, just for a different look i want.

It rides amazing as is, givin it's girth.

I just love the look of that offset wheel on the Sugar forks. From what I've been reading, my idea would definately decrease my trail, but not make it zero or neagtive. Seems you run into problems if you do that.

 

[sensor]

hope THIS helps....

 

[Dorian]

If your bike is taken apart then put it back together enough so you can figure out the current trail numbers. You may find that you have flop as it currently sits, and by welding on the new dropouts you may actually improve the steering. Because its a bicycle and not a motorcycle you can shoot for 0 to 2 inches of trail. See how many inches of trail you now have now, then figure out if the new dropouts will take you closer to where you want to be. You have to figure out what your baseline trail is first. With 0 trail you will have no flop at all and will be able to steer your bike with one finger.

Those Sugar Bear springers have really low trail numbers and no fork flop.............Dorian

 

[spinman]

maybe I'll make em, then get em spot welded on the back side. If they don't work, I'll just grind em off.

 

[spinman]

I don't have the frame around (p-coaters) so I rigged up this half-hiney rake & trail measuring jig.

current set up

proposed set up

looks like the proposed would give me negative trail, so back to the drawing board.

 

[g-ratter]

Looks like you can just shorten up the add on 1/4 inch or so and end up with 0 trail.

 

[Dorian]

If you have a factory bike it is probably close to the ideal trail. If not, Wait till you get the frame back and take the trail numbers again when you can mock it up properly, its easy to miscalculate with only approximate information.