Rake and trail on 1930s to 1947 Harleys with Springer forks

[PC_Hater]

Before I make a bad mistake modifying my 45 chop with 6" over teles to be a bobber with springers I am trying to find out what trail Harley used on the 45s and knuckleheads.
I know the rake figures, but after a lot of searching the internet and asking on various forums I am getting no answers at all about the trail!

45s had 23 degree rake from about 1936.
1936 Knuckleheads had 25 degree rake.
In 1941 they became 28 degree rake.
In 1947 they were 30 degree rake.
'High Speed Wobbles' is the reason the internet gives for the changes in rake. I tend to believe them.

But does anybody know what trail they had?
The 18" wheels and 16" wheels they used are supposed to be the same size at 26" diameter so the change in wheel size should not affect the trail...

I have the Victory Library 45 tuning books and intend to perk the old girl up a bit so getting things right will be important. At the moment max speed is just over 80mph.

George Brough of Brough Superior fame chose Springers for his fastest bikes because he thought they were more suited to fast bikes than anything else available at the time - and despite asking around I can't even find what rake he used never mind the trail!

[hrdtail78]

The lower rockers come into play.  It's the difference between the parallel line off the fork stem from the offset of the axle.  Sugar Bear might have some info you are looking for.

[PC_Hater]

No useful technical info on Sugar Bear's site. Nice looking springers and rolling chassis' though.
Unfortunately he uses solid rod for his forks, I'm an engineer so I would be unable to buy a set as a matter of principle!

[PC_Hater]

It seems the trail figures for Springers are a secret never to be told...
So I did the drawings instead.
On a WL45 with a 23 degree rake and a 16" front wheel the trail is 1.6".
On the late Knuckleheads with 30 degree rake and a 16" front wheel the trail is 4".

I assumed 'in-line' springers with the forks inline with the steering head. Using the offset springers will reduce the trail.
Standard Harley rockers have 4.5" between the centres of the rear rocker mount and the axle.
The rear rocker mount centre is 0.5" below the axle centre.

Using a larger diameter front wheel will increase the trail a bit.

So far the only difference I can find between a standard WL45 frame and the racing WR frame is extra strengthening tubes at the rear end. Short trail and high speeds must have been interesting!

[CraigArizona85248]

4" - 6" of trail is the sweet spot in my opinion. Stable at speed but not heavy at low speed. 1.6" of trail sounds scary. Of course the 45 side valve isn't super fast, so there is that factor.

My wishbone rigid frame is raked to 45 degrees. I run a 10" over glide front end w/ 21" wheel. This gives a trail of 9". Super stable on the highway. But heavy steering in parking lots.