A complex question about front end geometry

[Chuey]

Today, my friend Max brought my new part to check if it works. It works! It is a new steerer tube that enables me to put a Ducati Sport 1000 fork onto a BMW frame.

With the fork on the frame, it became clear that it is one inch shorter than the BMW fork.

The axle is in the center of the fork, whereas the BMW axle is on the front of the fork leg, so further forward.

The Ducati fork has less overall offset. In other words, it does not end up as far forward.

I think this will lead to a very low trail figure. What should I expect from that? We were thinking a faster turn in. Should that be less stable? Hard to turn? Wonky in turns, or at high speed?

I'm going to try to measure some of this out so I can ask better questions.

Chuey

[Garnet]

Well till Vanzen gets here and starightens things out, yes I belive you will have a quicker turn in with those forks. It will also be twitchier at speed. Weather the frame is stiff enough to aviod a tank slapper you will not know till you try it.

But the new fork in much stiffer structuraly, and that may help the rubber cow a bit.

I' m sure you've pondered all this in your heart already.

[vanzen]

... With the fork on the frame, it became clear that it is one inch shorter than the BMW fork.

The axle is in the center of the fork, whereas the BMW axle is on the front of the fork leg, so further forward.

The Ducati fork has less overall offset. In other words, it does not end up as far forward.

I think this will lead to a very low trail figure.

From what you have presented, resulting trail #s are unknown vs the stock #s –

What should I expect from that?

Short answer: Who knows ? Speculation will tell you nothing.
The measured numbers will at least be a clue –
but then, suitable trail #s will also always be dependent upon the specific chassis in question ...
FWIW: I have planned for a trail # of @ 3.69 inches – not too far from stock ,
but a good balance in terms of stability and curve-ability .... IMO.
But that will be what I want, and your expectations may vary ...

We were thinking a faster turn in. Should that be less stable? Hard to turn? Wonky in turns, or at high speed?

The typical expectation is that decreased trail #s result in quicker steering and less stability.
Always a compromise and / or a balancing act. Practical limits at each end might be easily exceeded –
So, you just can't have your cake & eat it too ...
Given a stock chassis, and again IMNSHO, the factory did pretty well with the balancing act.

[robtg]

With the axle it the center of the leg and less offset in the triple clamp you would have more trail.
The shorter fork will increase steering head angle a bit and reduce trail, but not by much.
Overall trail should be increased.

[vanzen]

...
But the new fork in much stiffer structuraly, and that may help the rubber cow a bit...

I submit the proposition that increasing the "stiffness" of the forks assembly
will not significantly lessen the forces generated or encountered by the chassis,
but rather will impart / transfer a significant percentage those forces elsewhere into the chassis -
for better or worse.
i.e.
Improved handling characteristics to be gained by the addition of those forks
will always be determined, defined, and limited
by the ability of that "least common denominator" of the entire chassis package
to cope with the forces generated or encountered.

[vanzen]

MEASURE

[Deleted User 61]

Thank you, Chuey, for making my motorcycling problems seem simple!

[robtg]

MEASURE

That is a very big red word there.

[melville]

With the axle it the center of the leg and less offset in the triple clamp you would have more trail.
The shorter fork will increase steering head angle a bit and reduce trail, but not by much.
Overall trail should be increased.

Yeah, what robtg says. I can tell you all about what happens on a bicycle with more trail--it's mostly good, with more stability, but a little more countersteering pressure needed to hold a line in a corner. That may rise to an annoying amount, or a bit of a countersteering workout, with MC levels of grip, speed, and mass of steered objects.

I always had a heckuva time explaining this to frame customers.

Me: Less offset = more trail.
Customer: But the touring bikes have more rake (offset)?
Me: Yes, but a different head angle.

A couple times I had to get out the string and tape measure to convince a customer of this.

[vanzen]

With the axle it the center of the leg and less offset in the triple clamp you would have more trail.
The shorter fork will increase steering head angle a bit and reduce trail, but not by much.
Overall trail should be increased.

So, like this, right ?

The axle is in the center of the fork, whereas the BMW axle is on the front of the fork leg, so further forward.

The Duc increases trail by virtue of a lesser ("0") axle offset.

The Ducati fork has less overall offset. In other words, it does not end up as far forward.

The Duc increases trail by virtue of a lesser fork offset.

With the fork on the frame, it became clear that it is one inch shorter than the BMW fork.

And the Duc decreases trail by virtue of a shorter length (decreasing rake).

To know what you actually have:

MEASURE