Measuring trail at full braking

[BackMarker]

I just replaced forks and shock on my '01 M900, and some suspension guru mentioned on the phone that if I reduced trail too much, the front would fold under heavy braking (I'd like to track the bike) . . . so

How do I effectively measure what the trail would be under full braking?

I'd like to know how much I can raise the rear without worrying about this . . . no idea if it's a realistic concern or not.

BTW, it has original triples w/25mm offset, but the forks are off an '07 S2R - the overall effect is to lower the front by 15mm (shorter fork tube). The rear now has an Ohlins DU440 shock - did not raise the rear but half a thread turn on the hoop heim joints.  Has a steering damper for WTF.

[bikepilot]

What do you mean by "front will fold."  Generally too little trail will make it a bit unstable, tend to oversteer and tuck the front end under in turns. 

When running shorter than oem forks is not a bad idea to make sure the tire won't hit anything under hard braking.  Note that the forks will flex back just a bit when braking hard so measuring with them collapsed won't be quite the same, but will get you pretty close.  A bike will decelerate at just over 1g and most of that is done on the front wheel so figure bike weight + rider weight as the amount of force pushing the front wheel backward

[BackMarker]

I think he meant that trail would go near enough or past zero so that the front wheel would caster backwards.  Maybe he was drunk, or a fake guru.

I've got 130mm of fork travel, so I'll just put a 130mm block under the rear wheel and measure trail as normal.

And from what I've read, lowering the front 15mm is not too extreme . . . but can I raise the rear height any?  Any extra clearance for the low-mount pipes would be nice.

[Raux]

i've read on here people raising the rear at least an inch... so you have at least 10mm left to go

[stopintime]

I did some rough calculation a while back.

One inch, front/rear or combined, reduces rake by one degree.

At full compression, another five inces - still almost 20 degrees rake.

I don't know how seriously those numbers influence trail,
but I can't imagine a wheel flipping.

These are rough estimates, I'm not an expert, so do more research if you want to get more peace of mind.