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