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I will take a crack at clearing the murk. Maybe I'll make it worse. Caution, this is NOT simple, and by no means do I have a complete understanding of this ... I am a regional-level roadracer who happens to own some tools (and have a mechanical engineering degree...) but I am not a WorldSBK race-team suspension engineer.

Set aside the obvious effects of lowering upon cornering clearance. Obvious bad side effect is obvious.

I am going to use a phrase "rider-aboard static ride height" a lot. The geometry with the bike sitting by itself doing nothing doesn't matter. What matters is what the geometry of the bike is, with the actual rider on board, in normal riding position, wearing whatever gear the rider wears, with all fluids and a nominal fuel load, etc. There is a suspension-fully-extended geometry (bike lifted up off the ground), and then a "rider-aboard static sag" which is the amount that the suspension compresses at both ends. The rider-aboard ride height, with the sag applied from the bike's weight and the rider's weight and whatever normal loads it is carrying, is what matters - that's the situation as you are rolling down the road. That's the starting point. Then the rider starts accelerating and braking and cornering, and superimposing those situations on top of that rolling-down-the-road situation.

If you lower the front of the bike by lowering the rider-aboard static ride height (by any means, I don't care what), then unless you lower the rear of the bike by also lowering the rider-aboard static ride height by the same amount, you are going to change the angle of the steering head, and that affects the rake, trail, etc. If you lower the front more than the rear, it will steepen the steering head and shorten the trail, reducing steering effort and stability (trending towards "nervous"). If you lower the rear more than the front, it's the opposite (trending towards "steers like riding through glue"). Consequently, at least at first-guess, if you change ride height at either end then change it by the same amount at both ends, subject to fine-tuning once you get a handle on the knock-on effects.

If you lower the rider-aboard static ride height by any means, you are lowering the height of the swingarm pivot above the ground, and that affects how chain-pull force affects the suspension. The technical term is "anti-squat". Acceleration involves a load transfer off the front suspension onto the rear. Ordinarily this would extend the front suspension as the fork springs now are carrying less load, and compress the rear suspension, and thus all this changes the steering head angle (see above) - but the "anti-squat" from the chain-pull force counteracts this force without requiring the rear suspension to compress (as much) so the steering doesn't get affected as much. Lowering the bike reduces anti-squat (because the swingarm pivot height is lower). It means the bike will be allowed to pitch back under acceleration. It also means the loading on the tire doesn't come in right away when you hit the throttle ... it has to wait until the suspension compresses. That leads to more tendency to spin the tire coming out of corners. The increasing rake angle during acceleration also wants to make the bike run wide coming out of corners. It mucks with the steering in all sorts of strange ways. Basically - recognising that a shorter rider may be obligated to lower the bike in order to be able to ride at all! - you do not want to lower the bike any more than absolutely necessary. (In my roadracing application, I found it necessary to RAISE the whole bike - to get more cornering clearance, and to get more anti-squat.)

Now, on to the topic brought up by this thread, what's the difference between internal fork modifications and just sliding the forks up in the clamps.

What you don't want to have happen, is for the front fender or the front wheel to EVER collide with bodywork, the bottom of the lower triple clamp, the radiator, etc. The mechanical travel limit of the suspension has to be before such bad things happen plus a few millimetres of clearance just to make sure.

If you slide the forks up in the triple clamps, you are moving that mechanical compression travel limit together with the forks, because that is something that is built into the internals of the forks.

This is okay as long as you maintain the few millimetres of clearance between fully-compressed and collisions.

If you want to establish this (before modifying fork internals), it can be done by supporting the front of the bike off the ground in a way that doesn't obstruct wheel travel, then taking off the fork caps and removing the springs, and simply raising the front wheel up until it hits a mechanical limit, and then measuring how much clearance you have. If you have two (minimum) or three (easier) people, you can do this by first having the front of the bike up on a steering-stem paddock stand in order to remove the fork caps and springs, but leave the handlebars in place because you're going to need them, then carefully lower the bike off the stand using two people to hold up the handlebars so that it doesn't all come crashing down. Then you can simply set the bike down on the fully-compressed forks which will now most certainly be against the mechanical compression stops. Then use two people to lift the bike up again and re-insert the paddock stand. You could now, if you wish, reposition the forks in the clamps (and re-secure the handlebars as needed) and re-check it to make sure you still have clearance. After this procedure, you will have lowered the front as much as it can safely be lowered without modding the internals. I haven't done this with the R3 because in my roadracing application, I found it necessary to raise, not lower, in which case it's not an issue.

Now, let's suppose you have a really short rider, and the above did not achieve what was needed. The only course of action remaining is to further reduce static ride height by tinkering with fork internals. Let's examine some options and consequences.

First understand that ALL such modifications are going to reduce the amount of suspension travel available in compression before "bottoming out" - hitting the bump stops. It will bottom-out easier. That's bad. It means you will no longer have suspension travel available in certain situations where you could really use it - like hard braking or cornering on irregular surfaces. Now, it might not necessarily be as bad as one might think, because riders who have very short legs also tend to be pretty small and light in general, so perhaps they automatically might not be compressing the suspension as much. One of the ways to control "bottoming" is to use stiffer springs (higher spring rates). Of course, this has its own share of side-effects.

OK so option 1 might be shorter but higher-rate fork springs ... what actually matters here is the combination of the length of the fork spring plus the spacer between the fork spring and the rest of the works; some custom-length spacers can also work to shorten the effective length of the combined assembly, thus leading to the bike sitting lower on its forks because the spring+spacer is shorter with the same load applied to it. Simple, right?

Not so fast. The fork is mechanically capable of a certain total amount of travel between the compression limit and the extension limit. If you shorten the spring+spacer then you shorten the amount in compression available before hitting the compression travel limit but since the total travel is available, then you lengthen the amount of available extension before the extension travel limit is reached.

So what?

Remember that paragraph up there somewhere about not having enough anti-squat leading to the steering head angle changing too much under acceleration? Well, one of the other ways the steering head angle can change too much under acceleration ... is by allowing the fork to extend too much under acceleration.

And now we come to ... Top-out springs.

In the absence of top-out springs, during acceleration, your forks would spend a lot of time topped-out against a hard limit, and it would allow the geometry of the bike to change quite a bit before the forks extended far enough to hit that hard limit.

So ... instead of having a hard limit, forks have springs that come into effect as the forks are extending beyond the normal ride height. This softens the topping-out effect and allows the geometry of the bike to be maintained more consistently.

Hence ... The modification discussed in post #1 of this thread ... of lowering the forks not necessarily by shortening the main load springs and spacers (the obvious route), but rather by increasing the effect of the top-out springs.

Is this going to be the be-all and end-all that allows any arbitrary amount of lowering of the suspension without having any bad side effects whatsoever? Absolutely NOT. The bad side effect of not having as much cornering clearance before the footpegs, or other parts, start dragging, remains. The bad side effect of not having as much travel available in fork compression before bottoming-out, remains. The bad side effect of lowering the swing-arm pivot, and thus reducing the amount of anti-squat, and thus probably making the steering somewhat less consistent under various conditions of accelerating and cornering (and accelerating out of corners), remains.

BUT ... If this is what it takes to allow a rider with short legs to ride at all without falling over frequently because they can't reach the ground ... so be it ... as long as they are aware of what's going on!

My general recommendation to avoid lowering any motorcycle any more than absolutely necessary for a short rider to be able to operate the bike safely ...

And the other thing is that if this is a street bike, the vast majority of riders don't ride hard enough for the various anti-squat effects and so forth to matter all that much. The cornering clearance might not even matter.

But, I am coming at this from a roadracing background, and my R3 sits higher than standard. And, even though I'm not WorldSSP300 material - far too olde, far too heavy - I still make adjustments that affect basic geometry by only a few millimetres at a time.
 

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While a proper race set-up does not make for a good street set-up and vice versa ... The physics involved doesn't change. If you compromise the available compression travel, and that leads to the forks bottoming-out during hard braking when a van cuts you off at a junction, not having a wee bit of compliance left over could lead to having a bad day if the pavement isn't perfect.

There are situations where stiffer spring rates can lead to both better ride quality and better grip ... and that's if too-soft springs are leading to the suspension hitting travel limits at inappropriate times. This applies to both street and track applications. And, obviously, this has to be within reason. If 7.5 N/mm springs are allowing bottoming when the bike is ridden over a representative section of road/track, perhaps try 8.0 N/mm ... but not 80.

MotoGP and WorldSBK have wheel-travel sensors and data logging to establish where the suspension is operating within its travel range and what's happening when the bike does something that the rider doesn't like, anywhere on track. An O-ring or zip-tie installed around a fork tube or shock rod will at least tell you how much compression travel you are using ... and even in the absence of having that, a strategically-placed dab of grease on the inner fork tube or shock rod can reveal the same.

Street or track ... suspension bottoming is bad news.
 

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Not disagreeing. For most street riders, the stock stuff is just fine. Don't mess with it. Yamaha built it that way for a reason, and it works. If I had one of these bikes as a street bike, I would probably leave it stock ... it works.

For street riders it's really only the out-of-the-norm situations that perhaps warrant changing things from stock. (Riders that are heavier than what Yamaha designed the bike for, riders who have legs considerably shorter than what the bike is designed for, etc.) Just beware what you are doing and the potential for adverse effects. If my post scares people off doing modifications because of how everything affects everything else ... maybe that achieved something.

And, my race bike IS raised up both front and rear ... in order to preserve steering-head geometry. It is not necessary to get more suspension travel ... just raise it up, to get more cornering clearance (primary desired effect) and less rear suspension bottoming and more anti-squat (secondary desired effects).
 
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