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Front Sag


Julian

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I finally got to measuring my front sag and I need some help to make sense of the numbers... 

The measurements are as follows:

Forks fully extended: 137 mm

Under the weight of the bike: 120mm

With me on the bike: 110mm

The front suspension travel given by Yamaha is 5.1" or 129 mm, so how did I get 137mm? Is it because of the top out spring? And which number should I use in the calculations, the one I measured or the one given by the manufacturer? 

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M. Hausknecht

What is the 137mm measurement? Is that between the fork dust seal and the bottom of the lower triple clamp with no weight on the forks? If so, that measurement should exceed the front travel number so you don't smash the dust seal into the triple clamp on full compression, and that's what you've got; so far so good.  Beyond that, the 137mm measurement doesn't mean anything by itself. If you raise or lower the forks in the triple clamps that number will change but your two sag numbers should not. You're then getting 17mm of unladen sag and, with you 27mm of sag, which is ok but you might find the front a bit stiffer than you'd like for street riding. 

There are no magic sag numbers, like "25% of travel" or some such that you'll see in Youtube videos, but they are useful as a guide to help you understand what you're feeling from the suspension when you ride.  At one end of the spectrum, you don't want to experience bottoming out except perhaps rarely, and at the other, you want your springs to absorb bumps rather than transmit them to the sprung portion of the bike, like you. Between those two extremes there is room for personal preference. Some like a cushy ride (more sag), some like a firmer platform (less sag).

Is a problem you're trying to address?

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49 minutes ago, M. Hausknecht said:

What is the 137mm measurement? Is that between the fork dust seal and the bottom of the lower triple clamp with no weight on the forks? 

The measurements are of the shiny exposed part of the fork. If I use that higher number that means 17mm static sag and 27mm rider sag like you say. AFAIK the stock springs are 0.88 and on sites like Racetech and Sonic they recommend 0.8-0.84 for my weight so I think the stock springs are close enough for my weight. 

What I would like to have is less front dive under sharp braking, but I think that is caused more by the weak compression damping. 

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4 hours ago, Julian said:

What I would like to have is less front dive under sharp braking, but I think that is caused more by the weak compression damping. 

You can use a more viscous fork oil to increase dampening, to slow compression and rebound.  And you can increase the fork oil level to increase the air spring force, to prevent bottoming out.

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Increasing oil level will also decrease your sag, which may not be what you want.  You can shorten your fork spacer to compensate.

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M. Hausknecht

Yako beat me to it!

I have a couple suggestions. First, you reference "sharp braking". When I read this, I think "slamming on the brakes", you know, like a panic stop in a car with ABS. I hope that isn't what you mean because it will tend to cause the forks to collapse. Even when you need to stop or slow ASAP, the front brake should be applied progressively. Not slowly exactly, but progressively so that the initial application loads the front tire and then you're free to squeeze the lever further until you lift the rear tire at which point you're not going to stop any quicker unless you hit something. How progressively? I've never measured but maybe about 1/2 second or so.

Beyond that, there are two minor, inexpensive changes that should help short of replacing or supplementing fork internals. First, I'd check the fork oil level and compare to what the manual calls for. Adding oil, including going beyond the quantity set forth in the manual, will firm the forks up in the last inch or so of travel. Raising the fork oil  level by 25mm should produce a noticeable difference. Try it; if you don't like it, just remove some or all of the extra you added. Second, you can drain the fork oil and replace with heavier weight oil. This will increase the damping in both directions, for better or worse.

Edited by M. Hausknecht
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Julian, your sag numbers are on the low side ( low numerically, high in relation to ride height), so it doesn't appear your fork dive is spring related, as you said. 

Thicker fork oil like Yakko mentioned will give more damping, but I found my fork dive issue wasn't really as bad as I thought. It was the lack of rebound damping in the rear shock allowing the rear suspension to unload/extend quickly and caused the whole bike to pitch forward. 

Both ends of the bike need work to be perfect, but the shock is the biggest offender IMO. 

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4 minutes ago, shinyribs said:

Both ends of the bike need work to be perfect, but the shock is the biggest offender IMO. 

Julian: When I practiced emergency braking with the stock 2018 suspension, the front end dive was very unpleasant for me, but the rear end was probably rising also, making me feel like I was almost being pitched off the bike.  So increasing the rear shock damping may also help a bit.

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nozeitgeist1800
9 hours ago, shinyribs said:

Julian, your sag numbers are on the low side ( low numerically, high in relation to ride height), so it doesn't appear your fork dive is spring related, as you said. 

Thicker fork oil like Yakko mentioned will give more damping, but I found my fork dive issue wasn't really as bad as I thought. It was the lack of rebound damping in the rear shock allowing the rear suspension to unload/extend quickly and caused the whole bike to pitch forward. 

Both ends of the bike need work to be perfect, but the shock is the biggest offender IMO. 

you beat me to it. i recently replaced my rear shock and holy hell what a difference it makes with how the front feels. not nearly as terrified as i was going into bumpy corners. a lot less front dip when downshifting or braking as well. still gonna thicken up the front oil, and install the ohlins preloader/spring combo kit thing to see how that feels, but the rear shock made a huge difference

Edited by nozeitgeist1800
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  • 1 month later...
On 12/2/2021 at 6:46 AM, Triple Jim said:

The 2018-current MT-07 has adjustable rebound damping.

I'm running a 2018 mt07 rear shock on my 2017 fz07. Vast improvement in both rebout and damping, combining this with 15w in the front fort got my bike much more balance. Be advised if you're light the 2018 spring is stiffer and could give you trouble with sag. Which as others said you are a smidge on the right side. 

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