Does tire size affect sag? - Sportbikes.net
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post #1 of 17 (permalink) Old 12-26-2012, 11:42 PM Thread Starter
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Does tire size affect sag?

If I change from a 190/50 rear to a 190/55 rear, will my sag change to a point that I need to reset it..or should I be happy and leave it alone assuming its dialed in.
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post #2 of 17 (permalink) Old 12-26-2012, 11:52 PM
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Yes. It's pretty minimal, but it does. You're throwing weight forward, so the rear sag will be a bit less and the front a bit more.

For example, in a Q2 the nominal rear diameter is 25.32 vs 24.95". So .37" in diameter = .185" radius, or 4.7mm of height at the axle.

I'd get used to the new tire behavior before I fooled with it, personally. After running a 190/55, I'll never put a 190/50 on a bike again.

KeS

Last edited by kevin_stevens; 12-26-2012 at 11:58 PM.
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post #3 of 17 (permalink) Old 12-27-2012, 01:06 AM
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Minimal would be correct if not an understatement. You're talking about a .7" difference in diameter, or .35" difference in actual height change. Minimal enough that I highly doubt you'd ever be able to notice the difference in weight shift or sag. The bike will handle differently, but more because of the different tire profile than the suspension settings.
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post #4 of 17 (permalink) Old 12-27-2012, 02:58 AM
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It will change ride height more than anything. KeS is correct.
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post #5 of 17 (permalink) Old 12-27-2012, 06:02 AM Thread Starter
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Thanks! I kinda figured that it changed it..but that it didnt really matter on a practical matter. However I lack working knowledge/practical experience with motorcycle suspension setup so that's why I made the thread(that and a search came up empty)
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post #6 of 17 (permalink) Old 12-27-2012, 06:45 AM
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post #7 of 17 (permalink) Old 12-27-2012, 08:29 AM
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The tire diameter does change ride height and geometry - but I am not so sure that it changes "sag" / preload. Let me explain. Basically when you set "sag" you are setting the preload adjusters on your bike to change the suspension range or travel. Changing preload has the secondary effect of raising or lowering your bike to fit in that desired range. It does not effect the spring rate or your compression or damping settings on most bikes.

If the tire you use increases in diameter, it changes the measured relationship between your wheel axle and the ground - which could make the bike "taller" by a couple of mm. However, it has no effect on the suspension range and preload settings - which are essentially adjusters that put more or less pressure on the springs inside the fork legs and typcially outside of the rear shock. The tire "height" has no appreciable effect on preload at all.

What tire hight does do, is change the ride height of your bike. This could potentially, relating to the changes of the tire height on your bike before you change tires, change your geometry - making your bike turn faster or slower or minimally effecting rake, trail, and squat / anti squat properties. Thus, to make up for these changes, you would want to either raise or lower your fork legs and or adjust your rear suspension linkage or ride height adjuster in the back of the bike to maintain the same geometry you had before the tire change.

Sag/ Preload does effect the ride height of your bike - but in a way that does not change the height of the wheel axle off of the ground. What you worry about when changing tyre brands or models is a change in geometry in your bike. Sag only changes the ride height as a secondary characteristic of varying the suspension stroke / range.
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post #8 of 17 (permalink) Old 12-27-2012, 11:47 AM
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Yeah, I kind of glossed over a step. The tire change doesn't affect your sag directly - it raises the rear axle and ride height slightly. THAT throws weight forward and thereby increases sag in front and reduces it in rear. Thanks for clarifying.

KeS
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post #9 of 17 (permalink) Old 12-27-2012, 04:42 PM Thread Starter
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Quote:
Originally Posted by kevin_stevens View Post
Yeah, I kind of glossed over a step. The tire change doesn't affect your sag directly - it raises the rear axle and ride height slightly. THAT throws weight forward and thereby increases sag in front and reduces it in rear. Thanks for clarifying.

KeS
This is what I envisioned when asking the question.

I am aware of the geometry change, that is an added benefit to the change..I'm primarily doing it for the better tire profile.
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post #10 of 17 (permalink) Old 12-27-2012, 09:13 PM
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post #11 of 17 (permalink) Old 12-28-2012, 12:35 AM
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post #12 of 17 (permalink) Old 12-28-2012, 02:47 AM
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the hardness of a sidewall can affect sag, as well, can't it?
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post #13 of 17 (permalink) Old 12-28-2012, 03:11 AM
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Quote:
Originally Posted by Shamrock627 View Post
the hardness of a sidewall can affect sag, as well, can't it?
Technically not as spring sag is simply the difference in spring length between loaded and unloaded. Will it possibly have a impact on overall suspension performance yes although for the average road rider even an enthusiastic one its probably not worth any serious consideration.
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post #14 of 17 (permalink) Old 12-28-2012, 07:51 AM
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can you set the suspension up while on stands? or do you have to have 3 people to set it up.... was reading a bit about it a while back and it sounded like you needed the rider, a person to hold the bike upright (since the rider has feet on pegs to be "race-like") and another person doing all the measuring.... please enlighten me since i would love to set mine up before the next track day.... i have plenty of time til that happens but oh yes... it WILL happen.

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post #15 of 17 (permalink) Old 12-28-2012, 11:42 AM
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It depends what you mean by using stands. Normally to set sag you need to take a series of measurements based upon the loaded and unloaded height of the suspension of the bike. Remember that you are measuring from two points on the bike - not from the ground. Sag is set by "preloading" the suspension so that it does not top out or bottom out the suspension. When you do this you can have the secondary salutory or negative effect of changing your geometry.

So, no, I don't think you can or should use stands to set sag. My instinct is that raising or lowering the front or rear of the bike while setting sag will screw a bit with your measurements - which can be small and are effected by stiction / friction in the forks and shock. I get the idea though that you want to use the stands so that you don't need people to hold the bike up.

So, sure you could change the adjusters while the bike is on stands - but I don't think it would be a best practice to do this on stands.

I think what Kevin was trying to say earlier by the way, is that suspension adjustements have primary effects and often also have secondary effects that change based upon what adjustement you are making and what kind of bike / quality of suspension you have. (Sorry Kevin, I don't want to speak for you - but I think that this was part of your point.)
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