Easy Rear Sag Measurement

5685 Views 25 Replies 11 Participants Last post by neuone, Feb 26, 2009

neuone

Discussion Starter · Feb 25, 2009

I've had my SV650 since 2005, and I have NEVER even checked the sag. I weigh about 155lbs and I just set the preload to 5 in the rear and left it there.

Well yesterday I put a '07 GSX-R 1000 shock on the back. Nothing I remember reading said that it was about half an inch shorter, but that's a side point.

I took the bike out for a short ride, and I like the feel of the stiffer rear spring for the sweeping corners. It's hell on a bumpy road though. I'm going to slowly tweak the high speed compression damping to see if I can smooth that out a little.

On to the sag: Not that I don't have friends, but I usually prefer to work on my motorcycle alone. Measuring the sag on the motorcycle is kind of hard when you have to sit on the thing while you're taking the measurement. This is what I did.

I used a yard stick, two 12 inch rulers (with metric) and some masking tape.

I taped one of the rulers on the side of the tail section with about a 2 inch gap between the pieces of tape. This two inch gap should line up above the rear axle. The yard stick goes behind this ruler between the tape and rests on the rear axle nut. The cotter pin through my axle just so happens to be in a stop to hold the yard stick in place. Here's an image:

Some may note that there is nothing protecting the paint. This is because the paint on my bike is in bad shape and I plan on repainting it soon.

Then I marked the location where the horizontal ruler crosses the yard stick, this is the initial position and will be the mark you are measuring to with the second ruler.

Next, all you have to do is sit down on the bike. I put the kickstand up, and applied the front break. I tried to make sure that I wasn't bouncing the bike when I got on so I would get as accurate a reading as possible.

The line with the circle at the end is the initial position of the yard stick so all you have to do is measure from the edge of the ruler to the initial mark. Presto, one man sag measurement!

Thinking about it later, I probably should have made the initial mark with the bike standing vertically, but with this mark, and the GSX-R shock, I'm only able to get 25 mm of sag, which seems decent. I plan on getting some shorter dog bones to raise the height back to stock: I can touch flat foot now and I don't actually like it.

I hope this helps people out!

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30x26

· #2 · Feb 25, 2009

that's not the way to measure

you need 3 numbers:

A = distance with rear wheel lifted from the ground
B = distance with vertical bike
C = distance with vertical bike + rider (in riding position)

then for street riding adjust preload until you get: A - C = 30-35mm
(if spring is correct A - B will be 10-15mm)

BossMan

· #3 · Feb 25, 2009

30x26 said:
that's not the way to measure
you need 3 numbers:

A = distance with rear wheel lifted from the ground
B = distance with vertical bike
C = distance with vertical bike + rider (in riding position)

then for street riding adjust preload until you get: A - C = 30-35mm
(if spring is correct A - B will be 10-15mm)

+1

You'll probably need someone to help you as well.

dmmcd

· #4 · Feb 25, 2009

This is a creative idea, I like it, but as stated above the total sag needs to start with the wheel off the ground. Just tip the bike up on the side stand and push the yard stick down while the rear wheel is off the ground, and mark that. Then stand the bike upright (completely vertical) without you on it and mark it again. Then sit on it, with all your weight, feet on the pegs (lean lightly against a wall or something) and TRY to mark the ruler again, or see what it is reading.

I always work on my bike alone, so this is a good idea. Race Tech sells a device that allows you to do this, but it costs money.

I guess you could use your current measurement and shoot for 15 to 20 mm, as that should be the rider sag, or C-B from above.

neuone

Discussion Starter · #5 · Feb 25, 2009

OK cool, that's good to know.

dmmcd: I like your idea about tipping the bike to the side, but I have a hydraulic motorcycle lift I got for about $100 and I made a platform that bolts to the bottom of the engine casing and lifts it from there.

But thanks for the correction!

507phantom

· #6 · Feb 25, 2009

i've never set the sag on my bike as well, just because i don't know how. so now i've got a question.

does rear wheel off the ground mean with a swingarm stand? seems like there wouldn't be any difference except for tire deflection (maybe thats exactly why?). or does the rear need to be up without having the suspension compressed at all?

BossMan

· #7 · Feb 25, 2009

507phantom said:
i've never set the sag on my bike as well, just because i don't know how. so now i've got a question.

does rear wheel off the ground mean with a swingarm stand? seems like there wouldn't be any difference except for tire deflection (maybe thats exactly why?). or does the rear need to be up without having the suspension compressed at all?

No weight on the rear tire at all. So NOT on a stand.

A popular method is to flip the pegs and put jackstands under them to support the rear of the bike.

herbn

· #8 · Feb 25, 2009

so how often do your rides get "interupted" with your plate like that? i hate the way plates look, these days they should just have a little easy pass type of thing where your plate is,no tickets in the mail though ,they read it with radar and then pull you over for giving you the ticket.

tcl

· #9 · Feb 25, 2009

The way I do is one-personed is I tape an collapsible antenna to the tail then extend it down to the axle and use a loose loop of wire (aka paperclip) to attach it to the cotter pin for the axle bolt. I then walk around to the back of the bike and lift the rear off the ground by the grab bar. I measure the length of the now extended antenna. Then, I sit the antenna on top of the cotter pin (but not attached) and go sit on the bike, which collapses the antenna. I now measure the collapsed antenna. Accounting for the mounting positions, etc, length of extended antenna- length of collapsed antenna= rider sag.

dmmcd

· #10 · Feb 25, 2009

tcl said:
The way I do is one-personed is I tape an collapsible antenna to the tail then extend it down to the axle and use a loose loop of wire (aka paperclip) to attach it to the cotter pin for the axle bolt. I then walk around to the back of the bike and lift the rear off the ground by the grab bar. I measure the length of the now extended antenna. Then, I sit the antenna on top of the cotter pin (but not attached) and go sit on the bike, which collapses the antenna. I now measure the collapsed antenna. Accounting for the mounting positions, etc, length of extended antenna- length of collapsed antenna= rider sag.

Another good idea. I hadn't thought of that before. I have one of those collapsible magnetic pointers that would work well.

This is not quite as accurate as having a friend measure, because it will record maximum displacement, so if you bounce on the seat it will be off.

Some say to lift the bike, let it down slowly, take that measurement. Then bounce hard on the bike, let it come up. Take that measurement and average it with the first measurement. I guess that is the most accurate way, but I tried it once and got such a small difference that I didn't feel it was worth the trouble.

Gerard7aa

· #11 · Feb 25, 2009

Loosen up your high speed comp. It will soften up the ride.

tcl

· #12 · Feb 25, 2009

Gerard7aa said:
Loosen up your high speed comp. It will soften up the ride.

You say "high speed" as if there is a "low speed" which, AFAIK, there is not on the discussed shock- only "compression" and "rebound".

Gerard7aa

· #13 · Feb 25, 2009

MMMM I know that. He said high speed so I said high speed. Just trying to keep it with his syntax.
No biggie.

neuone

Discussion Starter · #14 · Feb 25, 2009

herbn said:
so how often do your rides get "interupted" with your plate like that? i hate the way plates look, these days they should just have a little easy pass type of thing where your plate is,no tickets in the mail though ,they read it with radar and then pull you over for giving you the ticket.

Well I used to have my plate like this:

I got pulled over 3 or 4 times but never ticketed. I would just make up some story about how the plate mounting bracket had *just* broken a couple days before and I had to double sided tape it back in place ( and I kept the tape, in the package, under the rear seat...).

The way it's mounted now is legal in Oklahoma, though I need to put the lights on there too.

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neuone

Discussion Starter · #15 · Feb 25, 2009 (Edited)

tcl said:
You say "high speed" as if there is a "low speed" which, AFAIK, there is not on the discussed shock- only "compression" and "rebound".

On the 2007 GSX-R 1000 rear shock there is an adjustment for high and low speed compression damping. High speed compression is when you hit a bump in the road. Low speed compression would be from taking a long sweeper.

tcl

· #16 · Feb 26, 2009

neuone said:
On the 2007 GSX-R 1000 rear shock there is an adjustment for high and low speed compression damping.

I know high -vs- low speed definition. I did not know the OEM shock had high and low speed adjustments. Are you sure of this? I found no reference to it in the Suzuki website and it would be a pretty big deal if it had both adjustments.

EDIT: Ah, I found it and stand corrected.

efnar

· #17 · Feb 26, 2009

30x26 said:
that's not the way to measure
you need 3 numbers:

A = distance with rear wheel lifted from the ground
B = distance with vertical bike
C = distance with vertical bike + rider (in riding position)

then for street riding adjust preload until you get: A - C = 30-35mm
(if spring is correct A - B will be 10-15mm)

I don't understand how A-B does anything. I've seen various sites suggest that this matters, but I don't understand why. When I ride the bike, I want the sag in the right place to make it so that the suspension doesn't run out of range, but it's not clear to me at all why I care how the suspension behaves when I'm not on the bike... If the suspensions doing much and I'm not on the bike, I have bigger things to worry about than the total available range of motion for the suspension...

As an aside, this doesn't appear to be a nod to stiction, as that would involve two B-C calculations, one after compressing the suspension and one after unweighting the suspension.

tcl

· #18 · Feb 26, 2009

efnar said:
I don't understand how A-B does anything. I've seen various sites suggest that this matters, but I don't understand why. When I ride the bike, I want the sag in the right place to make it so that the suspension doesn't run out of range, but it's not clear to me at all why I care how the suspension behaves when I'm not on the bike... If the suspensions doing much and I'm not on the bike, I have bigger things to worry about than the total available range of motion for the suspension...

You don't really set A-B but it is a good indicator that everything else is set well.

efnar

· #19 · Feb 26, 2009 (Edited)

tcl said:
You don't really set A-B but it is a good indicator that everything else is set well.

It seems to me that A-B for a given bike would be established by the weight of the unladen bike and the spring constant.

So, the range posted will work out unless you're very light (and thus use springs with low spring constant) or very heavy (and thus use springs with a high spring constant).

So (assuming that you've gotten the correct springs for your weight) I'm still not sure how this tells you anything other than "eat more you motorcycling supermodel waif" or "layoff the fast food, tubbo".

tcl

· #20 · Feb 26, 2009

efnar said:
It seems to me that A-B for a given bike would be established by the weight of the unladen bike and the spring constant.****

So, the range posted will work out unless you're very light (and thus use springs with low spring constant) or very heavy (and thus use springs with a high spring constant).

So (assuming that you've gotten the correct springs for your weight) I'm still not sure how this tells you anything other than "eat more you motorcycling supermodel waif" or "layoff the fast food, tubbo".

Exactly. If you have the proper spring rate for your weight, and set the rider sag to ~35 mm, you will have ~10 mm of static sag. If you have (eg) much too low a spring rate and set the rider sag to ~35mm, you could possibly have ZERO static sag.

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