Triumph Rat Motorcycle Forums banner

21 - 40 of 56 Posts

·
Registered
Joined
·
7,455 Posts
I actually wasn't agreeing to his last point about using shock mounting points as reference points. I had forgotten that he mentioned that. Which is why I use the axle nut and a plumb point above on the subframe. I was only commenting on his first part.
 

·
Premium Member
Joined
·
683 Posts
wouldnt the correct way be to measure the mounting points on the shock where it connects onto the swingarm and frame?
Kind of confusing, because if we were to measure the rear travel the same way we do the front forks, you would measure between the upper and lower shock mounting bolts. But the dimension we are after is called rear wheel travel and the only correct way to get that is to measure from the rear axle nut to a fixed point on a verticle line above it. Racetech also has a nice tutorial on setting sag and that is how they do it.

Sent from Motorcycle.com Free App
 

·
Registered
Joined
·
341 Posts
Don't over complicate things Felony is correct, directly above the axle as long as you measure in the same spot every time
 

·
Premium Member
Joined
·
1,801 Posts
Kind of confusing, because if we were to measure the rear travel the same way we do the front forks, you would measure between the upper and lower shock mounting bolts. But the dimension we are after is called rear wheel travel and the only correct way to get that is to measure from the rear axle nut to a fixed point on a verticle line above it. Racetech also has a nice tutorial on setting sag and that is how they do it.

Sent from Motorcycle.com Free App
yes, thanx for that. from the axle nut to a fixed point on the frame makes sense
 

·
Registered
Joined
·
277 Posts
I can see a reason for this. On smaller bumps where you want the softer compliance , only the bottom (soft wound) end of the spring moves. If you install it the other way around all the spring has to move on even small bumps.
On a smaller bump, all coils compress, just at different rates. With 50-pounds of force, or "hit", the soft coils compress 50-pounds worth. The strong coils also compress 50-pounds worth... it's just that they move a shorter distance because they're... uhhhh, stronger. All coils move at all times in response to all loads.

As I said earlier, you'd probably not know the difference during our street bike use. This doesn't, however, mean that there is no physical science that explains how & why they behave as they do.

StMcK
 

·
Registered
Joined
·
939 Posts
Discussion Starter #28
Mike, on the question of rear shock travel is it acceptable to bottom out now and again? I have a piece of string indicating the extremes of movement and it is usually using most of it and sometimes jammed right against the bumper. Thoughts?
kito you said in another thread that you are not getting that harsh thump feeling of the shocks bottoming, so are you certain that they are actually bottoming?

the rubber bumper's job is to stop any bottoming of the shock under extreme impacts/loading and it effectively increases the spring rate dramatically over the last 20mm of suspension travel.

as you say that the string is only sometimes pushed against the bumper and if you are not getting that thump feeling I would not be concerned, a suspension guru once said that if after a race you had used all but the last 1mm of suspension travel all is good ;)
 

·
Registered
Joined
·
939 Posts
Discussion Starter #29
wont this dimension be affected by whereabouts you measure on the frame and swing arm? eg if measured towards the rear of the frame and swing arm it will give greater length measurement for the three steps than if you had markers further up towards the tank

wouldnt the correct way be to measure the mounting points on the shock where it connects onto the swingarm and frame?
1/
quite right bonza, ideally you should measure as near to the end of the swing-arm as convenient, I can only apologize for this omission and blame a tired typing finger lol,
some like to use the wheel spindle but this can change after wheel removal or chain adjustment, I prefer to mark the swing-arm so that measurement 'A' remains constant and doesn't require re-doing for minor sag adjustments.

2/
if the bottom shock mounts were at the very end of the swing-arm and the top mounts were directly above this would so,
but sag is a percentage of wheel travel so we need to measure wheel travel.
 

·
Premium Member
Joined
·
1,801 Posts
this morning I measured the rear sag for my my new shock absorbers and was happy to find the static sag was 15mm, riders 20mm and total sag was 35mm. was very happy with this as verified the new shocks are ok and have things set up well

thanks for a very informative thread
 

·
Registered
Joined
·
11 Posts
If preload does not affect rider sag why would bikes be equipped with adjustable preload? The static sag should be incorporated in the design as static load is a known variable and spacer would be made to unladen sag.

I ask because I notice that my new thruxton fork is quite soft and am considering options. First will be to set sag correctly.
 

·
Registered
Joined
·
277 Posts
This is an old thread, but it showed up in my e-mailbox so here goes.

The other poster didn't completely understand how pre-load works. With rider aboard, if your front forks are sagging (depressed from maximum height) 3-inches, adding a 1/2-inch spacer on the springs will raise the ride height 1/2-inch. adding a 3-inch spacer will raise the ride height 3-inches, but will also "top-out" the forks, after which additional preload will not raise the ride height, but will raise the "threshold of movement" higher, making the fork VERY harsh on small bumps.

Assuming you have a pair of (for instance) 100-pounds/inch front springs in the forks, this means that it takes 200-pounds (100 x 2 fork springs) to move them through 1-inch of their travel. If you preload til the forks are topped-out, THEN add one more inch of preload, when you ride down the street it will take 200-pounds to move the fork from its resting (topped-out) position! The forks are made of concrete 'til they encounter a 200-pound force. A section of washboard bumps would break your teeth.

Adding pre-load to a spring doesn't change its spring rate. It does, however, raise ride-height. But as it does so, this preload also raises the "threshold of movement."

So, if you need to add 3/4-inch of rear spring pre-load to carry the occasional passenger, go for it. However, if you need that amount of preload just to get your solo ride-height in order, then you need to look at heavier springs. They allow proper ride-height without being pre-loaded to death, which causes incredible harshness during initial travel.

To apply this to Bonnevilles: I'm a big guy and I needed some big springs. Rather than empty my wallet, I went to my local independent Harley shop and perused their selection of used springs (they use spindly forks too!). I forget exactly what I paid, but it was something on the order of $25 for a set out of a NEW Super Glide that was trailered from the dealer to get a lowering kit.

Spring has sprung, Fall has fell
My preload's off and it's stiff as...
can be.

StMcK
 

·
Registered
Joined
·
65 Posts
Since its Free, have try to adjust my SAG,
currently installed YSS https://www.amazon.co.uk/YSS-ammortizatori-360trl-22-x-Bonneville-Centennial/dp/B01LY58YBA

i put on center stand with a half a inch of plank to ensure the rear wheels are free. Measured free (A), measured bike without stand (B) and measured with me (65kgs) sitting (C) with my wife holding.

No changes between A-B, try a few times, sometimes could get 1-2mm, thats it.
A and C difference are also only 1-2mm

Next, try my wife and me sitting and start rocking the suspencion and measure me individual, get the same results.

Is my Shock Dead?
 

·
Premium Member
Joined
·
683 Posts
I don't think your shock is dead, just too high of a spring rate. (Too stiff) The difference between A and C, or total sag, should be about 1/3 of your total shock travel. In your case I think 30 to 33 mm would be fine. Might want to give YSS a ring and discuss the issue you are having and see if they can recommend a lighter spring.
Cheers!
Edit: there is a chance that your preload is set so high that it has the shock topped out even with you on it, so back off the preload and see if that can get you within the parameters outlined above.
 

·
Registered
Joined
·
7,455 Posts
Sounds like either the spring rate is too stiff for your weight or the preload is set too stiff to begin with. If I remember for the rear of a bike, A-B would be somewhere between 0-10mm. One point to consider is that when you measure for sag, your measurement must be taken perfectly vertically. Your reference points must be in a straight line.
 

·
Registered
Joined
·
65 Posts
Thanks Sanckenil/Felony..... i dun want to disperse the idea my shock is dead = spending new shocks. currently the spring is set at almost 90%, have try compress the spring to 50% and do the sag, and the results are the same, logically it should provide some differences between A,B,C, however results are 1-2mm diff.

As you have rightly pointed out, it could be the spring as previous owner is like 130kgs compare to me, 60+kg !!!
 

·
Registered
Joined
·
7,455 Posts
That's a big difference. Spring rates work within a range of rider weights, but double the weight is probably out of your range. If you set the preload to the lightest setting or least, see what the results are. I imagine you need lighter springs.
 

·
Registered
Joined
·
518 Posts
.........As you have rightly pointed out, it could be the spring as previous owner is like 130kgs compare to me, 60+kg !!!
Your YSS shock springs will have a spring rating on them along the lines of 46-17-25-200, please let me know what this is............
 

·
Registered
Joined
·
65 Posts
Your YSS shock springs will have a spring rating on them along the lines of 46-17-25-200, please let me know what this is............
Boy Racer.... Where do I find the spring rating as from the box, these are the only details I have. I bought the bike with previous owner already installed. Thinking of changing back to original , just to see where went wrong OR if indeed it's the spring or shock went bust.
 

Attachments

21 - 40 of 56 Posts
Top