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Suspension sag

D

davem

Tuned and Synch'ed
Joined
Jun 7, 2010
Messages
73
Location
Lincoln
I thought I would set up the suspension on my Breva 1200 (not sport).
According to most accounts the front forks should sag 25-30mm from the rider sat on the bilke to fully unloaded i.e the front wheel off the ground.
With the preload adjusters fully wound down the sag is around 40mm at best.
The manual recommends 8.5 turns out from full preload which is obviously way out to what i'm getting.
Any one else set up the suspension sag and achieved better results.
 
Actually you want the front of the bike to sag about 25mm vertically, not along the fork tube. Sag is measured with the bike on both wheels, and the riders weight off the bike. Then with the riders weight on the bike both the front and rear suspension should sag about 25mm vertically. You may need a couple friends to help stabilize the bike and take measurements.
 
O.k. thanks for that.
Next question. To get the 25mm vertical measurement by measuring the distance along the tube must be a simple bit of trig. if I could only find my calculator. Can someone work it out for me?
 
Actually, most people recommend measuring the front sag along the fork tubes, while measuring the rear sag along the vertical axis. Also, 25 mm of sag is really at the low end. Again, most people recommend 25mm - 40mm. I would shoot for 32 mm of sag, but you can try 25 mm and see if you like it.


Keith Code explains it all, here:
http://www.youtube.com/watch?v=bBlqVx4mWUw
 
To get a 25.00mm vertical sag, it will be 27.58mm on the fork inner tube based on Breva 1200's 25 degree rake angle.

If I can still remember my high school trigonometry correctly.... :mrgreen:

Phang
 
Phang said:
To get a 25.00mm vertical sag, it will be 27.58mm on the fork inner tube based on Breva 1200's 25 degree rake angle.

If I can still remember my high school trigonometry correctly.... :mrgreen:

Phang
Click to expand...

I should even be better at remembering that than you ... I took Trig three times!

Think I eventually got a mercy past because the teacher was so sick of me. :laugh:

Bill
 
It seems Keith Code measured the sag with the front wheel off the ground.

With the Keith Code method the sag was 40mm with the preload wound right down which is just in the 25-45mm range he is recommending but with no further adjustment possible.

Then without lifting the front end the sag was 26mm.

It seems odd that the bike is so softly sprung. Any further ideas?
 
john zibell said:
You may need a couple friends to help stabilize the bike and take measurements.
Click to expand...


With the bike unladen, stick a zip tie in the fork tube, push it all the way down. Load up the bike and the zip tie will move up the tube. You can then take the measurement either running along the tube or vertically. No assistance required nor any need to share that nice bottle of red.

regards

Robert
 
Roblatt said:
john zibell said:
You may need a couple friends to help stabilize the bike and take measurements.
Click to expand...


With the bike unladen, stick a zip tie in the fork tube, push it all the way down. Load up the bike and the zip tie will move up the tube. You can then take the measurement either running along the tube or vertically. No assistance required nor any need to share that nice bottle of red.

regards

Robert
Click to expand...

I don't mind sharing good wine.
 
Hmm, I think you might have some stiction that needs to be accounted for.

Static sag compared to extended fork: 40mm
Static sag compared to static fork: 26mm

That would point to a free sag of 14mm, right? I know that free sag is normally measured rather than calculated, but I think in theory the calculation and the measure should come out the same.

And if you're getting 14mm of free sag with the preload cranked all the way, then either the springs are way to light for the bike, or there was some stiction confounding the measurements.
 
Rechecked unladen sag it was 13mm.

The cable tie method is pretty fool proof.
Fit the calble tie to fork tube, push against the fork seal and then sit on the bike this will push the tie up the fork leg.
Get off the bike and raise the front end so there is no weight on the forks.
Measure the distance from the tie to the fork seal this is the sag measurement.
Any Breva or Norge (not the new model) owners willing to give it a go and report back would be appreciated.
This will need to be done with the preload wound right down for consistant results.
 
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