Suspension Sag

[southwester]

I have been riding my 07 Rev 3 250 with the pre load in forks and shock maxed out as this is what it recommends in the owners manual for the forks for my weight (over 80 Kilo as I'm 15.5 stone) there is no mention for the shock so I maxed that out as well.

There was 3/4" bike on ground fork sag but no rear sag set up like this also I've had rebound screws front and rear completely undone.

Sag on the rear was about 3.5" with me aboard the bike

From what I've read the spring rates if stock arnt the correct rate for my weight which I'm not really keen on changing as I'm only a beginner and have chucked a possibly unnecessary good few quid at it already, but I would like to do what is do what I can with what I got.

Playing with the settings last week I wound out the fork preload adjuster all the way and slacked of the rear so I had about 3/4" bike on ground rear sag to see what the difference was, When I rode it Saturday the front wheel turned left and right while riding over rocks where it had held straight previously so I wound fork preload adjuster all the way back in again which sorted out that problem.

Sag on the rear is about 4" with me aboard the bike

I still haven't tried riding with adjusted rebound but with the fork adjuster screwed all the way out or in doesnt seem to make any difference whilst compressing them in the garage anyway.

So as the owners manual recommends the fork preload is maxed out for my weight where is a good starting point for the rear sag without me on the bike as that seems all I can control, also I will screw rebound adjusters to the mid position if they do, do anything.

Handlebars

I have had handlebars with rise vertical up to now but have rotated them forward a few degrees to see what difference this makes?

Any thoughts welcome

[wallrat]

Rear sag should be about 1/3 of the total travel so about 2.1" - that's measuring from axle to fender while on a stand, then with you + gear in riding position. Enlist a second person to help with this so you're not altering your ride position by trying to read a tape measure. More importantly, try to adjust the suspension so it reacts the same front and rear. Strange front wheel behavior is probably not related to sag and is more likely a result of having the front suspension not matching the rear.

Rebound is a matter of taste. Some folks prefer less rebound to give them extra spring action when hopping the tires. But too little and traction and control will suffer. Best to start in the middle and ride it that way for half a day. Then turn it 3 or 4 clicks one way and see if it feels better or worse. If worse than try 3 or 4 clicks the opposite direction from your original set point and see if it feels better or worse. Fine tune it until it feels right to you.

[lineaway]

I seem to remember 100mm-110mm on the spring length. It is far more important that the forks and shocks work together, than worrying about sag. Also in trials it is better to be too soft than slightly on the hard side. Push down in the middle at the seat. Both ends need to work exactly together.

[0007]

Pop quiz:

What is the purpose of "Sag"

As in why would I need it

Edited March 5, 2014 by 0007

[southwester]

I seem to remember 100mm-110mm on the spring length. It is far more important that the forks and shocks work together, than worrying about sag. Also in trials it is better to be too soft than slightly on the hard side. Push down in the middle at the seat. Both ends need to work exactly together.

100-110mm would of been the standard setting recommended by beta would it?

When you say front and back sus move together when pushing down on seat, is that they travel down the same amount or just move at the same time?

Thanks for replys

[lineaway]

The earlier rev 3`s Beta had listed the settings for the shock which was a great starting point.(Or just leave it there.) Pushing on the seat, both ends should work in unison.

[hoodie2]

I think the purpose of sag is to allow the wheels to stay in contact with the ground. The suspension compresses to deal with bumps but the sag also allows it to extend when you ride over a dip or a hole. The sag basically lets the wheel drop into the hole maintaining contact with the ground and therefore maintaining traction and steering. The sag will also give you the ability to pre jump or hop the wheels using the rebound from the partially compressed springs.

[guys]

Pop quiz:

What is the purpose of "Sag"

As in why would I need it

And the answer is...

[dan williams]

There is no purpose to "sag" It is merely how much the springs compress under the static weight of rider and bike. It has become a somewhat misunderstood measure of preload on the suspension springs. A general rule of thumb that becomes less useful with riders of different weight or springs that have sacked over time or been changed for accessory springs of a different rate.

I was planning on going into this in depth at some point with pictures and diagrams but I'll try and cover a pocket version here.

On a typical suspension application the springs and the weight of the bike/rider form a low frequency filter. As the wheels move rapidly to follow the terrain the frame reacts slower. The idea being that the quicker wheel movements are averaged out providing a smoother ride. At the same time the wheel weight and springs form a second resonant filter based on wheel weight and spring weight. This determines how fast the wheels react to changes in the terrain to stay in contact with the ground. The two systems are somewhat counter to each other as a lower spring rate/heavier chassis gives a smoother ride but a higher spring rate/lighter wheel maintains better contact with the ground. This is where "unsprung weight" becomes critical. Unsprung weight is essentially the wheel and any suspension components that are directly connected to the wheel on the bottom side of the shock. The lighter the wheel the lower the necessary spring rate to keep the inertia of the wheel from making it lose contact with the ground.

Trials bikes are usually set up with very soft suspension so the first thing Clydesdales like myself notice is how easy it is to bottom out the suspension. The cheap answer is to crank up the preload. It's not the best solution.

Preload is not spring weight. If I have an unloaded spring that takes 50kg to compress a centimeter it takes 100kg to compress it 2 centimeters, 150kg 3 centimeters. If I have the same spring preloaded 1 centimeter and I put 50kg on it won't compress. If I put 100kg on it will compress 1 centimeter, 150kg 2 centimeters. I've effectively lost the first centimeter of travel. What that means in practical terms is if I ride over a series of little stutter bumps that never put more than 50kg of load on the spring it might as well be a solid metal spacer. If you've ever driven a 1 ton truck with nothing in it around a bumpy corner you know the feeling well as all it does is skip across the road.

Assuming you are not built like Fujigas a heavier spring with less preload is better for two reasons. The first is less preload means the spring will actually react over smaller bumps. The second is peculiar to trials and specifically any maneuver that stores energy in the suspension to be released with the wheel leaving the ground, splatters, zaps, hops... With the spring preloaded you have to put in much more energy to get the suspension to move but you don't get it all back. If I jump on the above mentioned preloaded spring with 150kg of force that first 50kg gets transferred directly to the ground and only 100kg of energy is available to be recovered from the spring. OK it's more complicated than that as energy is stored in the tire but that's the general concept.

So sag is not an absolute measurement. More critical is balance between the front and rear suspension, how well your wheels are tracking on bumpy terrain, your personal riding style. Big hits and trick riding vs. slow turns and bumpy sections. If you can find any video of Jordi Tarres and Eddy Lejuene you will get an example of how different two world champions suspensions can be. Jordi's bikes were always sprung very high with minimal damping where Eddy's bikes were soft as a sneaker full of oatmeal.

If you're bottoming out a lot a little preload goes a long way with a rising rate suspension but be careful as it's very easy to dial in too much. Best approach is small adjustment, ride, small adjustment, ride. You will eventually find a setting that works for you.

[taff_d]

The Ohlins Tech who built my shock and Splat Shop told me too set the rear shock with 85mm of rider sag and the front forks with 45mm of rider sag, Could not achieve this with std Beta springs

so I have a Ohlins rear shock with heavy spring fitted and I've just fitted a spacer to the front spring but I have not tested this yet.

Edited June 13, 2014 by taff_d

[0007]

Dan put a lot of technical description in there and it looks to make good sense

IMHO I don't think sag has as much importance in a low ground speed vehicle as it does in a fast vehicle IE a MX bike or street bike

You can never really have 0 sag but to simply set it at the same spec as a regular dirt bike makes no sense to me