General Category > General Discussion
Help me understand Pre-load
bad Chad:
I know this will seem like an incredibly basic question to most of you, but for some reason I can't seem to wrap my head around the concept.
Let's stick to only rear shocks, to keep it even more basic.
Here is what I think know. Most shocks haver a preload adjustment, and by adding preload we are compressing the spring. This in turn makes the spring more resistant to compressing more under load, because it has already been partially compressed?
So a compressed spring is stiffer because it has already used up some of its finite compressibility, and thus this might be a good thing it you are adding more weight to the motorcycle. Does increasing preload/spring compression raise the ride height too? Is the only reason to add or subtract preload to better deal with varying weight?
Wouldn't one want to run with as little preload as practical?
Aaron D.:
Real quick-spring rate is measured in force needed to compress a given amount.
Preload does NOT change the rate, only starts the spring down the road of increased force.
So using fake, easy to understand numbers:
Spring rate 25 lbs/inch of compression
Spring with no preload will move 1" when you add 25 lbs to the unladen spring.
With 25 lbs of preload, adding 25 lbs doesn't move it-it only will move 1" if you put 50 lbs on it.
Wayne Orwig:
Preload basically just changes the ride height. Not enough and you bottom out the shock. Too much and you top out the shock. Changing the ride height can also impact handling.
The amount of compression of the spring is determined by the weight being carried.
A normal spring travels X number of inches per every pound added to it. It is linear, for a NORMAL spring. But progressive wound changes that, and the linkages on like a monoshock rear suspension change that relationship.
Or that is my understanding.
Huzo:
When you add preload, you are only raising the perch (the seat in which the spring sits), higher up the shock body.
Indeed if you put the bike on it’s centrestand, the spring will “top out” and will have more load prior to use, but that will only mean it “sags” less when you go to ride off.
The spring will still only have the same load when in use that it had before, your mass has not changed but your ride height will be greater because the shock compressed less to achieve equilibrium.
It is a complete fallacy and always has been, to suggest that winding on pre load “stiffens” the suspension.
There is a lot of material to clarify the myth. Google “spring preload, stiffness myth” or similar and all will be revealed.
LowRyter:
The general rule of thumb is the static sag should be set so that suspension travel compresses about 1/3 when the rider sits on the bike.
It's much easier to do this where the bike is on a shop stand and someone is measuring the suspension when the rider sits on it. It can also be done with mounting weights on the bike to mimic the weight of the rider. There are probably other ways.
That's why when you purchase a new motorcycle, the dealer should set the sag with the rider when the dealer delivers the bike (I know you Guzzi guys won't spend for a new bike and only buy used).
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