droop to compression ratio

[COChev]

hi,
i'm new to the board. i come from a 4wd background and would like to build a prerunner/dunerunner. i'd like to know a good droop to compression ratio (i think i'm saying that right) to run for weekend warrior but decent on the street purposes.

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[Bob_Sheaves]

If, by "droop to compression" you mean rebound to jounce limits, Detroit uses what is called "design point" as the median of static (not dynamic) travel. This is defined as half payload, 2 150lb people, full fuel, largest tires. I use the same (nearly) location for travel in concept studies, depending on the application (i.e. open desert, short course, road racing, circle track, etc.- each one of these is slightly different in requirements).

Best regards,

Bob Sheaves

[COChev]

ok i'll have to come up to speed on your lingo here. define for me static travel cause that sounds like an oxymoron. i thought static ment not moving. to try and clear up what i was getting at by droop and compression was this: starting with ride hight (truck with gas, 2 people, etc.) how much does the suspension droop from that point compaired to how much compression or up travel from that point. say for example with the truck sitting on flat ground how far would the suspension travel if the springs were compressed to the bumpstops compaired to how far it would travel if the rear of the truck was lifted up intill the tires came off the ground. i thought i read somewhere where it was like 2/3s of the travel was in the droop and 1/3 in compression starting at ride hight. obviousely the trucks weight and more specifically the weight of the front of the truck and the rear have to be taken into acount when selecting spring rates in order to acheive the correct ride hight.

am i making any sense? if not you all can cyber slap me

[Bob_Sheaves]

Hi again,

"Static travel" is defined as mechanical travel limits, excluding the dynamics of deflection of bushings, flexion of control arms, bending of shock rods, etc.....

Proper term for "droop" is rebound travel (mechanical limits of travel in a negative "Z" axis), proper term for "compression" is jounce travel (mechanical limits of travel in a positive "Z" axis). "Ride height" can be anywhere the designer sets it, but design point is a specific set of circumstances and conditions. The old 2/3 - 1/3 rule is a very rough guide for the person guessing at what to do, but there are far more variables to concern pros that designs a suspension that will perform adequately for a specific task.

You are correct, as far as you go, on the relationship of spring rates, travel extents, and "ride height". For a given "ride height" (Yes, Kreg, and others, this is a generalization, not a specific recommendation...LOL) the more travel you have, the lower the spring rate that is needed- due to free length of the coil type spring, as an example- to achieve that ride height. This does NOT mean that it is practical, because there are all these other variables floating around that have to be considered.

The answer to your question depends on what you want to do- calculate what is needed to do a defined job, or "hack and whack", in which case, the 2/3-1/3 rule is adequate.

Best as always,

Bob



ADDENDUM:

Some of what I am talking about is shown in the "A-arms" thread here under the "SHOP" heading. Just review the attachments and you will see a better visual explaination of the concepts....<P ID="edit"><FONT SIZE=-1>Edited by Bob_Sheaves on 05/07/02 09:46 PM (server time).</FONT></P>

[COChev]

cool. thanks for the info. i check out the thread you mentioned.