BP-51 front strut pre-load setting and wheel drop spec - Need a spring rate physics lesson

[kcjaz]

So after the installation of my BP-51 suspension (see BP-51 Install Complete thread), I have determined that the preload on the front struts is set at the factory setting of 20 mm which is for a diesel LC with a front bar and winch. My 200 is gas (U.S.) with T13 bumper only (no winch). The install instructions say the setting should be 5 mm and that should provide 40 - 50 mm lift over stock (not exactly sure what my actual lift was but believe its more than 50 mm). The manual also says that the front wheel drop should be 2.5" (63.5 mm). I lifted the front and could raise the frame about 3 1/8" (79.3 mm) before the wheels lifted off the ground so, 79.3 - 63.5 = 15.8 mm more than the target wheel drop spec.

Is the additional 15.8 mm of wheel drop directly related to the 15 mm error in setting the preload? Seems so, but I am having trouble making my brain understand the physics here. I don't understand how less preload would make the wheel drop decrease. In my head, it would go the other way if it did anything at all. The preload and the actual weight of the front set the resting height of the truck. If all I did was add weight to the front to get it to drop 15 mm from where it sits now, and then jacked the frame to measure the wheel drop why would it be any different than it was before I added the weight?

Why does the actual load (preload plus truck weight) change the unloaded length of travel (wheel drop) of the strut?

 

[turbo8]

Adjusting preload won't change your total travel.

Did you account for the tire compressing under load?

 

[kcjaz]

Adjusting preload won't change your total travel.

Did you account for the tire compressing under load?

Thanks. I talked to Ben at Slee and confirm the same. I couldn't see how it would. You have a good point about tire compression that would cause my method of wheel drop measurement to measure a little more drop than there really is due to the compression of the tire. After phone discussion with Ben at Slee here what I have learned:

1. Preload does not affect total travel of the strut. It only affects the resting height of the front of the truck. If I reduce my preload, I will lower the front of the truck. This will increase what I measure as "wheel drop" because the truck will start at a lower resting height. This is what I thought in the beginning. I was just misunderstanding the 2.5" drop as an upper limit and the preload somehow a means to reduce the wheel drop.

2. Total load (weight on the strut) could affect wheel drop if it were way too light. That would really mean you have the wrong spring for the truck which would lift it too high and in the extreme could reduce your wheel drop to something less than 2.5" which you would not want. Since we are talking about struts designed for 200 series LCs, this shouldn't happen unless you have done some weight reduction mode to your front. (would like to see what they would be!)

3. A wheel drop of >2.5" is what you want to verify. More than that is not bad. Its actually good. Ben said that the strut itself can not over extend and damage itself (implied "as long as it is installed correctly" clarification).

4. BP-51 front struts can be set at any preload between 0 and 20 mm. There is nothing wrong with, or bad that will happen as far a damage goes with any setting in this range. It only sets the height of the front. It is essentially esthetics and how the ride feels. In my case, my truck is pretty level without any load in the truck and I like the ride. I sag in the rear a little with weight in the back. I can fix this by decreasing the preload setting on the front struts or by adding spacers to the rear springs or by putting heavier springs in the rear. I suppose air bags would be another option.

5. The preload to vehical height ration is 1/2 : 1 meaning that 15 mm preload adjustment is 30 mm height change. In the end, every 200 will be a little different. The preload table in the install instructions is meant to get you close and hopefully good enough the first time as it is a bit of work, plus an alignment each time you change the prolead and the height of the front. My installer just left it at the factory setting. What is sad is that I worried about that and discussed it with him. The copy of the install instructions I gave him though omitted page 3 which is where the preload instructions are. He thought we were talking about the damping adjustments on the shock. Stuff happens. Otherwise he did a great job and really I may just leave it for a while. The 5 mm setting would probably be a little too much rake for my taste.

BP-51/general suspension experts please correct anything I have misstated or got wrong.