Spring perch preload when compressing sprig

I get the fact that you should always be aware that when you move the lower bracket on the strut you should always be sure that you retain the full suspension movement. But one thing is a bit confusing for me.

As said 7kg/mm spring needs load of 7kg to compress 1 mm. But knowing that a spring always wants to go back to it's uncompressed state, that also means that if you want to make the ride hight, higher, at certain level you will compress the spring because the shock can't extend more, so in this case you will stiffen up the suspension i supose because the spring delivers some kind of force due to being compressed and wanting to go back to it's uncompressed situation. Isn't in this situation better the just move the mounting bracket. Above this isn't it always better to lower as much as possible with the mounting bracket as long you don't limit bump travel by doing this, because if you lower with the spring perch while there is still enough place to to move the wheel up, you're actually compromising in suspension travel

Uh, from what I can understand of your query, you don't quite grasp how a spring, and 'coil-over' works as a suspension component.

I can't go into it right now, but some sketches on a sheet of paper is a great brain-stroming tool.

Start with a spring, add the top and botton retaing collars, then the mounting points for the coilover, then consider how the top point and chassis are directly related to each other as are the bottom point and suspension/wheel.

What i mean is, andre tells in the course that you should always change ride hight by the lower spring perch. Nut there comes a point were you still want the right hight to be higher, but the shock is completely drooped, so at that point, when you keep turning the lower spring perch, you're actually compressing the spring, which results in a force being aplied to both lower and upper spring perch, which from my understanding has to be 'defeated' first when you hit a bump, before the shock starts to travel, hence why i ask if it isn't better, to change hight with the lower bracket position, as long there is no bump or rebound limitation due to the tire/wheel hitting the wheel arch

Uh, no - it may be a problem with translation - although Europeans usually have a surprisingly good understanding of english, whereas most of the rest of us have the barest smattering of other languages, to our embarrassment.

You seem to be what "pre-load" is, and I'll try and clarify. NOTE, FORCE and LOAD should always be expressed in Newtons, with kilograms ONLY used to describe MASS, but I won't worry about that here.

With any conventional spring (there are also progressive and dual rate made) the compression force is in proportion with the amount of compression (or extension, but that isn't a concern here) within the elastic range of the spring. If the spring is compressed, or extended, to the point where the elastic range is, it will be permanently deformed.

In context, this means that with a specific spring strength and (vehicle) load it will compress the same amount. When pre-load is used, this just means the spring is compressed between the upper and lower seats, and may be expressed as either a distance or a force applied. This compression will apply a force to the spring in proportion to the distance the spring has been compressed compared to the 'free' length - this is the overall length of the spring when no forces are applied to it.

In order to further compress the spring from the 'pre-loaded' point, there has to be more force applied than that applied by the pre-load - this is NOT in addition to the pre-load but takes over from it. For example, a 7kg/mm spring may have 10mm of pre-load, that means that a force of 70kg is being applied and anything less than 70kg won't move the spring seats, but applying 77kg will move it 1mm, 140kg by 10mm, 147kg by 11mm - if you get the idea. If we change the position of the bottom of the spring, when it's in it's operating range between being held by the pre-load and coil bind, the spring's upper end will move it's position to balance the load against the spring rate.

The point you are bringing up seems to be what happens if you want to lift the vehicle by quite a lot - could there be a point where the lower spring seat has to be screwed up so far that it is put under so much pre-load that the spring force is greater than that applied by the vehicle. This is certainly possible, but it will be due primarily to the wrong spring rate and/or length being selected in the first place. In this instance a longer and/or higher rate spring with, if required by the latter, a helper/keeper to retain the spring in position at full extension of the coil-over.

That should have answered your question but, if it didn't, please continue and we should, hopefully, be able to help further.

Hi Shedley,

I understand this topic can be a bit confusing for people. I think we may need to do a video with a miniature test rig as an example to help explain it more clearly.

Gord has explained it very well already so there's not much left to be added. In summary, you are talking about adding preload the spring (when the damper it at maximum extension and unloaded). For any reasonable and practical combination of damper length and spring rate, this preload will be overcome by the car's weight when it is sat on the ground. Once the preload is overcome, the rate (stiffness) of the suspension is identical regardless of how much preload is used. The preload does not affect the stiffness of the coilover or the suspension.

Yeah i understand, so the force will actually 'be there' but it doesn't mather because the weight of the vehicle overcomes it, and in first place if this happens, or if the travel is massively compromised by setting height like this, it's more a fault of inproper shock body/spring lenght

Yeah i understand, so the force will actually 'be there' but it doesn't mather because the weight of the vehicle overcomes it, and in first place if this happens, or if the travel is massively compromised by setting height like this, it's more a fault of inproper shock body/spring lenght

Uh, yes.

You've got it Shedley! Nice one 👊

I understand the physics, but... when should I consider using the lower mount? Would that be first place to start, in order to make larger changes (lowering from stock), then overcome travel limitations via the spring perch?

Perhaps if you open a new thread, rather than continue this?

Different things mean different things to different people, and there are two main variation in the coil-over design.

Are you using a strut type suspension and the strut being of the type where there is a secondary threaded section that screws into the axle assembly - sometimes called the TEIN type? When you refer to "the lower mount", you're referring to the lower threaded section, with the "spring perch" (AKA spring seat) being the threaded adjuster the spring is actually acting on?

If so, the normal process is to adjust the seat (perch) to just take lightly apply pre-load to the spring when fully extended - perhaps 3-5mm, or 1/8 - 3/6", so the spring is captive, then adjust the thread engagement into the lower section for the desired ride height.

If not, and the body of the damper has a simple seat adjustment and is fixed in place, one is limited to adjusting the seat and/or spring length to obtain the desired ride height. Note, this may require a light, secondary spring to ensure the spring doesn't become unseated on full extension - the preferred option is to select the correct length spring and then, if required, use the secondary spring.

The coilovers I have are very similar to the ones in this course. They have a fully threaded body with a lower mounting section. They are BR Series from BC Racing. The best pictures are at https://www.modernperformance.com/product/BC_Coilover_Suspension_08-09_Caliber_SRT4/08-09-caliber-srt-4-suspension-and-chassis.

Ah, it's what I was thinking it would be. Take up the free clearance so the spring is just lightly pre-loaded - it's held between the top and bottom seat when fully extended, then adjust the ride height by adjusting the lower threaded section.

Hi Michael,

First thing to mention here is that if you already own our Motorsport Wheel Alignment course, we just released a new worked example (see the RX7 example) for that course last week that covers the ride height setting process in detail. As a coincidence, it looks like it's the same series of BC coilovers used as to what you are using.

If you have access to that, please go watch Step 1 of that worked example. If not, then I'll talk you through the process here 🙂