Mac Phearson Strut Droop Travel

How much droop should a mac phearson strut system have? its kinds hard to find information on this. 

Car in question is 986 boxster based tube chassis car. 

Hard to say just based on suspension design, but you want the LCAs to be close to level at static ride height.

There's isn't really such a thing as too much droop travel but there is definitely such a thing as too little. At one point my AE92's front suspension only had about 1" of droop travel (due to a super-hard single-spring setup) and that was too little, with a tender spring setup I was able to increase it to closer to 4" which helped traction out of corners and on rough surfaces.

Compression travel is more important for a track car, but definitely don't short yourself on droop. Correct answer is "as much as you can package without giving up compression travel".

LCAs don't need to be level. A line drawn between the pivot points, on the other hand, that should be close to level.

20-30% of the total travel set aside for droop is a good baseline. 

I generally have good luck setting ride height around 1/3 of the way into the travel on street stuff. I've definitely had too little droop before, and it makes the ride really choppy and really hurts traction. Too much never seems to be an issue, but usually macstruts don't have a ton of total travel so you end up running out of bump travel.

That depends on a couple of things:

Are you asking about droop limited by spring extension, or droop limited by shock extension?

The former is not how much it 'should' have, but how much it 'will' have. It's a natural byproduct of wheel rate vs corner weight, which is just another way of referencing the oft referenced suspension 'natural frequency'. This is what I would call the '1G' droop travel.

How much the latter should be, is as close to the former as possible without compromising bump travel too much.

How much bump travel is 'enough' is a whole nother can of worms that depends on numerous factors. The main thing I'd argue is that 'enough' bump or droop travel is not a measured in length, but rather as a percentage of the 1G droop value, to provide better comparative consistency across a range of cars. Kind of like using a measure like natural frequency rather than spring rate to measure suspension stiffness. That way it automatically takes into account rates and weights.  Based on both production car wheel travel ratios and well regarded (off-the-shelf) coilover travel ratios, typical street and track vehicles can get away with significantly less bump than 1G droop. From what I've seen, bump travel to a traditional bump stop can be pretty successfully be at least down in the ~60% of 1G droop range on street and track cars, and suspensions designed to use the bump stops to provide progressive rats can even be in contact at static ride height. Personally, I'd target more if possible though.

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Also are you asking about modular coilovers where the ride height is adjusted independently of the spring/shock travel, or unitized coilovers where the ride height is adjusted through preloading of the spring(s)?

If the former, droop travel is is really just determined by your spring/shock travel adjustment.

If the latter droop travel will be fallout of the strut body length/travel vs desired spring rate and ride height, and may result in more strut droop travel than spring droop travel. This is where helper springs come into play.

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I'm not sure where the 1/3 of shock travel for droop and 2/3 for bump oft cited recommendation comes from. If it's based on 1G droop, then it seems overly conservative for typical road and track use. That's saying a stock Miata, which I recall having a 1G droop of ~6 inches, should have another 10-12 inches of bump travel. Even a fairly stiff suspension having 2.5" of 1G droop would need another 5 inches of available bump travel... Which only really makes sense if the traditional (contact avoided) bump stop is also 2.5+ inches. On the other hand, if preload is used to achieve it, then it's a largely meaningless ratio without knowing more about what percentage of 1G droop it is preloaded to.