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  1. #11
    Join Date
    Jul 2018
    Location
    france
    Age
    49
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    Quote Originally Posted by jon1d View Post
    My measurements DAX SPAX shocks MKiv chassis 15 inch wheels, tyres 235/60 15 rear and 215/60 15 front. On level floor. With weight of the car suspension ride heights with fuel tank at 1/2 full. No added passenger weight. The wheel rim to rim is 16" 1/2 inch.

    Drivers side measurements only.

    Chassis rail bottom Rear in front of wheel = 5" 3/4 inch

    Chassis rail bottom Front in rear of wheel = 7" 3/4 inch

    Rear top wheel arch to top wheel rim = 7" inch

    Front top wheel arch to top of wheel rim = 6" inch

    My rear shock centre hole to centre hole is 13" 1/4 inch .

    I could not measure the front shock lengths yet.

    My car does sit higher as I have underslung exhaust.

    Some people might add your body weight in the seat and re-measure heights.
    Many thanks for these measures. My old dampers are same as you (spax) ie 15" open and 11.8" closed.
    Your rear ride height is 7,75" with a shock at 13.25" : it will give you about 1.5" compression travel (good margin to bump stop).
    If, with same dampers, we set the ride height to 6.5" Dax numbers (so 1.25" lower) and take into account the rear suspension motion ratio of 0.64, shock will go to 13.25"-(1.25*0.64) = 12.5" : in this case margin to bump stop will be only 0.7", quite small ?
    I'd like to keep the car low so i have to found some bump stop margin. I've found that 15" open lenght is unusefull in my case as springs have 0.4" of play when shock fully open so quite equivalent to a 14.5" open length shock. I think an improvement to all of uswhen planning to change dampers is to go to Avo or Protech or another supplier which offer us shocks with more travel. For info i think go to 14.5" Protech at rear which give a 11" closed lenght ie a 0.8" improvement vs spax ie a final 1.5" margin !

  2. #12
    Join Date
    May 2004
    Location
    Cowbridge, United Kingdom
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    63
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    10,400
    Don't forget that it's not the dampers that set the ride height it's the spring length and rate. (assuming you have the correct length dampers)
    The more you wind the spring up the damper thread the more preload you put into the spring and the higher the car will sit.
    The back end of my hawk is currently sitting too high for my liking even with the springs as loose as I can go.
    I want to let it settle a bit after a few miles before changing to shorter springs.
    Kev Davies
    South Wales Area Rep. UKCC Membership Secretary
    DAX Mk4, 383 Chevy Stroker, Tremec. SOLD
    Contemporary CCX 3-4028, 445ci Big Block FORD FE,TKO 600, Old School IVA'd and SOLD
    Dax De Dion LS2 and T56 IVA'd June '17 SOLD
    1965 Ford Mustang 289 Convertible SOLD
    In build- Hawk 289 Sebring Awaiting IVA

  3. #13
    Join Date
    Jul 2018
    Location
    france
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    Quote Originally Posted by kdavies3 View Post
    Don't forget that it's not the dampers that set the ride height it's the spring length and rate. (assuming you have the correct length dampers)
    The more you wind the spring up the damper thread the more preload you put into the spring and the higher the car will sit.
    The back end of my hawk is currently sitting too high for my liking even with the springs as loose as I can go.
    I want to let it settle a bit after a few miles before changing to shorter springs.
    As you say springs keep a main role in height and lots of people use preload to adjust ride height but shock open length have also a main role in ride height. In my case i have already choose spring rates according to desired suspension front/rear frequencies and want a small preload, so ride height will be impacted by open lenghts. On race dampers (which i can't afford) you set spring preload and you have an adjusting perch to set ride height independantly. I see the preload nut only as a little final adjustment.

  4. #14
    Join Date
    May 2004
    Location
    Cowbridge, United Kingdom
    Age
    63
    Posts
    10,400
    If you are using the open length of the damper as the method of setting your ride height then you will have no rebound in your suspension at all.
    Kev Davies
    South Wales Area Rep. UKCC Membership Secretary
    DAX Mk4, 383 Chevy Stroker, Tremec. SOLD
    Contemporary CCX 3-4028, 445ci Big Block FORD FE,TKO 600, Old School IVA'd and SOLD
    Dax De Dion LS2 and T56 IVA'd June '17 SOLD
    1965 Ford Mustang 289 Convertible SOLD
    In build- Hawk 289 Sebring Awaiting IVA

  5. #15
    Join Date
    Aug 2007
    Location
    Bournemouth
    Posts
    705
    The spring position on your shock absorber will determine your ride height. The shock absorber length will determine how much dampening you get in either direction. The perfect scenario, with weight on wheels is to have the sock absorber in the middle of its travel, which is difficult to achieve. So shock absorber length is also very important.
    The main difference between the Spax and Gaz are that the Gaz shocks have a wider range of adjustment to set the ride height, ie, the position of the coil spring.
    With Spax fitted I was out of adjustment on ride height and riding on the bump stops. The springs were adjusted as high as they would go but this caused the shocks to be very nearly fully compressed. Although my springs were very tired, I don't believe there would have been much difference in the ride height from when they were new. The Gaz set up has sorted all this out for me. I am no longer on the bump stops and I actually have suspension if I press down on a corner of the car.
    Shock open length has no bearing on the ride height that I can see.
    David.

    Life is driving the Cobra. The rest of the time is just waiting................till I fix it!

    Wessex region caretaker rep.

  6. #16
    Join Date
    Jul 2018
    Location
    france
    Age
    49
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    Quote Originally Posted by kdavies3 View Post
    If you are using the open length of the damper as the method of setting your ride height then you will have no rebound in your suspension at all.
    Not what i am trying to say (sorry for my english, i'am french) I'll give an example : according to your suspension goals (frequency, preload, shock bump/drop margins) you have to choose the right spring rate. For simplicity assume no preload and a 15" shock : the spring will start to compress from 15". If you chose a 14" shock and again no preload, spring will start to compress from 14" : result is a difference of 1" of final shock length or, with same spring characteristic, a ride heigth directly related to open length.

  7. #17
    Join Date
    Aug 2007
    Location
    Bournemouth
    Posts
    705
    The preload on the shock will have no difference to the fully loaded shock with weight on wheels unless the weight on wheels is not sufficient to overcome the preloaded the spring is set to. If that is the case then your suspension will only work if you are racing and cornering exceptionally hard. Good luck with coming out of that alive!
    The final loaded spring is purely on the weight that is compressing it, ie, the car. Not the weight compressing it plus preload.
    David.

    Life is driving the Cobra. The rest of the time is just waiting................till I fix it!

    Wessex region caretaker rep.

  8. #18
    Join Date
    Jul 2018
    Location
    france
    Age
    49
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    72
    Quote Originally Posted by Darnpistonbroke View Post
    The preload on the shock will have no difference to the fully loaded shock with weight on wheels unless the weight on wheels is not sufficient to overcome the preloaded the spring is set to. If that is the case then your suspension will only work if you are racing and cornering exceptionally hard. Good luck with coming out of that alive!
    The final loaded spring is purely on the weight that is compressing it, ie, the car. Not the weight compressing it plus preload.
    You said yourself, it's only the corner weight ABOVE spring preload that compress the spring. This is why changing preload affect ride height. As i've said my goals is very small preload as high preload has negative effect as you describe.

  9. #19
    Join Date
    Jul 2018
    Location
    france
    Age
    49
    Posts
    72
    In addition for example if :
    - your corner weight is 400lbs
    - assume motion ratio of 1 for simplicity (ie sprint force = wheel force)
    - spring stiffness is 200lbs/in
    - spring preload is 100lbs
    Spring compression will be (400-100)/200 = 1.5in
    If now you set preload to 400lbs, spring compression is 0 (your example).
    You can see preload as the corner weight from which spring start to compress but above preload the spring will compress according to its stiffness : preload as no impact on stiffness (while loops are not in contact).

  10. #20
    Join Date
    Jul 2018
    Location
    france
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    Posts
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    ... and with same example the 1.5inch of compression is deduced from shock open length that's why open length has a direct relation to ride heigth (if spring stiffness and preload are previously choosen which shall be the case when designing suspensions characteristics).

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