Spring calculator, ride high, sping rate/harshness...

[mariodschy]

Once again a spring thread...
I've read many hours, but i should have done this bevor i've ordered my new springs.
A few month ago i started a thread about sping rate, bump stop a.s.o.:
lotus-suspension-f42/bump-stop-lenght-sprint-t34965-15.html#p238883

I was looking for (fast) road springs and got the advice from Kelsport that i should try the following springs:
Front: 150lb/in free length 9"
Rear: 125lb/in free length 10"

I did take this advice against the (correct) advice that i got from Rohan and other luminary's here in this great forum, now i have the following problems:

1.) the ride height in front is about 1" to low (adjustable shocks are fully up)
2.) the spring harshness is absolut to high, may be it's ok for a race car but sure not for road

Sumary i need the car in front more than 1" higher (adjustable platform should not be on the upper end) and a much lower harshness, in my opinion fast road seems in front 90 to 100 and rear 80 to 90.

The wrong advice from Kelsport got me that I have to informed myselve a bit more about springs, i made this (very easy) excel file for calculating the right spring, based on the fact which are give from the Lotus manual (load) and on the fact that springs have almost linear characteristics. Please let me know your opinions.

Thanks
Mario

[rgh0]

Hi Mario

it sound like you prefer the softer end of Fast road. I agree that 150 lb front and 125 lb rear are more suited to the race track unless you are a hard core road warrior and have smooth roads where you drive.

Your spring rate calculator gives the correct free length for the same installed spring length and ride height as standard with a higher rate spring. Due to suspension geometry if your changing the ride height the spring installed length does not change in proportion. Also the wheel load and wheel rate is different from the spring load and rate due to geometry. The actual front and rear wheel loads and wheel rates are almost the same due to the different ffont and rear suspension geometry with the standard ratio of front to rear spring rates

cheers
Rohan

[UAB807F]

Hi Mario,

I don't know if this photo will help solve the ride height problem, but hopefully it will be of interest. I also have Kelsport springs, in my case I dealt with Pat Thomas before he retired and I ended up with very similar recommendations. (175/140 but very little difference in driver perception)

The first image shows a screenshot from my spreadsheet which gives the spring data from my elan, measured from the supplied springs.

The second shows where the damper platform ends up. It's the only shot I have but I can measure the height if it would be of help. This setting gives the lower arm as horizontal, which is as Pat suggested. The platform is obviously higher than with OEM dampers but the spring is nowhere near being coil bound at this setting and I can't say I've seen any problems.

It's a hard ride but I'd say it's still softer than a modern sports car - a Porsche with PASM on is much harsher for example.

Brian

The attachment Spring Details.jpg is no longer available

Spreadsheet details

[mariodschy]

Thanks for your coments.
@ Brian: If i look at your pictures i see that the lower arm is near a staight line with the cross member which means your ride hieght seems to be the OEM 6 inch.
What i not understand ist your spring rate (front) they have a rate of 175 and they are compressed about 1" so you have a load of about 180lbs. The factory manual shows a load of about 516lbs, regarding the fact that your ride high is about the original 6inch, the load should be not far away of the mentioned 516lbs.
If you look at the second sheet in my excel file you can find a similar discrepancy but in my case i got about 400lbs and i am 1" to low (gound clearance is only 5").
On Tuesday i'll get my new tyres (Fulda 155/80 *13 - 4,5J) and than i will test again with the original steel wheels, now i have the panasport fitted (165/65 *13 - 5J

Regards
Mario

[UAB807F]

Hi Mario,

The photo isn't very good but yes, the lower arms are horizontal between inner & outer bolt centres. The car is on 155/80x13 tyres and the ride height is around 6" with the car sitting level.

I did a quick look at the tyres you mentioned and the overall diameter is smaller than 155/80 by 33mm (578 vs 545mm) according to an online calculator. That doesn't account for a drop of 1" on the ride height but it should go at least half way in accounting for the lower height.

On the preload aspect, I just don't know enough about the subject to comment sensibly. I'd noticed the high OEM preload and assumed it was due to the lower spring rate, the higher preload presumably restricting the amount of travel you get once on the move.

On mine I didn't try to calculate the preload but simply set the car up at the ride height I wanted. With the front of the car on a jack I raised the damper platform until the spring was just compressed then slowly lowered the car on the ground and then increased the platforms evenly until it looked right. Drove it round the block to settle everything down, came back & did more adjustments based on a 6" height.

I've found another photo which, although the car's raised with the weight off the wheels, it gives you a better idea of where the platforms are set.

If your set up feels harsh, what have you got the dampers set on ? The damper settings made a difference that even I could feel so it might be worth slackening them off a bit ?

Brian

[rgh0]

Hi Mario,

...On the preload aspect, I just don't know enough about the subject to comment sensibly. I'd noticed the high OEM preload and assumed it was due to the lower spring rate, the higher preload presumably restricting the amount of travel you get once on the move......

Brian

The installed load on the springs has to match whats needed to hold the weight of car at its ride height. Thus regardless of the spring rate you need the 516 lbs of spring load to hold the front of the car up. This is higher than the actual weight of the car on the front wheels due to the spring being at an angle and the lever arm effect of the suspension.

The suspension travel is greater with a softer spring for a similar bump or corner load but the static load on the spring is the same regardless of whether the spring rate is higher or lower.

cheers
Rohan

[adigra]

Pat also tried to convince me that my car is so soft it's "dangerous" in his opinion, and that I should fit their springs. I declined as I think the ride of my car is just right for a road car on British roads.

[Apx]

I have this set up in my S4. When i spoke to Pat he asked me what i wanted, style of driving, details about the car and how i was going to use it, etc. He then recommended this set up and after playing with the setting and driving i have reached a set up that suits me for road driving, the improvement was dramatic. I have had no issues getting the required ride hight, ( less than 6 inches due to lower profile tyres). The ride is not so harsh, certainly not as harsh as my Elise. But the one thing i have learnt through the process of selecting the suspension components is " one mans meat is anothers poison". Having set the suspension in another car i firstly asked people with different set ups for a ride to help me with my decisions, ( the value of local clubs) it certainly helped.

[mariodschy]

Hi Mario,

If your set up feels harsh, what have you got the dampers set on ? The damper settings made a difference that even I could feel so it might be worth slackening them off a bit ?

Brian

I've tryed different damper settings but i couldn't reconice any difference because the spring rate is so high it seems that the dampers are fully looked (this is not possible on my SPAX gas dampers)
All four dampers are now on the full soft position, but it is still muuuuuuuch to harsh.

The installed load on the springs has to match whats needed to hold the weight of car at its ride height. Thus regardless of the spring rate you need the 516 lbs of spring load to hold the front of the car up. This is higher than the actual weight of the car on the front wheels due to the spring being at an angle and the lever arm effect of the suspension.

The suspension travel is greater with a softer spring for a similar bump or corner load but the static load on the spring is the same regardless of whether the spring rate is higher or lower.

cheers
Rohan

Thanks for your explanation, seems that i've understand it right

@Brian: seems there is really something wrong in your spring sheet, could you messure the real spring lenght in load position (car sits on it's tyres)

[UAB807F]

@Brian: seems there is really something wrong in your spring sheet, could you messure the real spring lenght in load position (car sits on it's tyres)

Hi Mario,

If it's the "204mm" that's causing concern, ignore it. That's not a calculated field, it's just notes that I made when I first fitted the springs. I only included that section so you could compare the spring details you have (wire/diameters/working coils/etc) with those I've got so we could be sure we were comparing the same items.

The 204mm came about because I fitted the springs on the extended dampers and then raised the platform until it felt reasonably tight simply as a starting point whilst they were off the car. As you might expect it was too low and hence the platform was then raised with the car's weight on them to get the lower arms parallel to the ground.

I didn't aim for a specific ride height, Pat told me to adjust them until the arms were horizontal and I'd be about right, although I did later measure the height for my notes.

The car's over my workshop pit at the moment for some pre-MoT checking so I'll try to get a measurement on the actual spring under load although I'm not sure how accurate it will be. I'd guess with the car weight it's going to be somewhere around 15-17cm ?

Brian

[mariodschy]

Hi Mario,

If it's the "204mm" that's causing concern, ignore it.

Brian

Yes that's it, the real length would be interessting.

Mario

[UAB807F]

Yes that's it, the real length would be interessting.
Mario

Hi Mario,

Yes,I can understand the concern. This morning I measured the spring length with the car on it's wheels. The number I get is 16.5 cm BUT that could easily be 16cm or 17cm. I'm measuring from underneath the car with a ruler and locating the top of the spring by feel.

But it seems about right ? I also include a new photo so you can see the coil spacing and although the platforms are high, they could go higher with no real issue.

A couple of (potentially silly) questions to clarify things in my mind...

When you adjust your spring platforms, is it not possible to get the lower arms horizontal ?

Do you have free movement on the damper rod before the bump stop to allow spring movement ? I can't measure mine in the compressed state but would guess it's around 5cm?

I'm just a bit puzzled by your comment about damper settings not being noticeable and wonder if you've not got enough spring/damper movement ?

[ecamiel]

Those spring rates should not be unduly harsh so you need to look elsewhere.
The "bump stops" may be coming into play too soon, Check and the shorten them as needed. Also, They are actually progressive rate springs and by carving the taper in them you can make their rate softer at first. This will also help the handling change which occurs when they engage.

Also check for coil bind, shock settings and "stiction" in the bushings if you have installed solid or urethane bushings.

I have much stiffer springs and still have a smooth, compliant ride. on the street.

Eric
64 S1 Hart

[mariodschy]

Thanks for your coments.
On Tuesday i'll get my new tyres (Fulda 155/80 *13 - 4,5J) and than i will test again with the original steel wheels, now i have the panasport fitted (165/65 *13 - 5J

Regards
Mario

Yesterday i had time to test the steel wheels with new typres (Fulda 155/80 *13 - 4,5J), front pressure 1,7 bar und rear pressure 2,0bar. First thing that was surpriseing me was the differenc diameter, the 155/80 R13 Diameter was 580mm and the diameter from 165/65 R 13 was only 540 (both types has full profil).
The original style tyres are significant bigger which makes the car ground clearance higher.
The testdrive:
It was a new experience, the difference to the 165/65 was enormous, the car now drives much smoother and softer. No doubt the harshness is to high and the car ground clearance is still 0,5" to low, but in this case i change my mind, means the spring rate for fast road:
front 100 to 110 lbs/in (for road warrior maybe 120)
rear 90 to 100 lbs/in

[mariodschy]

Now i've checked what springs are available, the closest are:

front 1,9"ID 100lbs free length 12"
rear 2,25"ID 90lbs free length 12"


I wouldn't go any lower, but in the other direction for front are only 125lbs available and this is to much.
Perfect would be 110 (not available) but better a bit softer and keeping my teeth...

My problem now is that i've used for calculation the loads which are given from the original data (front 75*6,89=516,8lbs and rear67,5*6,71=452,9) but when i used the messured datas from my installed "road worrior" springs i'll get a load of 405lbs in front, a bit difference is understandable because the ground clearance is to less. In rear i have a load of 403lbs and the ground clearance is near correct. Should i forgett this and take the original data? Whats you experience/opinion.

Thanks.
Mario