BLUE TII

iTrader: (9)

Doh!
Thanks for that, now it all makes sense.

mrselfdestruct1994

iTrader: (1)

I have used a motion ratio of .63 for the front and .75 for the rear as per the spreadsheets on the FCM website.

Also, no disrespect meant but you have made an error with your calculations. To calculate the shock travel, you need to divide the corner weight by the spring rate. To convert to wheel travel just divide by the motion ratio.

With 16kg springs there is 30mm droop travel at the front wheels. 11kg springs would give you 43mm.

BLUE TII

iTrader: (9)

The motion ratio will provide mechanical advantage to help the corner weight compress the spring.
That is why he divided the weight by the motion ratio before dividing the weight by the spring rate.

mrselfdestruct1994

iTrader: (1)

If you want to calculate it that way, the corner weight should be multiplied by the motion ratio, not divided. It is easier to calculate the travel at the shaft and then divide by the motion ratio.

A) (300*.63)/(16*.63*.63) = 29.76

B) 300/16/.63 = 29.76

ZDan

I've seen a range of motion ratios presented for the FD. I've been using 0.61 front and 0.69 rear but no reason to doubt your source for 0.63/0.75. This guy came up with 0.60/0.68: https://www.rx7club.com/race-car-tec...ratios-591954/
Similar ballpark, anyway...

Motion at the shock is the change in force in the spring divided by the spring rate.
Dividing corner weight (force at the tire contact patch) by spring rate is wrong for a couple of reasons:
1. Corner weight is not the load supported by the suspension, you have to subtract unpsrung weight
2. The spring is *leveraged*, so the force in the spring is NOT simply the weight supported by the suspension (corner weight - unsprung weight). Wheel movement is greater than spring/shock stroke movement, force at the spring/shock to resist motion of the wheel must be greater by the same ratio. This is the *definition* of leverage.

There are a number of reasons why you might have measured 30mm at the wheel with 16kg springs:
Binding of suspension due to tire not being able to slide laterally on the ground when car lowered.
Non-zero preload in the spring at full droop.
Not accounting enough for unsprung mass (probably ~73 lb, 33 kg)
Car lighter than assumed
Assuming 50/50 when it might be 49/51 or 48/52 weight distribution
Stiction/hysteresis in shock and bushings/bearings
Spring rate actually less than 16 kg

No, then you would get the paradoxical situation where you are resisting movement of the wheel with reduced motion at the spring/shock, but with LESS force. That's not how it works, just ask Archimedes! You divide (corner weight - unsprung weight) by the motion ratio to get force in the spring.


If motion ratio is 0.63, then force in the spring required to support a corner of the car is (corner weight - unsprung weight)/0.63

This is not "travel at the shaft divided by motion ratio".
Assume we're saying that (corner weight - unsprung) = 300kg.
In that case, you have
(corner weight - unsprung weight) divided by wheel rate (which is spring rate * MR^2 or 16*.63*.63). That gives vertical wheel travel. Multiplying that by MR of 0.63 you get shock travel.

(corner weight - unsprung)/MR = force in spring. Force in spring divided by spring rate = spring deflection *at* the spring, aka shock stroke.

What does your car weigh and what's the f/r distribution? What front wheels/tires/brakes do you have? *ANY* front preload at all (doubtful it's exactly zero)?

Combination of the factors I listed above (plus measurement error) are likely the reason you're only getting 30mm apparent droop when *theoretically*, with zero preload at full droop, no friction/hysteresis, zero lateral constraint at the tire when loaded it should be:
force in spring/spring rate, or:
[(corner weight - unsprung weight)/MR]/spring rate
(300kg/0.63)/16 kg/mm = 30mm

Vertical motion at the wheel: (corner weight - unsprung weight)/(wheel rate)
= (300kg)/[16kg/mm * (0.63)^2] = 47mm

I would bet that (corner weight - unsprung) is more like 270kg than 300kg, there's some preload at droop, and the rest is due to stiction, hysteresis, and lack of tire slip when car lowered onto the ground.

mrselfdestruct1994

iTrader: (1)

The force being applied to the spring does not change. The weight of each corner compresses the spring as dictated by the spring rate. Do whatever you want to the other end of the spring, it won't change the corner weight or the spring rate. Your calculations predict that your 13kg springs will have about 58mm droop travel. I think you will find it's closer to 37mm.

BLUE TII

iTrader: (9)

Okay, there it is. Example A) shows the term we are missing in this discussion is-

Wheel Rate
Definition- Spring Rate as measured at the wheel

As you show the formula is-

Wheel Rate is equal to Spring Rate times Motion Ration Squared

So in our example-
Wheel Rate = 16Kg/mm * (0.63>2)
Wheel Rate = 6.35Kg/mm

To find droop travel one would divide the Corner Weight minus the Unsprung Weight by the Wheel Rate.

300Kg/6.35Kg/mm= 47mm droop travel for 16K front spring and 300Kg Sprung Corner Weight... at the wheel.

Multiplied by Motion Ratio would be 29.61mm droop travel at the shock.

Now I understand how you are able to simplify the formula to Corner Weight divided by Spring Rate divided by Motion Ratio.

mrselfdestruct1994

iTrader: (1)

The motion ratio can be used to calculate the theoretical spring rate at the wheel, but to use that to calculate the droop travel at the wheel you also need to use the motion ratio to calculate the theoretical load at the wheel. This is what I've expressed in the first formula.

In your calculations above you've used the theoretical spring rate at the wheel, but you're using the actual corner weight instead of the theoretical load at the wheel. This is why you've come up with 47mm at the wheel instead of 30mm at the wheel.

Honestly the first formula is a stupid way of doing it and just confuses everyone. I made the same mistake as you before measuring the actual droop travel and realising my mistake. It's much simpler to calculate at the shock and convert from there.

Once more for good measure, there is 30mm droop travel at the wheel and 19mm at the shock.

ZDan

??? If the force being applied at the spring doesn't change, the car doesn't move. But I *think* what you mean here is that it's no different from the weight at that corner of the car. But it is different. Because of LEVERAGE. Again, load on the spring is corner weight minus unsprung weight divided by motion ratio.

AND the motion ratio.

If I get a chance I'm measure it this evening! Long way home from workie though and will require food and beer...

mrselfdestruct1994

iTrader: (1)

If you were able to put a scale between the spring and the control arm it would read 300kg regardless of the motion ratio.

ZDan

Where'd you get that crazy idea?! Again, LEVERAGE. The fact that the movement of the shock stroke is different from movement of the wheel means that there MUST be a commensurate difference in forces involved

This is pretty basic stuff! Consider the front lower control arm. The wheel is mounted at one end via the upright, around 1/3 in the coilover is pivotably attached, and the inside end pivots on the chassis. Push up at the wheel/upright end and that moment on the control arm is reacted by the coilover. The reaction force in the coilover must be GREATER than the force pushing the wheel up to be in equilibrium.

Are you familiar with the concept of a free body diagram? If so I encourage you to do one of the suspension and see how forces at the wheel relate to forces at the coilover.

mrselfdestruct1994

iTrader: (1)

Imagine if I set the car up on stands with the wheels off. I put in a 10kg/mm spring and an adjustable control arm so I can change the motion ratio.

At a motion ratio of 1, if I applied 100kg of force at the hub, the hub would travel 10mm and the spring would compress 10mm. It takes 10kg of force to move the hub 1mm.

At a motion ratio of .5, if I applied 100kg of force at the hub, the hub would travel 20mm and the spring would compress 10mm. It takes 5kg of force to move the hub 1mm.

Either way it still takes 100kg of force to compress the spring by 10mm, and calculating the spring travel is as simple as dividing the force by the spring rate. If you want to calculate the hub travel, divide by the motion ratio as well.

Please let me know if I have made any mistakes there. Maybe I am just extremely confused, but my measurements seem to reflect my calculations fairly closely. Your calculations show that a standard FD would have around 6.3" of droop travel at the front wheels which doesn't sound right.

ZDan

OK!

Yes. The spring rate is 10kg/mm and the wheel rate is 10 kg/mm. 100kg force => 10mm at the wheel and at the spring.

No. At a motion ratio of 0.5 the force at the spring is 2x the force applied, so the spring sees 200kg force. 10kg/mm spring deflects 20mm. Deflection at the wheel is 20mm/0.5 = 40mm.

When you give the wheel LEVERAGE over the spring, two things are happening:
1. you are increasing the load on the spring by 1/motion ratio
2. you are reducing the movement of the spring by the motion ratio

This is why wheel rate is spring rate multiplied by the SQUARE of the motion ratio. You are not only asking the spring to carry more load, you're asking it to do it over a smaller change in length!

But the forces are very different at the spring vs. at the wheel!

C'mon, man, think about using a prybar to open a crate. The force you apply at the end of the bar is MUCH smaller than the force at the panel you're prying off. SAME THING.

Long/short, the force * distance at the wheel MUST equal force * distance at the spring.

With a 0.5 motion ratio, if you apply 100kg at the wheel and the wheel moves 10mm, the change in distance at the spring is 5mm and the force is 200kg

It *would* have a lot of droop if there weren't a ton of PRELOAD built in. Stockish setups will have a lot of preload. Once you get beyond ~11kg/mm you don't need or want much if any preload as it just limits your droop travel.

ZDan

OK, I measured the wheel movement at the back of my car parked vs. with the rear lifted in the air at the diff. Wheel motion = 48mm.
My car with half a tank of fuel weighs 2900 lb and has 50/50 weight distribution, so rear corner weight is ~725 lb. 275/35-18 NT01 plus 18x9.5 PF01 + brake rotor/caliper + half coilover ~69 lb. Corner weight - unsprung weight ~656 lb = 298 kg.

Assuming 0.72 motion ratio (average of your 0.75 and my 0.69), force on the spring should be 298 kg/0.72 = 414 kg. Displacement at the spring = 414 kg/(11kg/mm) = 37.6mm. I put the coilovers together with the Ohlins prescribed 2mm preload, so motion at the coilover is reduced by 2mm, = 35.6mm. Dividing again by motion ratio to get motion at the wheel: 35.6mm/0.72 = 49.5mm.

Not so far off from the 48mm I measured.

mrselfdestruct1994

iTrader: (1)

I'm stupid, you're smart, I was wrong, you were right, you're the best, I'm the worst, you're very good looking and I'm not attractive.

Thank you for persevering and correcting me, your explanation makes sense to me now. I am not sure why I only measured 30mm of droop, I thought about the factors that you mentioned but I'm not sure if they would make up such a large error. I guess another issue could be that the floor may not be totally level.

I will get into the garage on the weekend and figure out what on earth is going on. Sorry for being so bullheaded, I hate when other people do that.

Valkyrie

That's 'cause stock springs on most cars need preload so they don't fall out of the spring mounts on account of how they're shaped.


OTOH, a straight-cut racing spring with no preload will rattle about, but it won't fall out of the mounts.

I'm not sure what the technical term is for how both types of spring end.

ZDan

All true I won on the internetz! This time...

Did you by any chance lift only one side of the car? If so the sway bars would greatly limit droop of the wheels even when they are in the air.

No prob, it's good to talk these things out!

ZDan

It's because without any preload, stock springs would bottom out under the weight of the car.

Even a relatively stiffly sprung stock car like the FD would use up all or nearly all of it's suspension travel just holding the car up statically.

FD R1 front spring = 4.8 kg/mm
Assuming 0.63 front motion ratio and 270kg corner weight - unsprung, if you started with zero preload the wheel movement going from no load to static weight would be:

270kg/[(4.8 kg/mm)*0.63*0.63] = 141mm or 5.58", i.e. bottomed out...

7krayziboi

iTrader: (15)

I'm glad this is a happy ending

mrselfdestruct1994

iTrader: (1)

When I first measured I actually had the correct formula and that was one of the first things I checked. Jacking up both sides didn't make a difference and I couldn't figure it out, so I became convinced my calculations must be wrong!

I've checked the garage floor and it's nowhere near flat, so last night I borrowed a set of scales and leveled them up using some linoleum tiles and a water level. Here are the corner weights with the coilovers set to the same overall length from side to side and a full tank of fuel.



Using these corner weights and with the floor leveled, I was expecting the front right wheel to have 33.8mm droop travel and I measured 34mm.

Here is the math: (((270 - 35) / .63 / 16) - 2) / .63

These are the corner weights with an 87kg driver, plus four alcoholic beverages inside the driver giving an additional 1.3kg.



The car is a Series 8 Type RB with an advertised kerb weight of around 1260kg. I've made the following changes but the weights are either estimates or pulled from this forum so take them with a grain of salt.

Spare, jack and tools -13.5kg
Fog lights -1.5kg
Battery -12kg
Exhaust -10kg
Stereo -10kg
Non-sequential and emissions -15kg
Coilovers -5kg
Floor mats -6.5kg
Clutch -2.5kg
Twin oil coolers +3.5kg
Wheels +6.5kg

I will report back after I finish the corner balancing and alignment.

Valkyrie

Are those numbers with the sway bars unhooked?

You could probably even out the corner weights a bit if you jacked up the front right and rear lefts a bit (or lowered the others).

Unless you've also adjusted the corner weights, in which case you'd probably end up with too much ride height difference.

How did you use your level? With a piece of wood or something?

mrselfdestruct1994

iTrader: (1)

Those numbers are with the sway bars connected but when I adjust the corner balance I'll unhook them first. I think they might not make too much difference but I'll let you know.

For the water level I taped one end of the tube to a ruler and submerged the other end in a bottle.

Alpine

I also have a set of Ohlin DFV valved by ohlin japan, my impression is the performance is = to it's price range. I was hoping it would perform to the $4-5k JRZ/PENSKE level but sadly it didn't.
For sure way better than the run of the mill $1k coilovers out there.

guys make sure you are taking motion ratio into account when calculating the suspension travel, and also remember only the sprung weight compress the spring =)

In terms of travel I find Ohlin DFV is fine for a street car, but for a track car, I find it on the limit of the travel range. I wished ohlin would've kept the original travel of the PCV, which had a helper spring.

So in summary, Ohlin DFV is perfect for street car/privateer track cars, but if you want to sprung the car like a boss, pay up and get the usual JRZ/PENSKE etc.

BLUE TII

iTrader: (9)

Ohlins makes competition level shocks as well.

The Road & Track series Ohlins DFV are just the lowest spec coilovers they make.

Alpine

Ohh yes those TTX didn't even enter my mind, those are on whole another level !