Suspension: Springs

[bryan]

So now we have read that the front suspension has a 2:1 rate and a 6:1 rate (and on another thread 4:1)... what the heck is the truth?! And please support your answer.

thanks

British v Yanks
British vs. Yankee’s in Spring Weights

[MCMAIN0121]

Gentlemen,
I apologize, I should have spent more time trying to dig things up. I will search the "British vs. Yankee spring weights" thread, thank you!

It will be interesting to see what the rate of the mechanical advantage is...

[Sneds 67 GT]

Not to highjack the thread, I guess this may be off text a bit but...

I have heard some stories that too heavy spring/shock combo can cause stress fractures in the front end, is there any real world proof of this, or just guys talking. I am close to suspension overhaul on my GT, this is something of a concern to me. In a stock 35 year old car, metal fatigue could be a possibility, I know lots of track cars have shock mounts reinforced. As my car buddies would say, how does the metal know its 35 years old?

Again, not to change topic, but if this is the case, should I be concerned about getting the suspension to stiff. Any one experience this problem?

[genericwood]

The motion ratio is probably closer to 3:1. A 1200 pound spring gives you a 400 pound(ish) wheel rate. In general, you don't need to worry about chassis cracks due to heavier spring rates. A monster sway bar could cause cracks around the mounts, but that is more a problem with the 750/101 cars.

Erik

[bryan]

Great! now we have 3:1! I'm not flaming your input genericwood, from everything else I have read of yours, you have a wealth of knowledge. It's just that for new guys like us, there are now 4 different statements about what the front suspension ratio is.

Mr. genericwood, can you provide us with info about why you believe that to be the case. Much thanks

[olycarguy]

stress cracks are a common problem I've seen on many 105 series cars. Stiff springs don't help...... but most of the cars I've seen with this problem are street cars. The combination of unibody construction and 35 year-old cars...... The problem is easily solved by making re-enforcement plates and welding them to the front end. T.H.

[genericwood]

Quote:

Originally Posted by bryan Mr. genericwood, can you provide us with info about why you believe that to be the case. Much thanks

[George Willet]

My caculations are based on measuring, remeasuring and then rechecking .

The distance from the inner arm pivot to the centerline of the ball joint is 12.75 inches, and the distance from the inner pivot to the spring centerline is 4.562 inches. So the motion ratio is .358:1.

Thus a 1200 pounds/inch spring has a wheel rate of 154 in/lbs.

Hope this helps.

[George Willet]

Incidentally, since the spring pan is at different angles depending on the bump and droop of the car, the spring would have less installed height at the front of the pan if the car is lowered, so the rate would be slightly higher.

And if the coils bind, the rate goes higher. This is why it is only indicitive to use the formulas for spring rate, since you don't know for sure which or how many coils will be effective at an installed height, thus the invention of spring testers, and they start the spting at installed height, and then compress one inch to get the inch/pound rate of the spring.

[60sRacer]

Bruce

Thanks for the formula and the data. I'm not surprised there are diffferences in 'similar' sport springs. Even if the formula is off because of differences in counting numbers of 'active' coils, at least it gives a reference between different springs.

It would be really nice to take some of the springs sorted by the formula calculations to a spring tester. That'd help calibrate the coil counting technique.

Another issue to think about in choosing springs is the car on top of them. I have found a lot of the latest springs from IAP and CL to be a bit too stiff for my car. I've a 67 Duetto which weighs under 2000 lb - not a lot of insulation, carpeting and other goodies in the old simple cars. Springs that suit a sporty ride in a 2600 lb S4 spider are a bit too much for mine.

Fortunately I have a set of old Shankle springs that are just about right. I'll have to measure them to see hwere they fall in your list.

Anyway, thanks for the data!

Robert

[Alfajay]

Most of the respondents to this thread probably know a whole lot more about this topic than I do, but let me throw in a couple of points:

In response to "Sneds 67 GT":

Yes, the front suspension mountings of 105 cars CAN develop cracks. I am currently restoring a '65 Sprint GT, and when I got the suspension and accumulated grease off, I was apalled to see cracks in the welds that connect the front crossmember to the frame pressings. These were visible on both sides. While I do not know the history of my car in detail, it does not appear to have ever been a race car, nor have high mileage. It had the original Girling brakes on it when I got it, which shouldn't have put huge twisting loads on the crossmember. How does metal know it's 35 years old? Well, metal fatigue results from the # of cycles of stress imparted to the metal. So, I guess the issue would be miles, or more precisely, the number of times the brakes have been applied, not years. I just welded up the cracks, and hope it will last for another 42 years (by which time I will be 99 years old!).

"Genericwood" had written:

"The motion ratio is probably closer to 3:1. A 1200 pound spring gives you a 400 pound(ish) wheel rate."

Well no, not really. While the ratio is about 3:1 (or, 2.79 (= 1/3.58) as George Willet has calculated more precisely) keep in mind that this factor must be SQUARED to get the effective rate at the wheel. So with a 1,200 #/in spring, your rate at the wheel would be approx 1/9 that, or 133 #/in.

Someone else had posted the formula for this recently on another thread. Putting it verbally, the motion ratio comes into play twice (hence the squaring): Once because the spindle-A arm geometry puts more force on the spring than the wheel sees, and again because when the spring compresses 1/3 of an inch, the wheel will move 3 times as far, or one inch (I'm rounding the motion ratio to 3 for the sake of simplicity).

[George Willet]

Good Sunday morning Robert. Some time ago I posted the spring rates for the popular suppliers when I had ready access to a spring checker. I'll get them out and repost them to this thread. They're in a binder in the shop, and it is 18 degreesF outside so I need a second cuppa coffee to brave the elements.

[George Willet]

And I want to get out my copy of Advanced Race Car Suspension Developement to double check the squaring of the spring rate to get wheel rate.

I shall return, G

[60sRacer]

Thanks George. I was wondering about the squaring thing too.

I guess cold is relative. Had some ice on the driveway this morning from the sprinkler run-off. First time it's been below 40 in - oh a decade..... Probably have to put on a long sleeve shirt today. Brrrr. Really really cold for So Cal. It was 82 last wednesday.

Robert

[George Willet]

I checked and the motion ratio must be squared: Jay Macro got it right.



Spring rates for 105/115 Alfas from some suppliers:

As tested on a spring checker in 1999. Some of the suppliers may have changed the product as of this date (2007)

Front: in inch/pounds
• Stock ’69 GTV : 370
• IAP red: 450 (they had a .685 in wire dia.)
• Centerline yellow: 975 (wire dia .790 in)
• Dave Rough: 630 (wire dia .685 in)
• Alan Ward has told me he uses and supplies spring rates in the 1200 (mostly) and 1300 range.

Rears:
• Centerline #PS411: 150
• Centerline #PS412: 185
• In a phone call to Alfa Recombi at that time , Julius said their blues were 185 (.500 inch dia wire)
• Shankle red Super Sport pn 4732B tested: 150
• Dave Rough: 175
• Rear springs, from 65/69 gtv, .433 in wire, with one and one & a half coils cut from open end: 180

Antiroll bars:
• Stock @ 222 in/lbs;
• Shankle bar @ 1 1/16” dia: 382
• 26mm ft bar: 306

FYI:
• From my experience, the addition or removal of rubbers/spacers under the front springs is a 3:1 ratio of ride height change, (1/4 inch makes a ride height change of ¾ inch). In the rear it is a one to one ratio.
• To reduce understeer in a corner; Carroll Smith told me to “keep adding front spring rate 10% at a time until the front end starting to “skate”, then reduce spring rate 10%”. (Works if the car is fairly well balanced front to rear to begin with.)
• An antiroll bar transfers weight to the opposite end of the car. Adding a rear bar on a 105/115 car “kills” the rear to help the front. C Smith “Work on the end of the car where the problem is”.
• Springs alone will not make a good handling car. Springs, shocks, antiroll bar, tires (size and compound), and a good driver (practice, practice, practice) can.  .

Hope this helps.