Rear shock placement

[yody]

okay, not sure what all that meant, but i would think i would rather the shocks mounted outboard wtih a slight angle, if you look at most leaf spring cars they have staggerd shocks mounted as far out as possible at a slight angle.

[Matt@RFR]

Norm, I was thinking the same thing...why not just jog the frame in and mount the shocks just shy of the tires and straight up? Granted, this only applies to full frame cars.

I don't buy the body roll theory. No matter what the body is doing, the shocks are traveling through the same motion as if you had one tire up on a curb and the other on the track, but going straight.

[MrQuick]

Most production cars have things designed with packaging in mind. Rear shock mounting recommended by Steve Smith. This is from Building the late model sportsman :
"Shocks are most effective when operating nearly vertical. A static angle of 10 degrees is ideal."
"The proper mounting position of the shock absorber in the rear is 8" below the center line of the rear end housing and 5" in from the rear face of the brake rotor. Upper mounting to let shock mount at 10 degrees."
On a coil over set up 15 degrees is ideal.
My understanding is that cosine is a factor in the equation to finding the decrease of leverage due to mounting angle, in this case dampening effect ... and not a offspring of your mother or fathers brother or sister. correction requested....

[blazer4corners]

Mr.quick the only problem with "The proper mounting position of the shock absorber in the rear is 8" below the center line of the rear end housing " is with 15" or smaller wheels is that it would put the shock below the "scrub" line. Why you would run 15" or smaller wheels?

[parsonsj]

Vince, cool. As Mr. Smith says: "On a coil over set up 15 degrees is ideal". That's what I saw too.

Blaze, all this information is based on circle track books (usually), since they race on live axles. They are limited to 15" wheels. You can probably take the statement and extend it to 17 or 18 inch wheels.

Matt: why don't you buy body roll as a factor? It seems obvious to me, but I always wanna learn.

jp

[Norm Peterson]

You don't want much shock angle because with more initial angle comes more variability over suspension motion (the angle becomes larger as the chassis drops, and its cosine becomes smaller at an ever-increasing rate). And since this effect decreases the effectiveness of the shock as travels further into bump, the least amount of damping would occur as you approach the bump stops. Conversely, the greatest damping effectiveness would occur about when the suspension is all but fully extended in droop and the tire is about to leave the ground. That just sounds backward, and the simple fix is to minimize the angle in the beginning.

It's somewhat more important with coil-overs, as wheel rate is a cosine^2 function of spring angle, which is now concentric with the shock.

For the street, mounting the lower end of the shock 8" below axle centerline height is too low for 16" wheels (17's are OK, but only barely) since the shock bracket needs to extend at least an inch or so below the shock mounting bolt center to keep the bolt from tearing out the hole.

You'd run 15" wheels if the racing class rules specified them, as I think most of the roundy-round sanctioning bodies do. Under those circumstances, everybody on the track accepts a good deal more risk with far greater understanding of the possibilities and consequences of things going wrong than does the driving public at large.

On edit - in roll there is rotation of the chassis, so the motion at the chassis side of the shocks is not purely vertical like it is in two-wheel bump. I suppose that you could force roll response to be very close to vertical at the shock mounts, but that would involve a very high rear roll center (strictly enforced by a Watts link or similar direct mechanical constraint), but that would create far worse problems than the bit of nonlinearity in the damping that it's eliminating.

Norm

[MrQuick]

what would the scrub line be for a 17 -18" rim set up.Add the 2-3 or 1-1.5? Wouldn't tire width play into that too??

[Norm Peterson]

Worst case would be if both rear tires went flat and you dropped right down on the wheel flanges, making tire width irrelevant. In that case, a 17" wheel has 8.5" on the nominal radius plus about 0.5" of flange height for 9" total. That's the limit on how far below axle center the bottom of the shock bracket should extend. In reality, you'd probably also have a couple of tire sidewall thicknesses worth of room, at least until you sliced the sidewall all the way through over the full 360* or unseated a bead.

Norm

[MrQuick]

ok after looking at several books by the same author I believe the 8" statement was a typo.I looked in his other book and it states
"with a 15" rim the lower shock mount should be located 7" below axle centerline. This would give a safety margin of a 1/2" if the car were to damage a tire and limp back into the pits on a rim."
Thanks for pointing that one out Blazer... sorry about the confussion. I believe I will be ok with the use of 18" rears. thanks again Norm

[Matt@RFR]

Norm, thanks for your comments. I said that I don't buy the body roll effect on shock angles because I can't visualise it. You mentioned something about body roll, but it went completely over my head. Can you dumb it down for me?

[Norm Peterson]

Matt

Cut out a picture of a car taken from the rear. Mark about where you think the chassis attachment points of the rear shocks should be. This doesn't have to be exact or to scale, since all we're looking for is a demostration. Then stick a pin through the picture about where ground level is under the rear axle (simulating the geometric roll center, also no need for precision). Not into the dining table, if you want to keep peace in the house. Now rotate the picture about the pin. Note that the marks representing the shock mounting points will move laterally as well as vertically. The axle does not roll (not nearly as much, anyway), so the axle ends of the shock remain (essentially) in the same place. In a left turn, for example, the LR shock chassis attachment point moves to the right as well as up relative to the axle (the RR shock chassis attachment point moves right and down). That means that the chassis motion at the shock attachments is not necessarily in a direct line with the axis of the shock, nor is it purely vertical as in two-wheel bump.

That the shock piston can only move axially within the body of the shock has no meaning. It's how much shock piston motion results from a given chassis motion relative to the axle that matters.

When I mentioned relocating the rear roll center to the same elevation as the chassis shock attachment points, that would change the center of rotation such that the motion in roll at the shock attachments would be purely up (left side in the previous example) or purely down. Then the roll motion at the shock ends would be similar to the two-wheel bump motion, just not 'up' or 'down' together.

Norm

[Fuelie Fan]

I was laying underneath our company's pinto project, and I swear those things are laying at 45 degrees. I honestly think they were trying to add damping to control lateral motion of the axle!

How many degrees of roll are you guys designing for in your spring selection?

[Norm Peterson]

Right now I'm at just under 3* per g with the car in my sig. Add another 0.7* per g of apparent roll from tire deflection.

And it's still too soft for auto-x, even with 1.3+/1.5+ ride frequencies. I'd like that 3*/g to be much closer to 2*/g for reasons of camber control.

OT - I had a Pinto during the 70's. Occasionally I wonder how well it would fare in Street Mod . . .

Norm

[Matt@RFR]

Thanks Norm, that was perfect. I now know why I'm not designing chassis.

[yody]

maybe he has some real life experience, and just didn't read a book?

[spanky the wondermuffin]

yes i can hardly imagine what it must be like to build a car and get it out on the track regularly.

[Norm Peterson]

I hope that I'll be given a little latitude here, and that after this the thread can go back to topic without being locked. It seems that I've been publicly questioned as to credentials. I think I need to make a public response to set the record straight, and it will be best noticed by the appropriate individual right here. Spanky, feel free to e-mail me if you care to discuss this further.

Line item style just so that we can stick to things topic by topic . . .

man norm you see to disagree with everything steve smith writes.have you ever read any of his books?he has designed inumerable winning race cars.and he gives away good information for the price of a book.

My reading goes well beyond the books of just one person. Just the chassis/tire section of my automotive library is a stack of softcovers 8" deep plus a couple of hardcover titles. On the old board I posted the list, but I'll mention William & Doug Milliken, Fred Puhn , Don Alexander, Carroll Smith, Paul Van Valkenburgh, and Bob Bolles as a few of the authors. Books by Steve Smith would long since have been there too, had I ever found his titles in any of the local bookstores (I have looked specifically for them).

i've never heard of you,can you tell me what cars you build and where they win?do you have any books out that we all might benefit from?

Try to keep in mind that the questions raised in fora such as this one are asking for a variety of opinions from members of varied expertise. Presumably, from the range of responses and the ensuing discussion a better-informed choice will be made.

In the truckarm thread I did hint at my background. I've been an engineer (structural analysis) for nearly 35 years. Engineering teaches you to first get an understanding of things and how they work. Then to make things work efficiently. That becomes an approach to life that you don't lock up in the desk drawer or the tool box at the end of your work day.

What I've been paid to do has been similar enough to chassis and suspension topics that I don't think it's much of a stretch for me to get involved in such discussions. I've committed several automotive engineering topics to spreadsheet solutions, and let me say that doing that forces a deeper understanding of things than just reading somebody else's book.

My cars have all been daily drivers, but I've had some autocross success with them.

My 'published work' consists of posting on forums such as this one. Like most forum members, I imagine. If I rounded up all my 2750+ posts (yes, I do keep track, this one makes it 2759), distilled them all down to discussions of a few separate topics, and had it printed, how would that make any difference?

Why do I feel like I've written a cover letter? Is anybody hiring?

but i think it is time you prove him wrong,not just insist he is.

Help me out here. I'm not too sure what you think I'm claiming that he's all wet about. If it's the lower shock mounting point, I'm coming from the point of view that, in the event of flat tires, hard points should not be permitted to contact level ground. That's a legitimate concern for street-driven vehicles, including PT cars (and a legal one as well, if I'm not mistaken). But it's not necessarily a requirement for a dedicated track car, so this is likely a case where you should not directly transfer a track design right down to the dimensions over to the street.

How about going through my logic and pointing out where I'm making an error, or perhaps not presenting enough information. I'll listen. I might even change my mind, if you bring a well-reasoned explanation (this need not be your own original thoughts, but it does have to be more than just a quoted table or list of numbers without the equations or other data to back them up). And I won't even get *all mad* (hint: you'd probably get yourself banned and put on blood pressure medication before that would happen).

And as an off-topic (but I think relevant) excursion, could you try to teach your keyboard that it's OK to put a space between the end of one thought and the beginning of the next? Let me catch my mental breath between separate items, and let me believe that you're not about ready to spit nails. Try to sound a little more patient, or at least a little less argumentative on a non-technical level.

7.5 below centerline of axle.11-15 deg.staggered if leafs.in front of axle with coil spring.behind axle for coilover.

And what I previously posted completely disagrees with this?

Norm

[dennis68]

Spanky, not that Norm cannot defend his opinions on his own, I just wanted to vouch for his experience. Norm is very respected on all the boards he posts on, not just this one. His opinions are based on real world experience (yes, he actually races) AND theory. Try doing some research over at c-c.com, which is a who's-who in the road racing circles. You'll find Norm and many others who's opinion and tech are undisputed.

It's easy to argue with theory when that's all you have. As with anything else in the world, just because somebody puts it on paper and sells some books doesn't mean it is fact and not open to interpretation.

I have 0 engineering skills or "book smarts", however I do have 15 years of hands on as the suspension expert in my field. In 1994 Dodge launched a new pickup and with it came a world of suspension trouble (check the TSB, it's almost 30 pages) which I spent the next 10 years troubleshooting and correcting. I am still learning and find that usually you gain far more insight when you listen to others opinions (unless that absolutely no merit) than to question their background.

BTW, I do not ever recall a thread being locked on the old board, this has always been and will continue to be a very relaxed board. We do not typically have any name calling or put downs/questioning of one’s tech background when offering comment. Lets not start any new trends.

the idea that a shock bracket needs to extend below the skock eye is wrong.

What?!?!? If the shock bracket does not extend beyond the mounting location the shock will rip out of the bottom almost immeadiatly (ask me how I know).

[dennis68]

No desire to argue here. I'm here to learn as much as the next guy and offer what I can from my experiences.
I have NEVER built anything based on what somebody says in a book. I'll ask a dozen people who I believe know the answer to my question then research it on my own. After all that, I'll build it my way. I was told by a dozen "experts", some from the machine shop, that my last engine wouldn't even make it out of the stall. It was supposed to be way too radical for daily commute. It wound up being the strongest pulling car I've ever driven (in a long list including A/C Cobra's, Vipers, Vettes, and early B/B Mopars) so I don't put much stock into what others say, especially in books. I do however appreciate their input so I can try and make an informed decision. (Norm, Tommy, Mark, Mark and the others, please keep your opinions and experiences available to the masses. We do appreciate it).
Enough of that-It was asked what the optimum shock angle would be, I think the answer has been almost unanimously no more than 15 degrees from perpendicular to the ground. Asked and answered, unless someone has some first hand tuning knowledge of changing the angle--

Spanky, Shift key once in a while and a space after punctuation make reading much easier.

[yody]

"It wound up being the strongest pulling car I've ever driven (in a long list including A/C Cobra's, Vipers, Vettes, and early B/B Mopars)"

Dennis, what kind of times does your car run? or is this comparison a buttdyno? i didn't realize you had an 11 second el camino, doesn't a stock viper run low 12's?
besides that spanky seems to have a very strong opinion, but the prob. here is that this is the internet. So spanky what can you go in depth more about your recomendation? and also i understand you are a racer, but surely you can't just do everything based on one book? I didn't know who you were, but now i see. Give us some info so we can learn instead of arguing who has more experience. i am not on anyone sides here, the only thing i noticed is that i thought norm would of had more experience on cars on the track. but i dont think any of this matters much, we are all here to learn