[1971BB427]

Spring twist is more about length, and height of the vehicle, as the height gives the vehicle more leverage on the spring. In some cases the greater arc is used to raise the vehicle's height, so it will then put more leverage on the spring, but not if the spring is mounted in such a way as to keep the weight lower. That's why many makers mount springs alongside the frame when large stacks of higher arc springs are used. It lowers the center of gravity, and keeps the vehicle's weight from working against the spring stack.
Some arc will give the spring more lateral support, but too much will indeed cause more twist, as there's just no way to keep the force against it low enough. Plus the spring needs to get longer as it's arc'd if it still has the same mounting points. Keep the length the same, and put some curve in it, and the spring will have a bit more resistance to twisting due to the curved shape.
You can test this with small spring material and a tension gauge. A flat piece of spring stock is easily twisted when gripped at the ends, but bend it and then try to twist it, and the tension will be greater. I've seen test sheets from the local spring shop where they use something similar to a big torque wrench to do these tests, and accurately measure spring twist.

[aosborn]

Originally Posted by MARTINSR

Now again, just throwing something out there I don't understand. Wouldn't more arch have more sideways movement? I would think the straight spring of a 69 F body would have less sideways movement than a large arched spring because that large arch can "swing" side to side at the bottom of the arch like a pendulum. Even if the spring is a mono leaf as I think it is on that 69 Firebird, wouldn't it being flat make all the difference as far as side movement of the rear axle?

I personally am not a fan of poly bushings because I like the cushy ride and lack of noise of the rubber. On my Gran Sport I have poly on one side of the sway bar links for instance so I get a little bit of stiff without making it totally stiff with the poly at both ends.

Brian

You are correct, as the spring moves through an arc the axle will move longitudinally and laterally . This effect can be utilized, as I stated in an earlier post, to your advantage. Primarily, the angle of the spring as it is mounted in the car (as viewed from the side) will dictate how the axle rotates in a bump. Most designs when the body rolls over in a corner will make the axle turn the rear of the car roll into the turn (roll understeer) rather than out of the turn (roll oversteer). That is why if you look at the mounting of the leaf spring from the side, the front eye is lower than the rear eye that is mounted in the shackle so the spring is angled down toward the front. You can also see this effect when you lower a leaf spring car, the wheel is frequently no longer centered in the wheel opening, it has moved forward.

Very rarly is a leaf spring designed to be flat when installed. If the spring is flat when installed, it has likely lost some of its ability to support the load and needs to be rearched or replaced. If it is flat at ride height, it will reverse arch in bump and can tend to make the rear axle steer outward on a turn (roll oversteer). I am making several assumptions in saying this, but it is generally true.

Another intersting feature of many Hotchkiss suspensions is that the axle is mounted forward of the center of the spring. That helps in roll understeer, and I also have to believe that it helps with wheel hop under braking and acceleration. The traction bar effect.

Lastly, it was discovered by GM back in the 60's that having one shock mounted forward of the axle and one rearward helped control wheel hop too!

Just like everything else in the car, there is usually more to it than meets the eye.

Regards,

Andy

[aosborn]

Originally Posted by lakeroadster

Do have personal experience doing this? Can you show us a working example of a leaf spring suspension with a panhard bar?

Yes, I have installed Panhard bars in several cars with Hotchkiss suspension. I have done it for the reasons I and others have stated in prior posts, to reduce lateral deflection and reduce or solve tire clearance issues. I used to have some photos posted on this site, but they seem to be missing and I think there was an example there. Most recently I built a 35 Ford 2 door sedan that had a Chassis Engineering leaf spring kit and I installed a custom Panhard bar just to tighten things up. It wasn't a road racer by any means, but adding the bar just makes the car more precise and tight without losing the ride. I'll see if I can come up with some photos.

Regards,

Andy

[MARTINSR]

Interesting stuff, as noted, gets more complicated than one thinks!

On the flat spring, how about a Ford 4x4 which has a reverse arch spring on the front? How the heck does that work?



Here's one out of the truck, it's almost flat with no weight on it! What is the thinking engineering wise with this?

Brian

[aosborn]

Originally Posted by 1971BB427

Spring twist is more about length, and height of the vehicle, as the height gives the vehicle more leverage on the spring. In some cases the greater arc is used to raise the vehicle's height, so it will then put more leverage on the spring, but not if the spring is mounted in such a way as to keep the weight lower. That's why many makers mount springs alongside the frame when large stacks of higher arc springs are used. It lowers the center of gravity, and keeps the vehicle's weight from working against the spring stack.
Some arc will give the spring more lateral support, but too much will indeed cause more twist, as there's just no way to keep the force against it low enough. Plus the spring needs to get longer as it's arc'd if it still has the same mounting points. Keep the length the same, and put some curve in it, and the spring will have a bit more resistance to twisting due to the curved shape.
You can test this with small spring material and a tension gauge. A flat piece of spring stock is easily twisted when gripped at the ends, but bend it and then try to twist it, and the tension will be greater. I've seen test sheets from the local spring shop where they use something similar to a big torque wrench to do these tests, and accurately measure spring twist.

I am not saying that the spring can't be twisted within its length when not installed in a vehicle. If you anchor it solidly at one end and twist the opposite, it most certainly will twist. And what you say about the arc increasing that effect is also very true.

What I am saying is that when installed in a vehicle and the spring is mounted at the frame on one end (in a rubber bushing), solidly tied to the axle near the center, and then tied to a shackle (in rubber or poly bushings) at the opposite end, you would have to rotate quite a distance before any spring twist would come into effect. All bushings would have to be fully compressed before the spring itself would begin to twist.

Then tie that same axle with one leaf spring mounted to another leaf spring on the other side of the vehicle, there is no amount of twist between those two springs (which are now tied together) that will be a factor in allowing the body to roll about the suspension. The rate of twist in that configuration would be massive and the shocks would not be able to control it. That is why I say that the ability for the body to roll about the suspension of a leaf spring suspended car is due largely to the arc of the spring and the bushings. The angle of the springs while mounted in the car (in side view) is the last major determining factor in allowing the body to roll.

An interesting way to test this would be to set the car on jackstands with the suspension loaded on three corners. Remove the leaf spring on one side of the rear axle, and with a floor jack under the end of the axle with no spring and move it up and down. That way you could easily see where the deflection is and also determine how much travel you have before the axle binds in rotation. If you were real ambitious, you could test with rubber bushings, then poly and then aluminum or brass, and see the results. My guess is that it would be a significant difference in axle travel between compliant and non-compliant bushings. If you did all aluminum bushings in both the front eye and the shackle, I would bet that you could hang the rear end nearly level with no support on the opposite side. I might try that the next time I have the opportunity.

(Warning!! I am now going a little off topic) To take this to the next level, try the same test with a parallel four link suspension with a track locator vs a longer and more lateral Panhard bar and see what happens. Or even a parallel 4 link with no lateral locator attached and see how much travel you have. After doing that you will likely see why I and many other folks that design and build suspensions prefer at least a triangulated 4 link for a car that has to turn, or my personal favorite, a 3 link rear suspension with a panhard for a live axle car. (Sorry, no more drifting off topic, I promise)

Regards,

Andy

[aosborn]

Originally Posted by MARTINSR

Interesting stuff, as noted, gets more complicated than one thinks!

On the flat spring, how about a Ford 4x4 which has a reverse arch spring on the front? How the heck does that work?



Here's one out of the truck, it's almost flat with no weight on it! What is the thinking engineering wise with this?

Brian

I would guess with the reverse arch on the front spring, it goes along with what I was saying about roll understeer in turns.

With the other example, it is "nearly" flat, but not quite. You would have to see it installed to find out the whole story. Another explanation would be that is simply a very stiff spring for the application and it doesn't deflect much.

[NEW INTERIORS]

Originally Posted by aosborn

Herb Adams in his book Chassis Engineering points out that since leaf springs are so stiff as to not need a lateral locating device such as a panhard bar (for a race car),

Not to sure I'm following you.. You say it will help, then you post something like this....

[1971BB427]

I'm sort of confused too. If you say a spring can only twist slightly, and is held ridgid by the spring perch and the mounts at each eye, then it seems your panhard bar would be of little affect.
And then the quote from Herb Adams about not needing a panhard bar confuses even further?

[aosborn]

Let me try to clear this up. Herb Adams is talking about a race car where rubber bushings are taboo because any compliance in the suspension cannot be easily controlled or have predictable behavior. Consequently, hard or solid bushings are used in the shackles, and as Herb points out, he recommends a spherical bearing on the forward eye to allow rotation. With that scenario, it is his opinion that a lateral locating device wouldn't usually be necessary. So far so good...

With a street car, those hard bushings and spherical bearings would transfer too much noise and vibration, not to mention that maintenance on those type of joints would not be practical. That is why OEM's use rubber. They want comfort first and performance second.

With rubber bushings comes compliance, and that can be okay until your clearances get tight (tire to fender for example) or you put on tires with more grip. You do all recall I'm sure how crappy tires were back in the late 1960's? With more grip comes larger forces and then you come up upon the weakest (most compliant) part of the suspension, the rubber bushings.

As we have covered very thoroughly I think, some rubber bushings are necessary, and some bushings can be upgraded for more precise control of the suspension. One can assume that most of the lateral movement in a leaf spring suspension comes from the rubber bushings in the shackles. It would also stand to reason that the longer the shackles, the more lateral movement. If you just had to install those trick polyurethane bushings, put them in the shackles and not in the front spring eye.

All I am saying is that if you want noise and vibration isolation AND more precise axle lateral locating, you can use a Panhard bar... The Panhard bar can also help to lower the roll center of the rear axle, but that is a whole other story...

Hope this helps make it more clear.

[NEW INTERIORS]

So when you start out to get max performance and handling out of your car,, Aren't you looking to make it a disguise race car for the street.. Or something like that.. When your trying to get every little bit of handling out of your car,, (And this is very little extra, If any) your not out to build a family car..

I had a feeling you would throw in RACE CAR... I don't think he's asking to make it better to go to the store..

[aosborn]

Originally Posted by NEW INTERIORS

So when you start out to get max performance and handling out of your car,, Aren't you looking to make it a disguise race car for the street.. Or something like that.. When your trying to get every little bit of handling out of your car,, (And this is very little extra, If any) your not out to build a family car..

I had a feeling you would throw in RACE CAR... I don't think he's asking to make it better to go to the store..

There are race cars, and there are street cars. If you want some of the benefits of a race car (better handling, more power etc) you couldn't do it without looking to the racing world for answers. You just need to tone it down a bit for the technology to work on the street. You should know all about this NEW INTERIORS. Take for example the Pro-Street cars you like to build. It is all race car technology toned down for the street.

I'm sure your cars go well in a straight line and do fine around town. As fast as you seem to like to go in a straight line, I like to go that fast in around corners. I drag raced for years and then I went to road racing school. Then I traded my 11 second 57 Chevy for a 455cid, 4 speed 76 Trans-am and have never looked back. Instead of having fun 11 seconds at a time a few times an afternoon, you can have fun for hours! But as you like to say "that's just me".

To each his own, that is what makes the world an interesting place.

[MARTINSR]

Originally Posted by aosborn

There are race cars, and there are street cars. If you want some of the benefits of a race car (better handling, more power etc) you couldn't do it without looking to the racing world for answers. You just need to tone it down a bit for the technology to work on the street. You should know all about this NEW INTERIORS. Take for example the Pro-Street cars you like to build. It is all race car technology toned down for the street.

I'm sure your cars go well in a straight line and do fine around town. As fast as you seem to like to go in a straight line, I like to go that fast in around corners. I drag raced for years and then I went to road racing school. Then I traded my 11 second 57 Chevy for a 455cid, 4 speed 76 Trans-am and have never looked back. Instead of having fun 11 seconds at a time a few times an afternoon, you can have fun for hours! But as you like to say "that's just me".

To each his own, that is what makes the world an interesting place.

I have to tell you I got a little bit into the road racing stuff just a little, when to Jim Rice cart racing school and that opened my eyes to how much I didn't know (it also means I know NOTHING still being I learned so much in an day at that school) but really, my Gran Sport can hang pretty good all because of some handling improvements sway bars, tires, the right shocks, quick ratio box, etc. It is fun that is for sure.

Brian

[NEW INTERIORS]

Originally Posted by aosborn

There are race cars, and there are street cars. If you want some of the benefits of a race car (better handling, more power etc) you couldn't do it without looking to the racing world for answers. You just need to tone it down a bit for the technology to work on the street. You should know all about this NEW INTERIORS. Take for example the Pro-Street cars you like to build. It is all race car technology toned down for the street.

I'm sure your cars go well in a straight line and do fine around town. As fast as you seem to like to go in a straight line, I like to go that fast in around corners. I drag raced for years and then I went to road racing school. Then I traded my 11 second 57 Chevy for a 455cid, 4 speed 76 Trans-am and have never looked back. Instead of having fun 11 seconds at a time a few times an afternoon, you can have fun for hours! But as you like to say "that's just me".

To each his own, that is what makes the world an interesting place.

When I was younger,, I grew up running some hard corners in my 73 Camaro.. Very bad road with a lot of cornering... Fun to run but at that time I was young and dumb.. I do hear where your coming from and do know you know... I just didn't see where it would make that much difference.... I know my 73 would handle very nice in some very tight cornering... The 69's much be way different then the 73's was.. I didn't see where it needed to be done on mine anyway...

[1971BB427]

Never got into Autocross, but it is fun to watch. I built my '71 Camaro to be street driven, but handle well, and it does whatever I want, and usually more. I find I chicken out before the car has handling issues.
We like to take a drive in the summer up the windy Columbia River Gorge scenic highway, and push through the 10-20 mph curves as hard as our pucker factor will allow.

[MARTINSR]

My test for cars was an exit off the freeway in my town that I took to go home. This exit is now gone, changed to new version of freeway exits (isn't that amazing how they are changing them all?).
Anyway this was one of those tight hairpin like corners that goes off to the right , up the hill of the overpass then back over the top of the overpass.

I use to take that exit off the freeway at 50+ keeping it at 50 all the way to the top of the corner at the overpass. That sucker really handles pretty good. Blown away a lot of high end sporty types I'll tell you that, hit a corner with them on your bumper and when you come out they are way back there, then with the good old Nailhead to the floor, you are gone. Fun stuff, it sort of replaced the stop light drag races I had as a kid all the time which of course are now long gone and I don't want them back actually. That was stupid as crap. But to hit a corner out somewhere that no one is at and all you hurt is you? What the heck, it's a good ride. And yes, I have hurt me. In that Gran Sport hitting a freeway entrance too hard, I apexed the corner hitting it hard and was over my head and blew over the curb with my new Goodyears and Roadwheels, bent the left front wheel all to heck, along with the tie rod end. As I was sliding sideways across the dirt I pushed the right rear tire right off the rim. I changed the tire on the side of the freeway and still made my date. Yep, a lot better than draggin down the street safety for others concerned.

I dig the autocross at the Goodguys events but it is a little slow for me, just a little faster would be a lot more entertaining but it still is damn fun to watch. I don't plan on getting out there, those guys are way too serious for me I wouldn't stand a chance.

Brian