1968 Volvo project Pure Volvocity by 68EFIvert

14 minute(s) of a 519 minute read

With the help of a friend, I spent the better part of last weekend measuring the suspension of my car. That was a pretty involved process but I feel good about the measurements I took. I will the details down to sutton-racetechnology.com/ for a complete analysis and recommendations for adjustments if needed. They will also provide me optimal spring rates, sway bar sizing, custom tuning for my shocks (secret sauce as he puts it.) and alignment specs to get my car setup. I am pretty excited to see the results and I will share them when I do. It should put some closure to some of the questions out there.

the car is now going back to the body shop to finish up the metal and paint work. I hope that part is done this fall.

8-2-2016

I have a few updates but no pictures. The body work continues. Due to the fact that my car is about 5" wider in the back than stock I will be widening my rear valance I purchased from John Royal. I will also be making some minor changes to the design so that the fins are more aggressive. The front is starting to take shape. I will wait to tell the details of that until I decide if I like it or not. It is a pretty innovative solution to the problem and should look pretty cool. The valance will extend down quite a bit from stock. It will incorporate brake cooling ducts, as well as a lower opening for the oil and transmission coolers.

I heard a little bit about the suspension analysis and it looks like I will be making some changes to the rear of the car. I am not sure about the front yet as he is still crunching numbers. What I have learned is that the original design while works fine for low hp 4 cylinder engines will not effectively plant the torque of a V8. The upper links need to be at least as high above the axle as the lower links are below. My design, and I think the original design, has the upper links slightly above axle centerline. To make a 4 link work to its fullest I would loose the back seat. I am not willing to do that.

The plan at this time is to eliminate the upper bars and to add a torque arm to the suspension. The rest of the suspension can stay but I may change out the mounting points of the panhard bar so that it is adjustable on both sides. I will confirm with Ron Sutton before going to far with that aspect first. The torque arm will do a much better job of planting the tires from a start as well as exiting from corners. It will also allow me to add more rear brake bias which will help me get the most out of my 14" brakes. I don't think the torque arm will be very difficult to install and am looking forward to doing it. I will have to wait for that part of the build until it comes home from the body shop.

Here is a snapshot of some of the support that Ron has given me regarding the rear suspension. I really like how he explains everything in detail. He really knows his stuff!


There are four common types of rear suspensions utilized in Pro-Touring cars today ... 3-link, Parallel 4-link, Triangulated 4-link & Torque Arm. Ladder bars should not be seriously considered for any corner carving car, as they go into instant bind with body roll & offer practically no articulation.

The typical Torque Arm suspension is similar to a 3-link, using two lower trailing arms (or "control arms") ... but instead of the third link being on top (centered or offset) & pivoting ... it mounts solidly to the housing & extends quite far forward (closer to the center of the wheelbase) with its 3rd pivot point.

Assuming each type of rear suspension is set-up correctly, rod ends spaced away from brackets properly with high misalignment bushings & clocked correctly ... the 3-link & Torque Arm suspensions allow the rear axle to articulate more (roll angle in relation to frame) than the 4-links.

They all will bind at some point of articulation. The Parallel 4-link allows the least articulation before bind ... the Triangulated 4-link allows a little more articulation before bind ... and the 3-link & Torque Arm offer quite a bit more articulation before bind ... all things being equal.

A triangulated 4-link is simple, and fairly common as a factory style rear suspension in many cars. It could be argued it will handle more torque under hard launches than 3-links, but if you were going to drag race it with slicks, you would want a Parallel 4-link, not a triangulated 4-link. You want the push & pull forces going through the links to be parallel with the chassis … not angled within the chassis.

Torque Arm suspensions are also common as a factory style rear suspension in some cars. They are the simplest of the designs, allow a high degree of rear end articulation & can take high shock loads from hard launches. They can be made "a little" adjustable, but typically offer less adjustability than the other designs, as far as controlling the front Instant Center, rise leverage & anti-squat. If designed well & installed as instructed, these make a great all around suspension for the person that doesn't want to tune much.

3-links are very common in road racing, especially in full body cars like GT1 & the Trans Am series, because they allow for the most articulation & can be highly adjustable & tunable for track conditions. You also see them a lot on top AutoX racers.

3-links, Parallel 4-links & Triangulated 4-links can be made very adjustable if designed & installed with multiple or variable mounting points. But most "street kits" are sold with little or no adjustment to protect non-tuning novices from themselves. If you know set-ups or plan to learn, you may want to pick a system designed for adjustability. If not, pick a system designed for your application, install as directed & run it.

3-links can handle drag racing up to a point, but it wouldn't be my choice if the car was planned for super high hp, high rpm, clutch dropping, slick running, wheelie pulling launches ... as there are only 2 rod ends "pulling" through the top link to lift the whole car. 4-links can handle more launch load (like drag racing), because the force going through the rear end & rear suspension that "pulls" the top links(s) is spread over 4 rod ends.

Parallel 4-links, 3-links & Torque Arm suspensions require a device to keep the rear end centered in the chassis, like a panhard bar or watts link. A triangulated 4-link does not require this, as the 2 or 4 links running at an angle keep the rear end in the location you put it. There are pros & cons both ways.

A suspension with a panhard bar or watts linkage ... "can" allow for easy roll center changes, if the mounting brackets allow for adjustment. (Many ProTouring focused kits offered do not have adjustment capabilities) Again, decide if you want to "set it & forget it" (sorry Ron Popiel) or if you want a suspension that is tunable for optimum performance & varying track conditions.

For a track car, I can’t fathom not having an adjustable rear suspension. But if your PT car doesn’t have a rear suspension, that doesn’t mean you shouldn’t do track days. Rock on & have fun. Just know if the rear needs to be loosened or tightened up, to balance out the car for neutral handling, you have less tuning options.

For the best adjustable rear suspension for road racing, track car, or AutoX car, is the adjustable 3-link, as it has the best articulation. The adjustable parallel 4-link will work well as long as the car doesn’t require a high degree of roll angle for the suspension to work, but it's not my weapon of choice.

For the best non-adjustable rear suspension for road racing, track car, or AutoX car, is the Torque Arm suspension, as it offers good articulation. The non-adjustable 3-link & parallel 4-link “can work well” … providing the instant center location provides a decent anti-squat percentage for your application. Triangulated 4-links are rarely adjustable, but still ranks at the bottom as the push pull forces aren’t parallel with the car (which is desired) and the roll center is not separately adjustable, as it does not use a panhard bar or Watt’s link.

For drag racing, the advantage goes to the Parallel 4-link, with the Torque Arm suspension 2nd (for handling launches BUT not very adjustable), Triangulated 4-link 3rd & the 3-link 4th (only due to strength concerns).

For a "cruiser/driver" that will only occasionally see the track, with little or no tuning ... any of them will work fine ... but the Torque Arm suspension is best here & the triangulated 4-link 2nd ... which is why you commonly see these two suspensions in factory production cars. They both work fine in many hot rod & street performance applications. They are not better than the others, just simpler & effective. Plus these two allow you to keep the rear seat if that is important to you.

Offset 3-links

The rear end housing wants to rotate the same direction the driveshaft is … counter clockwise from the rear view, clockwise from a front view. So as torque is applied the left rear tire is loaded more & the right rear tire is loaded less. This makes the car want to “drive” to the right, a small amount, under hard acceleration. As you make left hand turns the car has more “forward bite” during corner exit … than right hand turns, which have less “forward bite” during corner exit.

The difference isn’t huge, but it exists. If it isn’t counteracted … the effect amplifies with increased power output.

For 3-links, the upper link can be offset to the passenger side to help counteract this torque on acceleration. Very few people can tell you accurately how far to offset it, because it changes with gear ratio & friction within your rear end. I have my own proprietary formulas I use, based on my knowledge of where the force comes from & high tech testing of dynamic loads. This allows me to calculate the amount of force difference from the left rear to right rear tire & offset the top link precisely to zero out any torque steer. I don't share this formula publicly, but I do offer this service as one of my 70+ tech services you can see HERE.

The formulas I’ve seen other people use involve rear steer, which makes no sense for handling cars. A rule of thumb is 8-12% of track width. In many race applications, it makes sense to make the top link mounts wide, so you can adjust the top link side to side to dial this in. Sometimes in the real world, packaging challenges play a role.

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Second, this is from farther down on the 1st page ...

First, calculate the max torque going through the rear end:
Peak Engine torque x 1st gear ratio x rear gear ratio = max torque going through the rear end
EX: 500# of torque x 2.64 1st gear ratio x 3.90 rear gear ratio = 5148# torque going through the rear end

Second is torque distribution:
(Distance from Axle Centerline (CL) to lower link rod end CL) vs (Distance from Axle Centerline (CL) to upper link rod end CL) = torque distribution

EX #1: 6" Axle CL to lower link CL vs 6" Axle CL to upper link CL = 50/50 torque distribution
This means 50% of the rotational housing torque is going through the lower links pushing the car/chassis forward & 50% of the rotational housing torque is going through the upper links pulling/lifting the chassis & loading the tires.

EX #2: 7" Axle CL to lower link CL vs 3" Axle CL to upper link CL = 70/30 torque distribution
This means 70% of the rotational housing torque is going through the lower links pushing the car/chassis forward & 30% of the rotational housing torque is going through the upper links pulling/lifting the chassis & loading the tires.

EX #3: 6" Axle CL to lower link CL vs 12" Axle CL to upper link CL = 33.3/66.7 torque distribution
This means 33.3% of the rotational housing torque is going through the lower links pushing the car/chassis forward & 66.7% of the rotational housing torque is going through the upper links pulling/lifting the chassis & loading the tires.

Third is max torque with torque distribution:
So if we use the 3 torque distribution examples above & the example 5148# torque going through the rear end, the max torque going through the brackets would be:
EX #1: 5148# with 50/50 torque distribution = 2574# max torque through lower brackets & 2574# max torque through upper brackets

EX #2: 5148# with 70/30 torque distribution = 3604# max torque through lower brackets & 1544# max torque through upper brackets

EX #3: 5148# with 33.3/66.7 torque distribution = 1716# max torque through lower brackets & 3452# max torque through upper brackets

Realize these brackets only see this amount of torque for an instant ... and these amounts are based on no tire spin, as with a racing slick or optimized suspension. The rotational housing torque diminishes rapidly, as the car gets rolling. The brief instant at initial power application is the only time the brackets will see these load numbers.

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OK, after that primer ...

There is a lot more than just the anti-squat percentage setting in setting up a rear suspension. The torque distribution discussed above is KEY! Remember the torque distribution that is lifting the chassis & loading the rear tires is one key ... a big key ... to how much traction we have for acceleration.

Torque arms & IRS always have 50/50 torque distribution. Is 50/50 optimum for every car & usage? Well ... there is no set-up that is optimum for every car & usage. The tire design plays the biggest factor in how much torque distribution we want loading the rear tires. The power level of the car is the 2nd factor. As a rule of thumb ...
* Stiff sidewall and/or harder tire compounds (like PT tires) need to be loaded harder for maximum grip.
* Soft sidewall drag slicks and/or super soft rubber compounds need to loaded less ... or we could experience tire shake (like top level drag cars do).

Because 3-Links & 4-Links can be designed with the top links & lower links mounted at DIFFERENT distances from the rear axle centerline ... the torque distribution is variable. At least until the brackets are built & welded on. I find that space above the rear axle housing in production cars ... or lack of it ... prevents us from placing the top link mounts where they need to be for optimum torque distribution.

Refer to example #3 up above in the torque distribution discussion. If we could mount your top links higher ... farther away from the rear axle centerline that your lower links ... are below that same centerline ... you'd have MORE than 50/50 torque distribution. That would be good. But if your top links are closer to the rear axle center line than your lower links ... you'll have LESS than 50/50 torque distribution. Not good.

Since ... in this case Darreld ... I have your measurements & know your top links are super low ... I can confidently advise you that your current 4-link design will not produce anywhere near optimum grip on acceleration. Your torque distribution loading the tires is far less than 50/50. Soooo ... a torque arm rear suspension with its inherent 50/50 torque distribution would be a major improvement in grip on acceleration. Not as much as if you cut out the trunk floor & placed your top link mounts 12"+ above the rear axle centerline (extreme race setup) ... but far better than you will have with the current set-up.