Suspension Mythbusting: Solid Axle vs. IRS

Same thing with upping the diameter of the Anti-Roll-Bars.  You are effectively increasing the spring rate on that side by transferring the load to the other side, reducing body roll, reducing the camber change on the outside wheel (with respect to the ground).

mad_machine said:

In reply to clshore :

basically you are talking about a "twist beam" as used by many FWD cars, but as a RWD DeDion axle.

 Somewhat similar, but with significantly different design goals and implementation:

The goal of the twist beam is minimal production cost, along with packaging that achieves minimal intrusion into the passenger/cargo space.

Wheel camber and toe are managed by a beam having minimal cross section, wheel 'caster' and fore/aft location is managed by a single

trailing link on each side, rigidly attached to the the hubs. Lateral wheel location is provided by bending in the joint between the trailing link

on each side where they join the beam.

This imparts significant bending loads that require both the beam and links to be somewhat heavy, along with the joints between them.

The goal of the Floppy-D is to achieve the camber behavior of a Live axle (and DeDion), combined with unsprung weight comparable to IRS.

Care is taken to avoid stressing major components in bending, instead resolving the forces by triangulation, and employing multiple links

with spherical joints to directly take the suspension loads into the chassis, as in IRS design, to save unsprung weight.

Cost and packaging are definitely secondary issues for Floppy-D.

Here's a question: Have any production cars employed 'twist beam' on a driven rear axle?

I know that there have been AWD versions of some FWD econoboxes that employed 'twist beam' rear suspension. 

I put a 3 link modified 8.8 in the Rice Rod because that's what another team was selling for $50 at the $2016 Challenge flea market.

My $2018 Challenge car will probably have IRS because that's what the donor car comes with. 

Neither are running now or in the forseeable future, so I don't have much to say on that 

Dr. Hess said:

Also, that whole driveline torque loading up the suspension -- more pressure on the contact patch thing, you are forgetting about the no free lunch.  That more pressure on, say, the left contact patch comes at the price of less pressure on the right contact patch. 

It's the opposite in my experience.  The anti squat equalizes the forces on the tires.  If one tire is "light", accelerating will push it into the ground.

The effect over rough roads is that the car stabilizes when you accelerate.  Sounds like what people describe 911s as being like, actually   Accelerate out of problems and don't dare lift.

clshore said:Here's a question: Have any production cars employed 'twist beam' on a driven rear axle?

I know that there have been AWD versions of some FWD econoboxes that employed 'twist beam' rear suspension. 

Smart cars have a transverse engine nestled inside a twist beam.

Blew my mind the first time I saw one.

Perversely, when VW converted some of their cars to AWD in the 1980s, they repurposed some Super Beetle parts to make a trailing arm setup.  From a handling standpoint, trailing arms have even more toe-loss issues than twist beams do, and VW invented "kinematics" in part to alleviate that problem with twist beams.  (Those funky ramped bushings that they used to use?  Those will yaw the beam under suspension load to negate some of the load induced toe out)  Nose heavy AWD cars need all the help they can get with rotation though so this is fine.  Makes them handle a bit spooky when there's 600lb of stuff in the back though.

In reply to Knurled. :

I'm not sure if what you are experiencing can be attributed to anti-squat or weight transfer.  And, with an IRS, you would likely have less problems over the rough roads to start with.

Has the same problems as a live axle in regards to the inside tire upsetting the outside.

I drew that up once. If you nail a bump big enough that camber of the opposite wheel is an issue, that end of the car is probably going to have daylight under both tires anyway.

A 2-3" bump at 72" is about 2 degrees, isn't it?  a 2 degree sudden (really sudden) camber change can't be a good thing for a tire/road interface.  If your starting camber was 0, adding 1.5 or 2 degrees positive is going to put you in the less grip part of your curve.  If you're starting at -1, you might get away with it.  Anyway, it may be more of the upset/sudden change/vibration in the outside tire than just the raw angle change and I fully admit not being an expert, just a student.

Lapping Hallett in the Esprit (RIP), I used to clip those bumps with the inside wheels and not really even notice them.  Not the turtles, but the bumps on the turns.  Can you do that with a solid axle?

In reply to maschinenbau :

I think the Suzuki SX4 uses a twist beam in the rear. 

I'm really curious how the Novak F500/F-Mod cars work in the rear suspension area. I totally understand how the elastomer pucks are both spring and damper but the four link in addition to the swingarm deal for lateral location seems like it would be well overconstrained. I get that the car has only maybe 2" of travel at most but I still think any bind is bad.

EDIT: On second thought maybe the key to this is the birdcage bearing deals on the axle. The axle mounts would then be able to twist relative to the other side unlike a conventional solid axle. Too many variable to run through my brain!

Dr. Hess said:

In reply to Knurled. :

I'm not sure if what you are experiencing can be attributed to anti-squat or weight transfer.  And, with an IRS, you would likely have less problems over the rough roads to start with.

We're not talking about accelerations hard enough for weight transfer to matter much.

Of course, it is tricky to separate because the anti squat allows you to generate more grip with allows you to accelerate harder which creates more weight transfer to create more grip with.  But you need that initial push.

I will allow that IRSs tend to be a lot heavier than a solid axle, which increases the amount of weight over the drive wheels...

Back in my old drag racing days, we tweaked the rear suspension to create lift, as in the body pushing the rear axle down when the body went up as opposed to body squatting and taking weight off the rear axle. Mopar drag cars had leaf springs in the rear, and simply clamping each leaf to prevent spring wrap changed the dynamic from squat to lift.. Coil spring rear suspension required a bit more engineering with bars. Of course, that was nearly 40 years ago, and I'm sure the ideology and engineering has changed dramatically...

I ran two race cars with the same power unit on the same tracks.  To maintain adhesion on the medium bumpy track surfaces we had, I needed to use an LSD on the live axle car, while the IRS car made do without that and did the same sort of cornering speeds.

Short answer - smooth track, not much difference.  Bumpy track, advantage IRS.

Both my modern sports cars have IRS; none of my 4 vintage sports cars have IRS.

I always found solid axle cars to be a lot easier to initiate sliding in the rear, but as a consequence stability in corners, especially when bumps were involved, was reduced.

That said, all my cars now have IRS, and I don't intend to go back to a live axle, unless I ever build a drag car. Even then, the new Dodge Demon runs an IRS, so it can't be that bad in the drag racing world...

I learned some things reading this thread. Explains why swapping from a Ford 7.5 to an 8.8 made the rear of my car ride worse. Everything's the exact same aside from 70lbs of extra unsprung weight. 

Dr. Hess said:

A 2-3" bump at 72" is about 2 degrees, isn't it?  

A 2-3" bump is a massive upset to the chassis no matter what the suspension type is.  At that point, at speed, damper tuning is more important to roadholding than mere camber figures. 

Smooth inputs need to happen at both ends of the suspension.

For reference, a 1" high bump at speed will make you feel like stopping to make sure all the tires still have air in them...

Knurled. said:

Excessive unsprung weight is still excessive unsprung weight.

 

That is why I went to moderately extreme lengths to try to get a 9" as light as the Mazda 7" that I am taking out.  I still need to find a scale but I am pretty sure the Mazda rear is 160lb with brakes.  I still have a couple places to shed weight but I already hit all of the cheap ones.

 

And why I want to build a Ford 7.5" for the RX-3.  Them suckers is light!

Just to complete this thought.

Old and busted on the right:  140lb as you see it.

New hotness on the left: 181lb as you see it.

Mind, neither rear has brakes, and the calipers for the 9" (VW/Lucas aluminum units) are waaaay lighter than the Mazda GSL-SE unobtainium, and the "GSL-SE" rear has a light Kia open diff instead of a limited slip, and the 9" has an un-lightened 5.43 that may get changed out for a lightened gearset in the mid-4s range. 

But the point is, the 9" parts should be more or less immortal, while the lighter Mazda bits were "I hope it lasts long enough to get me home so I can scrounge up some more stuff next weekend on car-part.com".

I think I have the only road race car (sedan) with an old Halibrand quick change that has a Jacobs Ladder. It has tremendous unsprung weight. The empty magnesium center section was heavier than I expected when we had it apart. Then add the shafts, spur gears, R&P, bearings(lbs and $s) and 1930s Ford truck axle tubes.I use curbs and rumble strips have gone over/outside the strips with no adverse issues.

The axle shafts started life with a 5 on 4.25" bolt circle.
 The splined end is the same as Volvo but the bearings
are much smaller. These were from some production car from the 60s or 70s but I don't know what.

Most of what I've gleaned from midget and sprint car sources isn't applicable or isn't quite right.
Some say the Jacobs Ladder is on the right for turning left. My car turns equally left and right in
the dry. Once long ago it wouldn't turn in the wet.

I wish I had the free track time for testing that I had 20 years ago when I did a lot of instructing.

I'm sure I can change the rear roll center with adjustments on the Jacobs Ladder. Exactly what does what I don't know. From what I've learned the way it's set up now the roll center is somewhere next door. The "straps"/ 2 tubes with heim joints on each end are close to parallel instead of a "V" pointing to the roll center.

I can drive it the way I have it now and have fun. I'm even surprised at some of the cars I've passed in corners with it.

In reply to Knurled. :

How the heck did you get a 9" that light!?

rslifkin said:

In reply to Knurled. :

How the heck did you get a 9" that light!?

Aluminum pumpkin.  The butt dyno equivalent of the scale says that the Yukon aluminum pumpkin with an aluminum Daytona-type pinion housing, WITH a 5.43 (heavy!) and unknown brand NASCAR takeout Detroit locker (light!) weighs as much as a bare iron 9" pumpkin and pinion housing.

It hurt to spend the money.  That hurt only lasts as long as it takes to pay for it.  The hurt of having a really heavy rearend lasts for years.

Knurled. said: The hurt of having a really heavy rearend lasts for years.

Giggle giggle

So what are some donor cars with really light rear axles? I've toyed around with the idea of putting on a four link system on the MGB but I also wanted to get rid of rear unsprung weight. That factory rear axle is seriously heavy for a car with less than 100hp.

In reply to OJR :

Keep in Mind not only is the QC Much heavier it also requires 50Hp more to turn the gears w/more Drag.

GTXVette said:

Keep in Mind not only is the QC Much heavier it also requires 50Hp more to turn the gears w/more Drag.

So a 49 hp engine can't even get the axle to turn? 

The spur gears are extra drag. It has a lot more ($s) bearings to add drag than a normal R & P but I'm sure not 50hp.

I can turn it by the driveshaft with one hand!

I didn't design, plan or build this. I'm just stubborn enough to keep it as it was. I have a copy of a grid sheet from 1982 that could have been it's last race before I got it in kit form in 1998.

Long ago the " something competition" or "Competition Something" shop (in CT) that built Paul Newmans first 510 built a different make sedan with this setup. Then Al bought that car and wrecked it in his first race. Then he built this car using many of the unusual for an SCCA sedan parts from the wreck.

The photo shows the Jacobs Ladder and the holes for adjustments.