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freetors
freetors Reader
4/10/18 7:39 a.m.

Looks like the theory to that is basically the same as a scrunched up panhard bar with a lot more rod ends!

OJR
OJR New Reader
4/10/18 9:33 a.m.
freetors said:

Looks like the theory to that is basically the same as a scrunched up panhard bar with a lot more rod ends!

 Yes it's like a panhard bar but much longer. You add the lengths of all the bits. This gives much less side movement than the arc of a panhard bar. The Jacobs ladder has 5 points with 3 holes each. The roll center can be changed from adjustments here. I'd like to know what does what before I change it more than I have.

 

OJR
OJR New Reader
4/10/18 1:35 p.m.

The rod ends....AARRRGGG!

I have left hand taps I've used on customer and other parts/projects.

All of the rod ends on this car  are right hand thread.

 

Knurled.
Knurled. MegaDork
4/10/18 7:43 p.m.
freetors said:

Looks like the theory to that is basically the same as a scrunched up panhard bar with a lot more rod ends!

 

Or a Watts link that has been folded in on itself.

Knurled.
Knurled. MegaDork
4/10/18 7:54 p.m.
freetors said:

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.

 

The Ford 7.5 is reasonably strong and light enough.  The reason I like it for the RX-3 is the RX-3 has a track width of only 50".  If you get an early Ranger axle, if you cut the (left?) side shorter and use two (right?) side axles, you end up with something in the 52" width range, with junkyardable/Dorman-able parts availability.

 

Alternatively, I also like the idea of grabbing a Trailblazer rearend and having axles made with 9" bearing ends.  Upside:  Magnesium centersection of dubious strength (but good enough for 100hp, eh?)  Downside:  You gotta get custom axles made no matter what, which means money.  Dutchman's pretty cheap at $400-ish for a pair depending on options.  But there's an upside to that:  You can have them made to YOUR specs.  So whatever bolt pattern or brake width dimensions you want.

 

For what it's worth, the Ivan Stewart era Toyota stadium trucks used magnesium case Dana 60 rearends.  4-link mounts on the case were bolted to it rather than welded.  Neat stuff.

Knurled.
Knurled. MegaDork
4/10/18 7:58 p.m.
OJR said:
freetors said:

Looks like the theory to that is basically the same as a scrunched up panhard bar with a lot more rod ends!

 Yes it's like a panhard bar but much longer. You add the lengths of all the bits. This gives much less side movement than the arc of a panhard bar. The Jacobs ladder has 5 points with 3 holes each. The roll center can be changed from adjustments here. I'd like to know what does what before I change it more than I have.

 

It looks to me like what you are doing by adjusting the locating points on the center piece is akin to adjusting instant center on a 4 link, except laterally.

 

The trig nerd part of me is foaming at the mouth regarding the possibilities!

AnthonyGS (Forum Supporter)
AnthonyGS (Forum Supporter) Dork
5/12/21 1:41 p.m.

This car had a solid rear axle.  It was so fast at Daytona and LeMans in the 24 hour races, they changed rules and the tracks (chicanes were added right after this car appeared on the scene).  Seriously, look it up.  The FIA and IMSA hated this car.  It's a good thing the car had transmission problems in many of the races or they would've hated it even more. 

No doubt IRS is better technology, but sometimes simplicity is beautiful too (and fast). 

asphalt_gundam
asphalt_gundam Reader
5/12/21 2:36 p.m.

Reading through all the comments I'm surprised that rear steer wasn't discussed. I'm no expert but rear steer is something that I've been observing and working with in my chassis adjustments. Mostly so far it has been counter steer or neutral. With the relocated chassis side upper arm I will actually get some rear steer.

AnthonyGS (Forum Supporter)
AnthonyGS (Forum Supporter) SuperDork
9/12/21 4:38 p.m.

In reply to asphalt_gundam :

No matter what has been said so far, I really like and appreciate your project!  It's a great example of doing something you enjoy.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
9/12/21 4:47 p.m.

In reply to asphalt_gundam :

Mazda made a running change in the solid axle RX-7s, moving the lower links' front pivot down significantly (20mm?).  This was to create roll understeer.  Chassis roll would cause the axle to steer to the inside, creating a nice safe negative feedback loop.

In practice, the years with this geometry ('84-85) feel less crisp in their handling compared to the early cars.  They also have more wheelhop because of the instant center change, and the geometric bind in the rear suspension got worse, so the '84-85 years were a lot more prone to ripping the upper link mounts off of the body.  It's a four link with parallel lower links and fairly short, angled upper links, and a Watts link, and the roll center described by the upper links is not the same as the one defined by the Watts, so everything binds up with axle articulation...  It worked well enough with the original pivot mounting points and the 18mm rear sway bar to keep it from articulating much.

frenchyd
frenchyd UltimaDork
9/12/21 7:41 p.m.
SkinnyG said:

Before building my Locost (and still now, 12 years later), I waffled back and forth between a stick axle and IRS.

My stick-axle Locost has been on the road, travelled 500km on the Trans Canada to autocross events and back, commuted as a daily for a time, and I've driven it everywhere, rain, shine, snow, even dirt roads (-that- sucked).

I've come to a couple conclusions:

1) If you want comfort, you really wouldn't pick a Lotus 7 or replica thereof.  Comfort is not their forte.

2) If, on the street, the stick axle is a limited factor, you are likely traveling faster than you legally should be going.  If it got you in trouble, it's going to be a big "off."

3) I'm not totally convinced an IRS is going to net a "lighter overall" weight of vehicle, and maximum lightness is what I want.

I don't believe that swapping TO an IRS is going to be worthwhile for -me-. If I was building a project and I had to source an axle, I'd probably go IRS, but the stick works well enough, and if it came with your donor vehicle, or it's already under your project vehicle, it will work fine.

An IRS can be lighter than a "stick" axle. But the right parts need to be selected.  The chief weight loss is the housing. Linkages and springs can be the same.  Plus you can  that the heavy disk and calipers and mount them inboard. Reduce unsprung weight.  

BA5
BA5 Reader
9/13/21 7:06 a.m.

I see a lot of pictures that look like this:

When people talk about a solid axle being lighter than an IRS setup.  But that's misleading.  The comparable setup for a solid axle is really more like this:

I'd certainly grant that a solid axle CAN be lighter than an IRS, but as in all things it comes down to how it's designed.

Plus the difference in unsprung weight (IRS will usually be lighter there) and not transferring motion to the opposite wheel goes a long way.  Sure a solid axle can be made to work if there are limitations on budget, design, rules, etc.  But it's not what I would pick if I were making a clean sheet optimal design.

frenchyd
frenchyd UltimaDork
9/13/21 7:10 a.m.

In reply to BA5 :

Once I start to see solid axles in Formula  1  Indy car etc. I'll agree with the premise. 
     Other wise if the rules limit it to  solid axles I'll be looking at IRS. 

AnthonyGS (Forum Supporter)
AnthonyGS (Forum Supporter) SuperDork
9/13/21 8:30 a.m.

In reply to BA5 :

Yes that top photo is misleading.  That's a 99-04 Cobra IRS which had serious design limitations.  It was designed to fit a solid axle chassis with no modification to the chassis.  This caused the control arms and linkages to be less than optimal.  


Next to it is an 8.8 rear end without the control arms.  It's installation does not match your 2nd drawing.  The 2nd drawing looks like a 3rd or 4th gen F body installation.

Sadly for the sake of this thread, the Mustang 8.8 is probably lighter than the Cobra IRS even with control arms, springs and shocks which are not shown.  

Tons of 99-04 Cobras were solid axle swapped for street / drag use.  

 

 

APEowner
APEowner SuperDork
9/13/21 9:04 a.m.

This thread really highlights the compromises that are inherent in pretty much any design decision.  One of the compromises we haven't' touched on is the evil twin of anti-squat on acceleration which is wheel hop on deceleration.  An extreme example of that can be seen on NASCAR Cup cars. To be fair the truck arm suspension in those is there due to history and the rules not because it's a particularly great suspension but any suspension design that relies on engine torque to push the axle down on acceleration is going to lift the axle on deceleration.

asphalt_gundam
asphalt_gundam Reader
9/13/21 9:24 a.m.

I'd be interested in a very in depth article on solid axles. Even over on the pro touring side of things there's lots of argument about which suspension design is the way to go. 3 link (centered or offset) vs 4 link (parallel or triangulated) vs torque arm vs truck arm and watts link vs panhard bar. All have pros and cons, all can weigh different and vary in complexity. 

I think the only real setback that can't be gotten around in the IRS vs Solid is transfer of bump influence from one wheel to the other. I have a ford 9" rear now and it is much heavier than the stock GM 7.5 was. However...going to an aluminum center, light weight ring and pinion, light weight locker, aluminum daytona pinion support, drilled axles, light weight brakes, and a fabricated housing I've researched to be 45-50lbs of weight savings (still around an estimated 120-130lbs with out arms for an rear axle that can support 700HP+). Half of which is rotating weight. Floater style housings can be built with radius drive axles to have camber and toe. Then comes in the various pros/cons of each individual suspension design. A long arm 3 link and torque arm seem to be the top contenders for a track/auto x type car with the torque arm being easier to package in most cars. The other designs have proven that they can work well too. This is where rear steer comes in as something that can be tuned on a solid axle easily vs an IRS. 

I agree that IRS is the best performance option in a track setting but there are plenty of cars out there that don't have that as an option without massive work in cutting/welding/fabrication. I find it hard to believe that an IRS could be a viable option in the cost/work vs performance gain when starting with a factory solid axle car.

Back to the beginning of my post I would be very curious to see some back to back, real world (track, autox, and street) testing of IRS vs 3 link/panhard vs 3 link watts, vs 4 link pan and watts, vs torque arm in fully prepared scenarios on the same (or as close as possible) car(s). 

asphalt_gundam
asphalt_gundam Reader
9/13/21 9:31 a.m.
APEowner said:

This thread really highlights the compromises that are inherent in pretty much any design decision.  One of the compromises we haven't' touched on is the evil twin of anti-squat on acceleration which is wheel hop on deceleration.  An extreme example of that can be seen on NASCAR Cup cars. To be fair the truck arm suspension in those is there due to history and the rules not because it's a particularly great suspension but any suspension design that relies on engine torque to push the axle down on acceleration is going to lift the axle on deceleration.

One of the cars that got into the wall back at Heartland with Gridlife was a truck arm suspension...he'd told me it was twitchy on the brakes and it went around on him in a decel corner. 

I have my triangulated 4 link set up to load the rear on accel because otherwise the car goes nowhere. On the brakes it does get a little light but in places that help to get some rotation during trail braking. I'd suspect that a torque arm is going to be a whole other feel than either of these.

APEowner
APEowner SuperDork
9/13/21 9:43 a.m.
asphalt_gundam said:
APEowner said:

This thread really highlights the compromises that are inherent in pretty much any design decision.  One of the compromises we haven't' touched on is the evil twin of anti-squat on acceleration which is wheel hop on deceleration.  An extreme example of that can be seen on NASCAR Cup cars. To be fair the truck arm suspension in those is there due to history and the rules not because it's a particularly great suspension but any suspension design that relies on engine torque to push the axle down on acceleration is going to lift the axle on deceleration.

One of the cars that got into the wall back at Heartland with Gridlife was a truck arm suspension...he'd told me it was twitchy on the brakes and it went around on him in a decel corner. 

I have my triangulated 4 link set up to load the rear on accel because otherwise the car goes nowhere. On the brakes it does get a little light but in places that help to get some rotation during trail braking. I'd suspect that a torque arm is going to be a whole other feel than either of these.

As you've discovered it's generally faster overall to dial in the anti-squat on accel and drive around the deccel compromise.  It does help the car rotate on entry and by playing with the brake bias you can adjust how much.  You just have to be sure that your downshifts don't completely unload the rear axle.  That's what usually gets people in trouble.

frenchyd
frenchyd UltimaDork
9/13/21 5:34 p.m.

In reply to APEowner :

For track racing it's generally cheaper to put brake someone than to put accelerate them. 
It's not as ego satisfying  or as simple to just blow by them under power  but it's a heck of a lot harder to comeback past you when you take their line away and require them to slow and wait for you. 
   I've also found guys with more power than others seldom are as good at  figuring out tactics. Then even with the same brakes as you they will not always be in a position to use that extra power.  

alfadriver
alfadriver MegaDork
9/13/21 5:39 p.m.
APEowner said:
asphalt_gundam said:
APEowner said:

This thread really highlights the compromises that are inherent in pretty much any design decision.  One of the compromises we haven't' touched on is the evil twin of anti-squat on acceleration which is wheel hop on deceleration.  An extreme example of that can be seen on NASCAR Cup cars. To be fair the truck arm suspension in those is there due to history and the rules not because it's a particularly great suspension but any suspension design that relies on engine torque to push the axle down on acceleration is going to lift the axle on deceleration.

One of the cars that got into the wall back at Heartland with Gridlife was a truck arm suspension...he'd told me it was twitchy on the brakes and it went around on him in a decel corner. 

I have my triangulated 4 link set up to load the rear on accel because otherwise the car goes nowhere. On the brakes it does get a little light but in places that help to get some rotation during trail braking. I'd suspect that a torque arm is going to be a whole other feel than either of these.

As you've discovered it's generally faster overall to dial in the anti-squat on accel and drive around the deccel compromise.  It does help the car rotate on entry and by playing with the brake bias you can adjust how much.  You just have to be sure that your downshifts don't completely unload the rear axle.  That's what usually gets people in trouble.

For the issue you are covering, design in anti-wedge into the geometry.  The third member will be offset from center, and pointed a little down- but dealing with engine torque is very possible.

AnthonyGS (Forum Supporter)
AnthonyGS (Forum Supporter) SuperDork
9/13/21 6:03 p.m.
asphalt_gundam said:

I'd be interested in a very in depth article on solid axles. 

Back to the beginning of my post I would be very curious to see some back to back, real world (track, autox, and street) testing of IRS vs 3 link/panhard vs 3 link watts, vs 4 link pan and watts, vs torque arm in fully prepared scenarios on the same (or as close as possible) car(s). 

Thanks to Ford and the aftermarket you can actually do this very test in a Fox Mustang from 79-2004.  It'd be a labor of love and cost quite a bit of money though.  If I had a sponsor, I could pull this off.  It'd take quite a bit of time.  At the end of the day though, I think all the solutions for a Fox Mustang would be closer than you think.

The factory quadrabind would be the worst no doubt.  A Griggs or Maximum Motorsports torque arm, panhhard bar, modified 4 or 3 link would be a big leap forward.  There are even Watts links available for the Fox chassis (or were).  A Cobra sourced IRS with MM mods might be a bit better but it'd definitely ride better.  None are optimal though, because Fox Mustang chassis. 

AENfor43
AENfor43 New Reader
9/13/21 6:09 p.m.

Shown is a suspension bushing set for a Nissan 350Z, a rear IRS car engineered with soooo many bushings. YIKES! PROTHANE wants to remind, that with more stock rubber bushings under the race car, the LESS CHANCE of everything keeping in alignment during racing. Alleviate the problem by replacing them with performance urethane ones, which are firmer in the right areas, for better traction. 

Shavarsh
Shavarsh Reader
9/13/21 7:14 p.m.

No weight savings here! Hopefully a more comfortable ride=more driver confidence in this case. We will see!

jerel77494
jerel77494 New Reader
11/1/21 4:27 p.m.

In reply to ncjay :

Very highly developed for the application and those tall sidewalls (and relatively low tire pressures) help.  Someone tried IRS and couldn't afford the developement time.  Also, there was no info to fall back on.  They were completely on their own.

jerel77494
jerel77494 New Reader
11/1/21 4:30 p.m.

In reply to frenchyd :

So msch depends on packaging now.  The front suspension on Formula 1 cars has a high front roll center which is usually undesirable, but what they gain in other areas must offset it.

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