FB Rear Suspension Geometry Problems/Options/Solutions
 

What have others out there done to fix their rear suspension geometry when lowered?
I would (and anyone else reading I'm sure) appreciate more of an answer than "I have a trilink, works great" or similar things. Just looking to have some good info so anyone looking to modify the rear suspension would be well informed and have a good grasp on the issues and solutions everyone has already gone through. First off, I would like to say thanks to everyone who has helped me on my way to understanding the issues/solutions available.
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Basic overview of your factory RX-7 Suspension:
Lowered car, factory 4 link suspension with a watts link.

The lower trailing arms are straight, but the uppers angle inwards from the rear end, and are much shorter than the lowers. This causes a few problems. When you lower the car, the axle will pivot and cause extreme wear to the axle pinion and trans tail shaft. Also, when you enter a corner the axle will pivot at an awkward rate and finally hit a point where it will bind up, normally causing snap oversteer. The Watts link is not centered on the diff and the bars are unequal length. This causes the rear end to act slightly differently on either a left or right turn.

Hyper4mance2k explains this more in my previous thread.
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Some solutions I have read.

PB&J racing mod.
http://www.pbandjracing.com/rear_suspension.html

Adj. lower trailing arms, (or/and upper) which would fix the pinion angle problem on a lowered car, but would not address the watts linkage/ upper link bind issues. Would the driveshaft still plunge into the tranny with only this?
http://mrcmfg.com/catalog/product_in...products_id=67

Gforce Engineering Trilink is a good overall fix. They have done the research and make a great "bolt in" kit.
http://www.gforceengineering.net/products.htm

I've seen some FB's that have custom fabricated a 3-link, most of which go through the floor to make the third link a straight piece. I also see Granny's Speedshop has a torque arm option (though only for the Ford 8.8 retro, I don't see why something like this couldn't be made for the stock RX7 rearend.
http://www.grannysspeedshop.com/ <- click the FB, then Rear Suspension Mods

Does anyone produce/ have designs to do a properly designed 4-link suspension setup? I've never seen one on here. I have seen an adjustable Watts linkage kit.

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The Trilink is the popular choice on this forum, but I'm not convinced that I would want to go from a 4-link car with a Watts (though mazda didn't do a great job.) to a 3-link and Panhard. Is the Trilink popular here because of its actual performance gains, or because it is the most readily available option to fix our problem? (and bolts right in) I'm not knocking the product, hell I've never been in a car with it, I just want some honest feedback.

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My car:

I had another thread about a noise from my rear end, and I know I’m not be the only person having troubles from being low. My car is currently on Full Energy Poly Bushings, Re-Speed Race Coilovers (350/150 rates), camber plates, Stock Swaybars. Im currently running a Chevy Small Block with a T-5 World Class and the Grannys Driveshaft. (I've been considering going back to a 13B; there are a few V8 FB's around, but no 13B cars!) 15x8+0et Wheels. 225/45/15 Hankook. The car is for Auto-x (XP) and is driven hard every time its started. The car is currently lowered about half way down the rear Respeed adjusters (not sure how much that is lower than stock, but I have about 1finger clearance between the tire and stock rear fender. Not sure how I'm gonna fix mine yet.
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Post up any pictures, suggestions, questions, advice, etc. Thanks!

I'm just finishing up a tri-link that retains the watts linkage and uses urethane bushings instead of the heim joints that require maintenance and are noisy. So far the testing is positive, with much more controllable lift throttle oversteer at the limits.

My set-up:
Suspension Techniques springs (1"drop)
Suspension Techniques 1 1/8" front sway bar
Tokico blue shock
Stock rear bar
PB&J rear control arm and watts linkage mods.

Attachment 710916

Attachment 710917

Attachment 710918


Attachment 710919

Attachment 710920

Attachment 710921

This set-up is strong enough for a warmed over rotary, but I would be concerned with a high torque V8, however.
RXDad

Wow -- nice initiative. It's great to see people with the guts to experiment with new setups. How did you zero in on the pinion angle without any adjustability?

I fabed and tacked the center control arm with the 4 link upper control arms in place and the weight of the car on the rear. I then pulled the control arm out and sent it out to be professionally welded (control arm is 4130 chrome molly for strength and has to be carefully tig welded).

RXDad

Of course, no discussion of FB rear suspension mods would be complete without Viking War Hammers epic thread about his custom 4 link and tubbing of his car:
https://www.rx7club.com/1st-generation-specific-1979-1985-18/diy-minitub-4-link-808272/

RXDad

OK -- so you weren't trying to change the geometry to account for lowering -- just fixing the binding issue.

Maybe I just got lucky or something, but everything seems to be working great on my car. I even let others drive it at autocrosses and they have no problems. The car is very low (low enough that I have to drive it up onto wooden blocks before I can get a floor jack under it), but has very stable handling with just a touch of oversteer when needed. I haven't experienced the dreaded snap oversteer since I moved from Racing Beat springs to the Respeed coilovers and lowered the car.

Now, I did take my car in to Sears for a front end alignement last spring before we took it down to North Carolina, and they said that the rear of the car was slightly off set due to being lowered. As a result, they refused to align the car. However, looking at the rear wheels, it must be damn slight because nobody can see any difference between the left/right wheel offset compared to the fender. I ended up aligning it myself in my garage. Worked great, and the car handled like a dream on the Tail of the Dragon.

Like I said, maybe I just got lucky?

I do know some racers that swear by the benefits of the Trilink/panhard setup. I've raced against some with this setup, and they are indeed quick cars. But I haven't installed my own Trilink (which I've owned for a couple of years now) simply because my car handles perfectly already in my opinion. She does everything I ask of her with a great big smile on her face. :)

Either way, I'm sure that Billy will be able to shed some light on the subject, which must be a true problem for most of us because I keep hearing about it from different sources.

Correct, although just by luck there seems to be less variation in pinion angle over the suspension travel with the trilink than with the 4 link. The geometry from the longer upper control arm seems to help.

What are you running as a rear sway bar? Are your spring rates significantly different in the rear compared to the RB springs you were running?

So you're saying that you did have significant snap oversteer, with the RB springs and the coil overs corrected the problem?

RXDad

Correct. While I was on the RB springs (which did not lower my car at all, btw) I actually spun out in an intersection while I had a passenger with me. Not screwing around or anything, just a slight bump in the road and I was doing donuts.

For spring rates, I run a pretty low rate compared to a lot of others (275/150). But I make up with it through ride height and massive swaybars. I've got a Respeed front bar, and the rear is from ISC (set on strongest setting). This setup works great, and I don't rattle my teeth driving around town.

If your suspension is stiff enough (without getting too carried away) the suspension won't be able to move enough to cause any of the problems associated with the poor geometry. If you watch Kentetsu's video's with the RE-Speed setup you'll notice his car hardly rolls, pitches or dives while autocrossing. I think the only issue he may one day experience would be the axle rotation due to the super low stance. This may or may not happen, and would likely take a very serious bump before it did.

mine too. a possible reason is on street tires, it doesnt have enough traction to roll enough to bind.

ive never driven 3 link car, but i did have a panhard, and it made the transition between oversteer/grip way way less snappy.

The reason you're not experiancing problems is because you don't know you're having them yet. Once you make some serious power and get on some R compound tires then you'll really start to feel the issue. I wasn't even good enough of a driver until the end of this season before I had enough seat time to realize exactly was going on with the car. I'm just about at the limit of the factory rear suspension setup. When I'm aproaching the traction limits of the car in very high G turns the car doesnot slowly break lose like a properly designed system does. It snaps out into oversteer, now it is much more controlable then it was previously and it almost seems like it's more gradually reaching the limit, but it's nothing compaired to other cars out there. Try going for a ride in a really well setup car and you will see the drawbacks of our suspension. My bestfriend has a Charged Miata that puts 190 to the wheels and it is amazing how it approaches the limit and progressively inches to the limit of grip before it slowly starts to give way on street tires. That car on street tires drives better than my FB on Kumho 710's. Now all I'm running for suspension is coilovers and illuminas. 300/200 for spring rates corner balanced with a stock front sway bar and no rear bar.

p.s.
Even though it's not a suspension mod. The biggest improvement I have ever done for handling in my FB was installing an Autopower bolt in 4 point roll bar. I could only imagine what a full cage would be like. :drool:
p.p.s.
RXdad: awesome job with the third link! I think I might have to rip you off, but add in adjustability!

Another thing to consider. The reason Mazda went with a watts link over the more simple panhard is because when the suspension travels up and down it keeps the axle centrally located within the chassis. Where as a panhard does not do that. The suspension actually travels in an arch. Now the longer the panhard and flater it is the less teis is an siiue, but it still is one. Now with the Gfrce panhard I believe it only moves about 1/16" left to right, so you'd have to be one hell of a driver to notice that little of movement, but in order to correct this you'd have to design a bad ass Watts link or a Mumford link.
With a mumford link you could theoretically move the roll center below ground. I have no Idea why one would want to, but how cool is that... The old school GT3 cars ran mumfords.

Rad websites:
http://home.earthlink.net/~whshope/
http://www.performanceforums.com/for...#post840424393
\http://www.eng-tips.com/faqs.cfm?fid=768
http://www.fastcarsinc.com/3link.htm

Since you are competing in XP I am assuming that your car is not street legal, correct?

If that is the case then I would recomend that you think out of the box. Forget about fixing the stock suspension because your are not restricted to it. Basically, forget that you have an RX7 and work on locating suspension pickup points that allow you to lower the car, keep the suspension in geomtry, and make it adjustable to compensate for traction conditions.

There are some excellent books like "How to Make Your Car Handle" by Fred Puhn that you should pickup. This is an old book but the concepts are timless and continue to be relevant.

When I competed in C Prepared most of the cars were running some form of a Three Link rear suspension. I had a three link in my Mustang that had a single upper trailing arm connected to the cage and two lower trailing arms. There were several other three link designs and a few four links being used.

I ended up with about 2.5" of clearance between the axle tubes and the "frame-rails" when I finished. The suspension was adjustable for anti-squat and pinion angle. It worked very well, transfering enough weight to pull front wheels 6" off of the ground on corner exit until this was tuned out with rear springs and a sway bar. Eventually, I would have sectioned the frame rails over the axle tubes for more clearance to get the car even lower....all of which is legal in Prepared rules. Anyway - sold the car and bought my RX7....

I think C Prepared is a great place to look for inspiration for your project. Keep in mind that a car with 400-600HP with 12-14" wide slicks needs a really good suspension to get everything to work. In short they are full on racecars that are built without respect to what kind of car they were originally. If you get a chance to attend a National Tour, check out the CP Cars. Seek out guys like Ron Ver Mulm, he runs in EMod now, the man is a genius and he will talk to you.

All I have are some pic's of what the cover looked like inside of the car - nothing under the cover or car, unfortunately.

http://inlinethumb03.webshots.com/46...500x500Q85.jpg

All of the stuff you see that is available commercially for an RX7 is built to work on the street or within the confines of a restrictive rule set. The Tri-Link, for example, works really well on a low HP RX7 that is being autox'd or roadraced in Improved Touring. It is designed the way that it is because of the rules. If it were legal in IT and SCCA Produciton to run a straight control arm through the floor to roll cage structure, that is what you would see.

As others have mentioned the Tri-Link is not a good solution for an RX7 that has higher HP. The E Production cars in this area all started out running Tri-Links but they all found that it was not possible to get off of a corner quickly with that suspension design. In short, they couldn't use all of the power they had as early in corner exit as they needed to.

The solution is a 4 traction bar suspension developed by Charlie Clark at KC Raceware (www.kcraceware.com). This suspension features 4 near equal length traction bars that allow a E Production car to come off a corner at a much higher rate of speed. Contact Charlie if you want more info.

All that said, Charlie's preference for a rear suspension that is NOT hampered by rules restrictions is a 3 link like the one I had on my Mustang.

Hyper: Last year I competed in 4 events while on R-comps. The car handled just like it does on the Sumitomos, I was just able to go a lot faster before the sliding started. And my transitions from traction/sliding are very nice and controllable.

I've had well over a dozen RX7 drivers drive my car after autocrosses during "fun runs", and all comments have been very complimentary. I've even had driving instructors ride along with me, and all were highly impressed by the handling of the car.

While I value your experience and your input, in this case you are making some assumptions. :)

I didn't think you were on R compounds yet. Well that's just weird, because with my car it was a world of difference getting off of street tires. With the street tires I run, Dunlop Direzza's, it would rarely roll enough to bind before the tires would give way and start sliding. With the R comps I had so much more grip I was way faster but I could now approach the infinite roll point and feel it snap out my rear end. Mind you it was much more of a controlled snap than before when I was running stock or even Eibach springs. HRM... I hate winter.. I wish I could try things and test them out before race season begins.

Why would you change out the watts link to a Gfrce panhard when it was adapted from competition RX-3s from Japan? This panhard sounds like somebody is trying to reinvent the wheel. Maybe I am confused. Somebody please tech me?

because it's too high, and it's off center creating a retarded high and off center roll center. Even Mazda got rid of the factory location of the watts link on their race cars.

I think the key thing is that my car just doesn't roll much at all, no matter how hard I'm hitting a corner. I attribute this mainly to the Respeed front swaybar, ISC rear bar on highest setting, and (most of all) ride height.

The car honestly did not role noticeably more with the Rcomps than with the Sumitomos.

Like I said, I think I got lucky and just hit on exactly the right combination of things. :scratch: I dunno, I just attribute it all to Billy's outstanding advice. :)

I have antique tags on it... so its basically "Street Legal" in VA as long as I say it is. :lol:

But seriously, I do have a dashboard,factory seats,and all the plastic trim. I took out most of the tar and am using some super thin carpet in the rear hatch so it still looks clean. I need the 7 to be drivable on the street until I build my next fun street car (RHD Subaru Legacy Twin Turbo Wagon :nod:)


So, for a Full time auto-x car, would the best solution be a redesigned 4 link, or a well designed 3 link going through the floor? (thinking about a 300hp/tq car here.)
3 link would be stupid easy to make with a cutoff wheel and a welder.

I also believe that racers go away from the watts link because under the force of racing the watts link binds enough and the axle pivot breaks under the stress. Mazdatrix sells a pivot reinforcing part to try to prevent breakage.

Also, the top watts link bar gets in the way of most trilinks that attach to the top of the pumpkin.

The panhards are easy to implement, work reasonably well, don't break easily and allow for a wide range of roll center adjustment.

RXDad

I'm not that smart and that is exactly how I made my trilink.

Go back to Viking War Hammer's 4 link thread and see how much effort is needed for a well designed 4 link set-up. https://www.rx7club.com/showthread.php?t=808272

RXDad

Yea if you're running big ass front and rear sway bars that makes sense. Jim recomends using a modded stock front and rear bar on his setups, but he also recomends ubsurdly high spring rates. I went with his absolutely lowest recomendation of 300/200. He recomends as high as 750/500 if I recall correctly.

Jim Susko and I disagree on a few points. I know he's got years of experience, but I like to think for myself sometimes too. I did a lot of reading over the years, and finally discussed my ideas with Billy, and I think we did alright. I hear that Jim was actually at a race in Detroit this year, so next year I intend to make sure that we meet up over an autocross course at some point. That would be a lot of fun!!! :)

He likes high spring rates with a little swaybar, while I like lower spring rates with lots of bar. But of course, I doubt he's driving his racecar to work everyday either.

I was attempting a happy medium, but running an aftermarket front bar and the stock rear bar I was picking up the rear inside wheel, which doesn't help much for exit speed. Plus I think the shock from the tire loading back up when it got back on the ground was helping break my rearends.

I have one of those half 3D modeled. Wish There was more time in the day..........

-billy

Please share with us I really would like to see this model.

Now that is something I would like to see! :)

Anybody want to buy a Trilink/panhard setup???

Was thinking about it but now that idea is out the window. Still not really convinced with the panhard setup. Now waiting on the Watts link model prototype from bwaits_.

This is something I have became aware of but didn't know what was causing it. I especially noticed it on R-Comps. On my R-comps my car tends to understeer and then be backwards. No in between, lol. When I've raced on my street tires it has felt amazing because the car actually has decent oversteer, something that normally doesn't occur unless its after the word snap. I'd want to do something about it but I have no funds to play with this season, just going to have to drive it as it is.

What does street preppared allow you to do to the rear suspension? Also, there was talk of going with too stiff of springs, noob question but what is wrong with going with really stiff springs on an FB, other than ride quality/shock longevity?

For your viewing pleasure, a properly constructed Watts link:

http://i305.photobucket.com/albums/n...Watts_Link.jpg

RXDad

Hmmmm...this past Sunday I was autocrossing and was having problems exiting an off camber turn... I was spinning the right tire like mad!!

First thoughts were that I need to tighten up the rear LSD. Guess from reading this, I might be lifting that tire pretty good... I have the same rear sway bar as Kentetsu and I keep it tight..

Earlier someone was talking about they did not know if they made adjustable watts bars. I run Mazdaspeed adjustable watts bars on mine. Working good!!

The stiff sping vs. lighter spring debate can go on for hours... I am afraid to go too stiff because of loosing "compliance" of the spring. Where we run, it sometimes can get a little rough... Stiff springs seem to work when track conditions are perfect, but race over some rough asphalt or potholes, and you might find yourself just skipping like a rock on a pond!!

There you go, actual in the flesh.... Same basic principle but a good bit different.

Panhards are not bad really. The have successfully been used on MANY race cars over the years. Yes they do have the arc motion but it is a minimal issue compared to the stock watts issues.

-billy

So, you need to decide if the extra 10lbs of unsprung weight with a watts link like the one above is better than the 1/8" the rear will move side to side in its range of movement.

If you have anything but rod ends in the whole rear suspension your trying to pick the fly shit out of the pepper worrying about it. Even then you won't feel the difference if the roll center is in the same spot.

That's exactly it. As well as the watts link being a much stronger locater than a panhard. I have hear plenty of horror stories from slight off track excusrions destroying panhard bars.

i know what you mean, i'm really slow at it too, unless its so simple you didn't need to 3d model it in the first place

I did all my mock ups in MS Paint!!! Gangstar!

Ballin'

That's sexy. What kind of car is that in?

I'm looking forward to seeing Billy's solution. I certainly wouldn't mind having a setup like that, but it'll probably be a good while before I'm good enough to make use of it, so I'm in no hurry. :)

I think it's a Mustang. I grabbed it off of a random site to show what a watts set-up looks like.

There are lots of aftermarket suspension set-ups for Mustangs available. Google Watts Link and you'll see all sorts of ideas.

RXDad

I did not say I was slow at it :nod:
Just to much stuff to model, not enough time to do it all.


-billy

A Mustang - FOX and SN95 Chassis.

I helped install one of these on a A Sedan Mustang 10yrs ago. Nice piece but it was not any better than the panhard bar it replaced in terms of on track performance. In other words, the driver didn't win more races with it installed than he did before.

Hmmmm....this has not been my experience at all.

First, the stock watts link pivot will fail if not reinforced - been there, seen that. Second, to install a watts link on the back of a rear axle housing requires allot of metal (weight) and a significantly strong (and expensive) pivot bearing because they fail too.....seen that also. Finally, a panhard rod can generate a much lower roll center without putting allot of suspension parts in harms way.

Keep in mind that the roll center on a panhard bar setup is the horizontal axis of the bar itself. The roll center on a Watts Link is the pivot. The lower the pivot is placed on the vehicle the closer to the ground the lower arm in the Watts Link gets.

There are other solutions for a Watts link that helps this a little. For example, it can be mounted to the bottom of the rear axle housing and setup to pivot in a horizontal plane instead of a verticle plane (like the stocker). I have seen setups like this on GT cars.

But no matter how you slice it, a Watts Link adds weight, complexity and expense. A Panhard bar doesn't.

When I purchased my trilink/panhard, I was counseled by the previous owner of it to take steps to reinforce the mounting of the panhard bar. He told me that they are prone to being ripped right off the frame rail.

Another friend, from Detroit area (Bruceman/RotaryReserection (I think)) gave me the same advice upon hearing that I had made the purchase. I believe he was basing that advice on personal experience.

So, I think that Hyper's statement is correct on that point. But of course, the amount of metal needed to reinforce those mounts accordingly probably would not amount to much added weight.

Plenty of options out there to address all of these issues. But for now I'm going to stick with my own philosophy, which is "reduce the amount of rear travel through the use of strong swaybars and reduced ride height, and you reduce the issues associated with the rear end". This seems to be working well for me so far, so I'll continue to see how far I can push the car in it's current state.

whoops, try it without that too in there... i'm AM slow

I figured as much, just messing with you. :egrin:

.....

I also come from the lighter rates, larger bar group. We tried all sorts of heavy ass spring rates up front (450-550), modified stock sway bars and panhards with super low roll centers back in the day. Always went back to softer springs, stiffer bar and rear roll center around 1" above the front. This always felt comfortable and allowed the rear to roll about the front roll center and set the front outside wheel. Granted this was in a class with stock pickup points, 13" wheels, small spec tires, no HP and no brakes to speak of - Also, fast sweeping tracks in the Southeast. The pitch/roll scenario worked well for us.


-billy

What will the Torque Arm on say the Granny's kit fix? I know its good for straight line squatting, but is it gonna help some of these other problems?

I also agree that the 'fix' is complicated because it depends on alot of fators like suspension and power and even IF you can do alot about it in the class you are racing

IMO TQ arms suck. They're good for drag racing and that's it.

Considering that since 1988 the ESP National Solo II Championship has been won using cars that have torque arms in all but 4 years tells me that they work for more than just drag racing.

It would not be my first choice for an Rx7. A torque arm is heavy and puts allot of stress on a unibody. I mounted one on a mustang and the subframe conectors had to be reinforced quite a bit. An RX7 doesn't have as much meat to work with - the sheet metal is thin.

So, who is running the stock Watts with adjustable 4 Link?