Rear suspension geometry and anti-squat

XR4Ti / Sierra / Sierra Cosworth Discussions - Questions, problem resolution, general talk, technical tips and modifications.
Merkur Club web site
xr4x4ti
Newbie
Newbie
Posts: 14
Joined: Mon Apr 10, 2006 1:15 pm

Rear suspension geometry and anti-squat

Post by xr4x4ti »

Ok, I have sorta been out of the Merkur community for the last few years. But I have still been working on my car and slowly upgrading it. For those that don't recall, it is a 86 XR converted to a turbo 331 sbf and AWD. Lately I have been using the car as more of an ongoing science experiment and turning it more into a weekend race/drag car. My goal is to eventually get into the high 9's in the quarter while still being able to autoX it when I want to.

I recently went to the track and pulled an 11.008 @ 128mph, missing my goal of getting into the 10's this season. With that MPH I really should be running mid 10's, but my launch and 60ft time are really not that great at about 1.7sec. With AWD it really should be a lot better. A few things contribute to this. First I did some testing and determined that my viscous coupling in my transfercase is toast and allowing the wheels to spin. Second, I am running some crappy tires to sort of prove a point. Third, I got some videos of these runs and it is quite obvious that the car squats like mad and it really is not hooking the rear wheels.

The squat seems to be inherent to the semi-trailing arm rear suspension. Looking at a few suspension books and some online references, to dial in some anti-squat in a semi-trailing arm setup, the pivot point looking at the side of the car has to be above the center of the wheel when at ride height. This is definitely not the case in an XR. To make this better, I am contemplating adding some additional mounting holes to the cross member that would move the pivot points 1, 2, 3 inches higher to either reduce the squat or add anti-squat.

Has anybody ever done this? It looks like I will have to cut the floor in a few spots and add some clearance. I am totally OK with doing that.

Tim
XR4X4Ti

PS, this got me thinking. I use to have an E30 BMW and it had terrible off throttle (engine braking) over steer. The XR's don't have this even thought they both have a semi-trailing arm setup. This recently got me thinking, maybe the E30's mounting points are much higher to reduce the squat. If this is taken to far, it will cause "squat" under engine braking which would induce over steer. Hmmmmm
V8+Turbo+AWD=FUN!!
User avatar
Mike McCreight
Level 7
Level 7
Posts: 3987
Joined: Fri Oct 31, 2003 8:44 pm
Location: Ottawa! Well, Manotick, actually...

Re: Rear suspension geometry and anti-squat

Post by Mike McCreight »

Welcome back, Tim.. :notworthy
It's not a hobby, it's an obsession!
Grayson
Level 7
Level 7
Posts: 1318
Joined: Wed Jul 07, 2004 2:34 am
Location: West Chester, OH
Contact:

Re: Rear suspension geometry and anti-squat

Post by Grayson »

I'm trying to wrap my mind around the concept of "maximizing anti-squat" by ensuring the STA pivot points are above the wheel centerline. This would imply that lowered XRs would squat more than XRs set at standard ride height, which does not seem to be the case. Now, you can consider the extreme example where the STA pivot points are as far above the wheel centerline as possible, which would mean the STA is vertical, and certainly that would comically stave off squat. Downward movement of the chassis would seem to be maximized when the STA pivot point is at the same height as the wheel centerline, and all points in between probably go as a sine wave. However, if the STA pivot points were set below the wheel centerline, I would argue that it would have the same effect as if the STA pivot points were set above the wheel centerline by the same amount, so you could argue that lowering the car would also fight squat in theory.

I think resetting your STA pivot points has a lot to do with toe and camber curves, but my gut feeling, having never experimented, is that you're not going to get much in the way of squat modification by way of moving your pivot points a reasonable amount.
timxr8
Level 5
Level 5
Posts: 642
Joined: Mon Feb 12, 2007 11:39 am
Location: Miramar, FL

Re: Rear suspension geometry and anti-squat

Post by timxr8 »

Unless I'm mistaken, you aren't really concerned with "Squat", you are concerned with the change in geometry when the car "squats". That is what is causing changes in contact patch (on a IRS car, you will get it) and you lose grip. The problem with the XR and most other STA vehicles is that the static setting should be near 0 deg. Have you had an alignment? You could even go a little positive, so when the car "squats", the change in geometry will bring your camber to near 0. Have you fiddled with the suspension at all with different springs, shocks, and/or bushings?
Tim Spencer
1988 XR4Ti Duratec project car
2011 Taurus SHO daily driver...not stock.
thesameguy
Level 7
Level 7
Posts: 2625
Joined: Mon Apr 18, 2005 5:06 pm
Location: Sacramento
Contact:

Re: Rear suspension geometry and anti-squat

Post by thesameguy »

I'd imagine squat is at least a little bit of a concern - you're losing power and time while the rear end soaks up the launch. I would think (don't know) Grayson is on the right track and lowering the car would increase squat. Seems like stiff, normal height springs would be the easy way to fight squat. You can't address squat on an IRS car like you can a solid axle car.
User avatar
John Brennan
Level 8
Posts: 11630
Joined: Wed Jun 18, 2003 4:19 pm
Location: Scottsdale, AZ "Summer Is Coming"

Re: Rear suspension geometry and anti-squat

Post by John Brennan »

I always thought some squat was good on a street-based dragger because it encourages weight transfer rearward at launch, thus aiding traction. I think timxr8 is on the right track... or strip... bearing in mind that the OP still wants to keep the car able to turn and stuff, too-- and doesn't want to over-specialize.
This is my car, and these are my people!
2015 Fiesta ST
2020 Edge 2.0 Ecoboost
xr4x4ti
Newbie
Newbie
Posts: 14
Joined: Mon Apr 10, 2006 1:15 pm

Re: Rear suspension geometry and anti-squat

Post by xr4x4ti »

I'm trying to wrap my mind around the concept of "maximizing anti-squat" by ensuring the STA pivot points are above the wheel centerline. This would imply that lowered XRs would squat more than XRs set at standard ride height,.
Yes, as you lower a STA car it actually reduces the anti-squat or increases squat. In the case of a stock XR, I think it has squat, not anti-squat built in. This means that the torque applied to the wheels actually tries to pull the wheel up, vs push them down. This makes the car extremely safe, sort of like designing in under steer from the factory.

Take a look at this

Image

As you can see to plant the rear tires and prevent the unloading of the front tires you want the pivot point or instant center of the rear suspension to be above the wheel centerline.

However, if the STA pivot points were set below the wheel centerline, I would argue that it would have the same effect as if the STA pivot points were set above the wheel centerline by the same amount, so you could argue that lowering the car would also fight squat in theory.
No, the affect would be reversed, as is the case if you believe my theories 8) . The reaction torque would try to pull the wheel up rather then push them down.

I think where some of the confusion comes in is if you take a stock softly sprung and dampened XR and lower it with much stiffer springs and dampers it will in fact squat less because of the springs and dampers, not because you have made the geometry better.

Tim
XR4X4Ti
V8+Turbo+AWD=FUN!!
xr4x4ti
Newbie
Newbie
Posts: 14
Joined: Mon Apr 10, 2006 1:15 pm

Re: Rear suspension geometry and anti-squat

Post by xr4x4ti »

On a slightly related note.

Has anybody made their own 6 or 9 degree rear beam?
V8+Turbo+AWD=FUN!!
michaelb
Level 3
Level 3
Posts: 237
Joined: Wed Sep 13, 2006 7:49 pm
Location: CT, Torrington

Re: Rear suspension geometry and anti-squat

Post by michaelb »

I have the whole rear trunk area for a jig to make one just haven't gotten around to it yet.

Image
original rouse btcc set up
It has shims to change the angle, would that give you more or less anti squat? Or does the hole actually need to be moved?

It's been a long time since I've read how to make your car handle but can't you only get 100% anti squat or close to it with independent rear suspension and more then that if you have a live axle?
Roll cages= viagra for car
xr4x4ti
Newbie
Newbie
Posts: 14
Joined: Mon Apr 10, 2006 1:15 pm

Re: Rear suspension geometry and anti-squat

Post by xr4x4ti »

Well, I back on this topic.

I didn't have time to modify my rear suspension last winter and tried to mask it instead by putting on some sticky tires. I also replaced by Viscous coupling in my center differential and put in a helical LSD in the front diff.

All of that made virtually no difference at the track :twisted:

I did improve my MPH at the track considerably, so I am making more power, but my 60ft time was no different and I had some wheel hop. In the short term I can probably improve the wheel hop with some stiffer STA bushings.

I won't get to any big changes until this coming winter, but I want to keep this conversation going on how to improve XR rear suspension. I will be posting some links to some European forums, some diagrams from a few books I have recently purchased, etc.

More to follow...
Tim
XR4X4Ti
V8+Turbo+AWD=FUN!!
xr4x4ti
Newbie
Newbie
Posts: 14
Joined: Mon Apr 10, 2006 1:15 pm

Re: Rear suspension geometry and anti-squat

Post by xr4x4ti »

michaelb wrote:I have the whole rear trunk area for a jig to make one just haven't gotten around to it yet.

Image
original rouse btcc set up
It has shims to change the angle, would that give you more or less anti squat? Or does the hole actually need to be moved?

It's been a long time since I've read how to make your car handle but can't you only get 100% anti squat or close to it with independent rear suspension and more then that if you have a live axle?
Changing the angle of the rear STA suspension will not change the Anti/Pro-squat characteristics but it will change what happens when it does squat. As you lower the angle by moving the inner pivot forward, the change in toe and camber is reduced. This improves straight line traction.

The stock XR STA angle is 18 deg. This is not by accident and if you make a little sketch you will see that this puts the front view instant center directly on the opposite wheel. This seems to be the common thinking of STA design in the 70's when the Sierra was developed and after doing a bit of research several cars were setup this way. But by modern standards that is way to short of an effective swing arm length. Even modern SLA suspensions where the pivot point can be put virtually anywhere in space wouldn't have that short of a swing arm length. Something more like double the track of the car is more common. By lowering the angle of the STA from 18 deg to something like 9 or what seems to be common in the UK, 6, you dramatically reduce the amount of toe and camber changes as the rear suspensions goes through its travel. If you read up at the PassionFord forums there are many people who have converted to the 6 deg beams and can't speak enough about the improvement in traction in a straight line AND in the corners.

Tim
XR4X4Ti
V8+Turbo+AWD=FUN!!
GeneticRehab
Level 5
Level 5
Posts: 688
Joined: Fri Oct 01, 2004 7:02 pm
Location: Some were in the USA

Re: Rear suspension geometry and anti-squat

Post by GeneticRehab »

Check some corvette forums. The guys their are able to make it hook up good on a irs. You might have to make a sway bar simular to the front. You want your irs to simulate a solid axle. From my understanding on anti swat, you would need to make a sla rear suspention. The XR'S rear irs is kinda crude in design, when compared to other irs's. As we know when we lower our cars, the rear is thrown way out of spec. Research will be you best bet. Check out 5 Gen camaro irs. Those guys are able to put down 1000 rwhp and get good times. How far are you willing to go? I know you built a hellauos car. I believe you will ake it happen.
Jason Liss
1989 XR Still in pieces

1986 XR Sold!
CV12Steve
Level 7
Level 7
Posts: 1473
Joined: Tue Oct 30, 2007 3:57 pm
Location: Sacramento, CA area
Contact:

Re: Rear suspension geometry and anti-squat

Post by CV12Steve »

xr4x4ti wrote:On a slightly related note.

Has anybody made their own 6 or 9 degree rear beam?
Bought a 6° from UK. Not installed yet.
Stephen
xr4x4ti
Newbie
Newbie
Posts: 14
Joined: Mon Apr 10, 2006 1:15 pm

Re: Rear suspension geometry and anti-squat

Post by xr4x4ti »

I don't know what it is, but I have been obsessed with rear suspension lately. It is honestly consuming my life :shock: I am trying to figure out what I want to do to the car over the winter to improve the rear traction.

I was pretty convinced that I was simply going to make my own 9-6 deg beam and simply raise the pickup points a bit to get some anti-squat. I already have a 8.8 conversion with the Cosworth 930 joints, Torsen, etc. So, I didn't want to reinvent the wheel. But after doing some calculations, to add any meaningful anti-squat to the rear end, I would need to raise the pickup points of the trailing arms at by about 4 inches. Well, when you look under the car and ponder moving the pickup points up 4 inches it starts to look a bit comical :lol: You would have to cut some of the floor out under the rear seats and make some pretty huge bumps. Don't get me wrong, I am not against cutting the floor up, it is already all modified for the AWD system and I am convinced I could take the material out of the rear seats and you would never be able to tell once it is all back together. But, for all of the work involved to make that happened I am getting more convinced just to fab up a SLA (or H-arm) rear suspension and have much better toe control while adding anti-squat an being able to put the camber curve and roll center where I want it.

I am also seriously considering just grafting in a complete sub frame from another vehicle. The rear IRS from a 350Z is pretty awesome, is well supported in the aftermarket and pretty cheap. But, this would mean pretty much cutting the whole back of the car out, which once again seems like overkill.

I am open to ideas, I am going to start laying all this out in CAD and see what I can come up with.

Thanks,
XR4X4Ti
V8+Turbo+AWD=FUN!!
Choochoochoo
Level 4
Level 4
Posts: 363
Joined: Sat Oct 31, 2015 11:40 pm

Re: Rear suspension geometry and anti-squat

Post by Choochoochoo »

If you're cutting up the floor, why not just cut it all out and do some tube frame design to make the whole rear end however you want? It's a lot of work, but could be worth it.
2000 Dodge Durango 5.9 (gone, and missed)
2008 Mercedes Benz R320CDI
1985 Merkur XR4Ti
Post Reply