Rear suspension geometry and anti-squat

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m2motorsport
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Re: Rear suspension geometry and anti-squat

Post by m2motorsport »

CV12Steve wrote:
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.
Where did you buy it from and how much?
2003 Ranger Rally Truck
1986 XR4Ti soon to be rally car.
xr4x4ti
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Re: Rear suspension geometry and anti-squat

Post by xr4x4ti »

Hey Gang,

I am moving forward with this mod. I am trying my darnedest to not just scrap the whole thing and start over. I am sure most of you have seen my post on facebook, but I am going to document the build here. Maybe I am just old, but I don't think Facebook is the place to document technical things (or much at all :lol: )

I have been posting on Cornder Carvers to try to get some feedback from a larger audience outside the Merkur world as a sanity check. See here:

[urlhttp://www.corner-carvers.com/forums/showthread.php?t=49547][/url]

Besides just posting pictures, I am going to post some analysis of why I am doing it, the compromises involved etc.

Here is where I am right now:

Image

I am going to get it ripped apart today and hopefully get the yellow poly beam mount bushing pressed out so I can start welding.

My final goal is to make something like this:

Image

But, with some tweaks. I would consider just buying one of these beams form the UK, but I already have the 8.8 conversion, plus I want to do a few more mods.
V8+Turbo+AWD=FUN!!
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Re: Rear suspension geometry and anti-squat

Post by xr4x4ti »

So why am I doing this. Is there some science behind it, or is it just me wanting to cut things for the fun of it? :headbang

There is definitely some science behind it.

Even though my car is AWD, I have reached the point of having enough HP that I need to improve my traction. I can quite easily spin the rear tires before the transfercase transfers enough power to the front to prevent the wheel spin. Lets ignore the fact that there are potential improvements to the TC that could help the situation. I want to improve the physical traction of the rear first.

Why do the XR's and their cousins the European Ford Sierra, Cosworth, Saphire, Escort Cosworth have such a tough time putting the power down? If you look at the European forums this is a very common complaint. There many examples of people complaining how there 400hp Cossies are getting beat my 300hp MK2 Escorts:)

This is sort of the old joke about Supra's.

What is the difference between a 300hp and a 1000hp Supra? About .1 sec in the 1/4 mile :lol:

Ok, what did Ford get wrong and why? If you were going to design any IRS, where would you start. The obvious thing is the desired camber curve. I have researched this excessively and what has become obvious is that the thinking of how much camber gain the front or rear suspension should have has changed over the years. Cars use to be designed with much more camber gain, which in theory may make it handle better the extremes of cornering, but at the sacrifice of straight line performance and bump stability. If you analyse the Sierra rear suspension with its 18 degree sweep angle and trailing arm length you will see that the front view instant center (FVIC) ends up almost exactly at the opposite tire. This is not a coincidence and was the common thinking of the time. This results in a camber recover of 50% and about 1deg/in of travel. The more modern suspension books I have read, such as Chassis Engineering by Herb Adams, Race Car Vehicle Dynamics by Milliken, etc suggest that is on the extreme of high end of how much you want. A more realistic number is .5deg/in or about 25% camber recovery at ride height. A STA suspension has a linear camber gain, it constant over the entire suspension travel. This is one of the disadvantages of the concept. A SLA (double A arm) setup can be made to have 25% camber recovery near ride height and then much more at the extremes. This is a major advantage.

So, Ford didn't screw up, but their thought process is outdated. How do we fix the camber curve? By changing the sweep angle. The stock Sierra sweep angle is 18deg. If we want half of the camber gain we simply reduce the angle in half to 9 deg. Not to coincidentally this works out almost exactly to where Ford put the extra suspension pickups on the RS500!

It is curious why the common mod in the UK is to make a 6deg beam? That seems a bit low and results in about a .3deg/in of travel. Obviously if you wanted to maximize straight line acceleration, you would make the angle zero and have zero camber and toe change like a solid rear axle. Maybe that is what they are going for, biasing the design towards that. This might be because the cars still squat so bad unless you put spine jarring springs in them. If one can reduce the squat during acceleration, maybe more camber gain would be acceptable? That will be the point of my next installment. I will also post some spreadsheets that demonstrate some of the discussed changes when I get time.

Tim
XR4X4Ti
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Re: Rear suspension geometry and anti-squat

Post by Spaldino »

xr4x4ti wrote:So why am I doing this. Is there some science behind it, or is it just me wanting to cut things for the fun of it? :headbang

There is definitely some science behind it.

Even though my car is AWD, I have reached the point of having enough HP that I need to improve my traction. I can quite easily spin the rear tires before the transfercase transfers enough power to the front to prevent the wheel spin. Lets ignore the fact that there are potential improvements to the TC that could help the situation. I want to improve the physical traction of the rear first.

Why do the XR's and their cousins the European Ford Sierra, Cosworth, Saphire, Escort Cosworth have such a tough time putting the power down? If you look at the European forums this is a very common complaint. There many examples of people complaining how there 400hp Cossies are getting beat my 300hp MK2 Escorts:)

This is sort of the old joke about Supra's.

What is the difference between a 300hp and a 1000hp Supra? About .1 sec in the 1/4 mile :lol:

Ok, what did Ford get wrong and why? If you were going to design any IRS, where would you start. The obvious thing is the desired camber curve. I have researched this excessively and what has become obvious is that the thinking of how much camber gain the front or rear suspension should have has changed over the years. Cars use to be designed with much more camber gain, which in theory may make it handle better the extremes of cornering, but at the sacrifice of straight line performance and bump stability. If you analyse the Sierra rear suspension with its 18 degree sweep angle and trailing arm length you will see that the front view instant center (FVIC) ends up almost exactly at the opposite tire. This is not a coincidence and was the common thinking of the time. This results in a camber recover of 50% and about 1deg/in of travel. The more modern suspension books I have read, such as Chassis Engineering by Herb Adams, Race Car Vehicle Dynamics by Milliken, etc suggest that is on the extreme of high end of how much you want. A more realistic number is .5deg/in or about 25% camber recovery at ride height. A STA suspension has a linear camber gain, it constant over the entire suspension travel. This is one of the disadvantages of the concept. A SLA (double A arm) setup can be made to have 25% camber recovery near ride height and then much more at the extremes. This is a major advantage.

So, Ford didn't screw up, but their thought process is outdated. How do we fix the camber curve? By changing the sweep angle. The stock Sierra sweep angle is 18deg. If we want half of the camber gain we simply reduce the angle in half to 9 deg. Not to coincidentally this works out almost exactly to where Ford put the extra suspension pickups on the RS500!

It is curious why the common mod in the UK is to make a 6deg beam? That seems a bit low and results in about a .3deg/in of travel. Obviously if you wanted to maximize straight line acceleration, you would make the angle zero and have zero camber and toe change like a solid rear axle. Maybe that is what they are going for, biasing the design towards that. This might be because the cars still squat so bad unless you put spine jarring springs in them. If one can reduce the squat during acceleration, maybe more camber gain would be acceptable? That will be the point of my next installment. I will also post some spreadsheets that demonstrate some of the discussed changes when I get time.

Tim
XR4X4Ti


Excellent post. Thank you for the research and insight!

I always had equated the lack of straight line traction to the squat of the IRS, something I gleaned from Mustang Cobras and people swapping out the IRS for solid axle rear ends back when they first came out with IRS. Every single Mustang magazine at the time had discussion pieces, how to's, and talked about the topic in their features on some cars (my dad was subscribed to every major Ford mag when I was a kid, lucky me!). I know there were some products that supposedly kept the rear tires from bouncing under hard acceleration if you wanted to keep the IRS but I can't remember what those were right now and I can't remember if they adversely affected cornering capabilities as it all seemed focused on drag racing.

*EDIT*

I looked it up and a lot of the things the Cobra guys do to reduce wheel hop and get better straight line traction are things we do too like poly/solid subframe, diff, and control arm bushings, and increased spring rate. Other things for the Cobras are subframe connectors, diff case brace, better halfshafts, toe link bars, etc... not sure how applicable those are to our cars and I haven't found any mention of how it affects cornering.
88 Mono-white XR: Side exhaust, Walbro, MC2 Front Adjustable Coilovers, Rear Bilsteins, Full Poly bushings, MC2 Diff Mount, Rapido intercooler, Some sort of mild cam(?), Rear Disc conversion, 225s on 17x8 wheels.
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