website:www.grannysspeedshop.com...e-mail:grannysspeedshop@verizon.net
phone: 360-391-1208 or 360-853-8219
GSS Torque Arm / Solid Axle Conversion System
Before you begin a solid axle conversion, there are a few decisions to consider that can effect the direction of your project. For instance, you may be going after an import style "Tuner" theme which might require wheels with a lot of "backspace" or "positive offset" to achieve the right look... a stock width rear might be just what you need. If you are after the lightest possible combination for a racing application or just want a drag race look, maybe a narrower rear using deep wheels with very little "backspace" or "negetive offset" would be better for your goals. Here's a list of questions that should be answered before proceeding...
Performance expectations? Is a TASA conversion right for you?
How wide? stock RX-7 width or something narrower?
What style Ford 8.8 housing should you be using?
What brakes do you plan to use on the rear?
What gear ratio?
Do you really need C-clip eliminators?
The following section will explore each of these questions and provide some possible options that we hope will help you get the most out of your project.
Note that below when we refer to a width dimension with regards to a rear axle, we use either WF/WF (wheel flange / wheel flange) or AF/AF (axle flange / axle flange)...
Performance expectations? Is a TASA conversion right for you?
The '86-'91 RX-7's rear suspension works pretty well overall, and it's design geometry actually helps the RX-7's stock rear survive some amazing abuse. Unsprung weight is low, helping the car's ability to to stick the rear tires over rough surfaces. If performance over rough surfaces is most important to you, you might want to consider retaining the RX-7's stock IRS.
Here's a pic of severe wheelhop damage we received from Jason, looking into our TASA conversion for his 3rd gen car, and his comments...
......WF/WF takes into account the thickness of the rotor or brake drum's center, which can be as thin as .065 for a drum or as thick as 1/4" for a brake rotor, per side.
......AF/AF is the assembled axle flange/flange width only, and does not allow for drum or rotor thickness.
Important points to consider if you are trying to calculate fitment or just trying to get the most wheel/tire possible into the stock RX-7 wheelwells.
If you find that forward traction is an area that needs work on your car, there are a few things you can do to maximize your stock RX-7 IRS. The main problem is that the Mazda design allows the rear to "squat" under accelleration. This squat minimizes the rate of weight addition to the rear tires under accelleration, and allows the rear tires to gain camber as the rear suspension is compressed. A slow rate of transfer leaves some potential on the table when it comes to maximizing forward traction, as the tires may begin to spin before they receive all of their additional weight. Camber gain also leaves some potential on the table as it loads the rear tires more on their inside edge, resulting in a net loss of potential traction due to an un-even, and constantly changing, contact patch. Addressing these issues is possible, but the fixes compromise the stock suspension's handling, reducing it's potential from almost "supercar" status to a point closer to "drag race only". Here's a list of typical drag race inspired modifications for the stock RX-7 IRS...
...raise the rear of the car...
...stiffer rear springs...
...stiffer rear shock compression...
...increase rear tire's static positive camber...
...install lineloc on rear brakes to "pre-squat" rear suspension...
These solutions can get you down to a 1.4 or so 60' time with a well sorted car. The ultimate failure point of the '86-'88 limited slip rear is usually a stub shaft, which tends to fail in manual transmissioned cars as they approach the high to mid 10sec 1/4mi level of performance. If you go with an automatic transmission, the failure point is typically delayed to around the hi to mid 9sec 1/4mi level of performance. A pinion snubber is a must at these levels, and any wheel-hop will bring on component failure that much sooner.
..."Grant, Feel free to use the picture. Attached are a couple more. I was at Capitol Drag Strip in Crofton Maryland and dropped the clutch around 4K RPM doing a burn out on street tires. The wheel hop destroyed the unit and I have around 350 rwhp. Luckily the rear diff mounts held everything together so the parts were not all over the track. When it took the unit out of the car it fell into the pieces you see in the pictures..."
If all the above sounds like too much of a compromise for your goals, then you may be ready to step up to our Torque Arm / Solid Axle Conversion. Our design gives you all the tire planting ability of a ladder bar suspension, without the bad habits. The chassis can still rotate about it's roll axis, giving the driver essential feedback, an absolute requirement if yoy want to get the most out of your equipment. Excessively high spring rates or ride heights are no longer required for getting the most from your launch. Rear tire camber and toe also remains consistant, giving the driver reliable feeback when he needs it most.
How wide? stock RX-7 width or something narrower?
If your RX-7 already has the look you want or you are just doing a solid axle conversion to obtain a more favorable gear ratio and want the car's outward appearance to remain unchanged, the '86-'91 RX-7's stock WF/WF width of 59-1/2" may be just what you need. The 8.8 rear from an '86-'93 Ford Mustang provides a good starting point.
FYI, there are a 2 "low-buck" narrowed rear combinations that are possible using stock Ford axles. Both combinations will have RX-7 friendly 5 on 4-1/2" lug patterns.
What style Ford 8.8 housing should you be using?
...If you plan to use an existing 8.8 rear as-is, and keep the '86-'91 RX-7's stock 59-1/2" WF/WF width, there is really only one housing choice, the '86-'93 Mustang V-8 housing, and a few axle/lug pattern options...
...If you plan to narrow a 8.8 rear for your car, there are a few housing choices out there. The shape of the casting's nose will vary depending on the style of 8.8 you decide to use. As our Torque Arms are custom fitted to each style of casting, we will need to know what style you plan to use. The common choices are...
What brakes do you plan to use on the rear?
For simplicity, we prefer to use the original RX-7 rotors and calipers. This allows your brake plumbing to remain as-is, and even allows retaining your oem parking brake and linkage.
If your RX-7 already has 5 lug hubs/brakes, it likely has vented rotors as well. Since the stock Mustang disc brakes had solid rotors, we consider the RX-7's vented rotors superior. If you want to use Mustang brakes, drum or disc, fabrication will be required for e-brake cables.
If your RX-7 has 4 lug rotors and calipers, those can be retained as well. You will have a choice of re-drilling your Mustang axles to maintain the RX-7's 4 on 4-1/2" lug pattern, or re-drilling your RX-7's rotors for the Mustang 4 on 4-1/4" lug pattern.
A word of caution if you plan to use RX-7 rotors on your Ford axles, there may be a minor problem that will need corrected...the axle's flange diameter will likely be larger than the machined inside face of the rotor. Either machine the inside face of the rotor larger to 6.100", or machine the outer diameter of the axle's flange down to 5.850". Machining the axle flange's O.D. is preferred, but either remedy will do.
What gear ratio?
Your ultimate choice for rear gear ratio will depend on many factors. Most are compromised to obtain the best overall combination for performance. Here's a simplified list of common available ratios and what we feel they are generally best suited for in a typical V-8 powered, stock wheelwell RX-7...
If you happen to be digging thru a wrecking yard looking for an Explorer rear to narrow, useful info can be found on a sticker on the driver's side door jam. This sticker will have an axle code which indicates the original rear gear ratio and diff type. The Explorer axle codes are as follows...
Do you really need C-clip eliminators?
Sanchioned drag racing have long had blanket rules requiring c-clip eliminators when a car surpasses a certain level of performance. The same rule applied to all cars and all rears, regardless of specifics. These were good rules that fit the times in the old days, but we feel the rules are now outdated, and should be modified. The problem with c-clip rears was that when an axle broke, the entire axle, wheel, and tire typically came out of the rear and left the car, a very dangerous situation at any speed. Most all cars in those days had drum rear brakes, and nothing was left to retain the outer part of the broken axle. In the modern times, many cars will use disc brakes in the rear. Wih the OEM iron rotors, there is just no way that an axle can come out of the rear unless it rips the entire caliper off with it. We feel that there should be an exemption for disc brake rears using stock rotors. But rules are rules, so if you plan to participate in sanchioned drag racing, eliminating the c-clips will be necessary, regardless of drum or disc rear brakes. C-clip eliminator kits are out there for around $150 or so, and basically just convert your outer wheel bearings to a press-fit retained style. They are also typically leak prone, and more trouble than they are worth if you have rear disc brakes. In our opinion, if you are using drum rear brakes on a fast car, they are worth the investment and trouble. If you are using stock disc brakes on the rear, we would not use nor recommend c-clip eliminators unless they were forced upon us. If you are narrowing an 8.8 rear and are using drum brakes, lightweight rotor hats, or plan to participate in sanchioned events, the best current solution that we recommend is to build the rear from the ground-up as a hybrid, using 9" style housing ends, bearings, and axles.
If one of your goals is to build the lightest possible car your budget allows, a narrower rear axle combined with the deeper wheels will result in shorter axles and shorter axle tubes for a given track width, shaving unsprung weight without adding to the rear wheel's rotating weight.
......NARROWED 53-1/4" AF/AF WIDTH USING STOCK FORD AXLES...6-1/4" narrower than the stock 2nd gen RX-7 rear (3-1/8" per side), made possible by using 2 right side Ranger axles (26-1/4" long), resulting in a 9/16" pinion offset. These axles are 28 spline and use passenger car sized bearings. Narrow the only left side of a 4.0 Ranger 8.8 housing, or both sides of most any passenger car 8.8 housing. Will require using a 28 spline differential.
......NARROWED 56-1/2" AF/AF WIDTH USING STOCK FORD AXLES...3" narrower than the stock 2nd gen RX-7 rear (1-1/2" per side), made possible by using 2 right side Ford Explorer axles (27-13/16" long), resulting in a 9/16" pinion offset. These axles are 31 spline and use larger truck sized axle bearings. Narrow only the left side of a '91-'94 Ford Explorer rear housing. Will require using a 31 spline differential.
......STOCK WIDTH OPTION #1- stock length '86-'93 Mustang axles and stock 4 lug wheel pattern...This one comes stock with a 4 on 4-1/4" lug pattern and drum brakes, both of which can be utilized for a low-buck conversion. E-brake cables and proper rear brake proportioning will need additional fabrication / attention. 4 lug Mustang wheels will be required for the rear, unless the axles and drums are re-drilled for an RX-7 lug pattern.
......STOCK WIDTH OPTION #2- stock length Mustang axles converted to 5 lug...If your budget permits, an aftermarket Mustang 5 lug conversion kit can be used to get a 5 on 4-1/2" lug pattern, which happens to be the same as the 5 lug RX-7 pattern. For a low-buck alternative, 29-5/32" long 5 lug axles from the left side of a Ford Ranger 7.5 / 8.8 (or right side of a Ford Aerostar 7.5 rear) can be used (both the 7.5 and 8.8 Ranger rears use the same 28 spline axles and small bearings). These Ranger/Aerostar axles are 3/32" shorter than the Mustang axles, no problem for drum brakes, but if you convert to disc brakes, they may require minor shimming to obtain proper caliper placement.
......'86-'93 Mustang / full size car...has 2 external ears, one on each side of the pinion housing. Uses 2-3/4" dia axle tubes and small axle bearings.
......'91-'01 Explorer / full size truck...has one external ear on the right side of the pinion housing. This housing is about 10lbs heavier than a Mustang version, mostly due to larger 3-1/4" diameter axle tubes that are almost 1/4" thick. Also uses larger truck axle bearings. This housing has a very large external ABS sensor and guard lugs located above the ring gear. The ABS sensor will need to be removed, the hole plugged, and the guards cut off and ground down to provide clearance for our upper watts linkage rod, which passes over the rear's center section.
NOTE-...although the stock Explorer rear is 59-1/2" wide, it is not suitable to use in a RX-7 if a 59-1/2" width is desired. The Explorer used un-equal length axles resulting in a whopping 2-3/4" pinion offset, not practical for our purposes. Only start w/ an Explorer rear if you want a rear 56-1/2" WF/WF or narrower. If you want to use heavier Explorer style center with the larger axle tubes and maintain a stock 59-1/2" WF/WF width, the best option is to narrow a wider 8.8 from a full size truck.
......'84-'85 Full size passenger car...has no external ears. This is the lightest 8.8 housing of all, and uses smaller 2-3/4" dia axle tubes.
......3.08...recommended for daily drivers seeking best overall fuel economy
......3.27...good compromise for a balance of economy and performance
......3.55...good gear for average street/strip car using non-overdrive transmission
......3.73...excellent choice for a fast street/strip/track/drag car with 130mph or better 1/4mi potential. We feel this is the best all around gear for real supercar performance
......4.10...drag only unless you have an overdrive transmission.
41....3.27 w/ open diff
43....3.08 w/ open diff
44....3.73 w/ open diff
45....3.55 w/ open diff
D1....3.27 w/ TracLoc diff
D2....4.10 w/ TracLoc diff
D4....3.73 w/ TracLoc diff
D5....3.73 w/ TracLoc diff
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Granny's Speed Shop
POB 814
Concrete, WA 98237