Wider Rear End

[gtopel72]

I read in another thread about a member installing a BMW IRS in his GT. I believe the BMW was a 3 series. Since then I've been looking at BMW rear ends (hey, I'm allowed, I'm not married). My question: What happens to the handling when a wider than stock axel is used on a GT? Has anyone tried this? I believe the 3 series rear end was used because it was very close in size to the GT rear end. That, and the member got it from a junk yard and very cheap. What about a 5 or 7 series? I know anything is possible but, would that be dangerous?

[opelnut10]

When you make the rear track wider than the front you run into serveral problems. The worst problem is it will make the car handle very funny except going in a straight line. If you must make something wider, make the front wider it will handle must better.

[GTJIM]

The GT's rear track is already wider than the front: 50.6" v 49.4"

Of course, using wheels with greater offset on the rear of a wider diff will bring things back into kilter a bit. ET28 on the front (standard offset) and say ET48 on the rear (FWD offset from modern FWD car). This narrows the wheel centre width by nearly an inch a side (48 - 28 = 20mm X 2 => 40mm about 1.5").
Unfortunately, that is about as much as there is in it by this method and the widening of the front by just using spacers just messes up the steering geometry. However, the GT can probably (all liability excepted!!) tolerate up to two or three inches wider rear track than front at normal road speeds - ie: below 80 mph!

[gtopel72]

Thanks for the responce. I'm guessing the wider the rear stance is probably makes the car handle, or at least want to handle, like a tricycle?

[RallyBob]

Originally Posted by gtopel72 Thanks for the responce. I'm guessing the wider the rear stance is probably makes the car handle, or at least want to handle, like a tricycle?

It will make the car understeer worse than it already does.

[jtb]

Originally Posted by gtopel72 I read in another thread about a member installing a BMW IRS in his GT. I believe the BMW was a 3 series. Since then I've been looking at BMW rear ends (hey, I'm allowed, I'm not married). My question: What happens to the handling when a wider than stock axel is used on a GT? Has anyone tried this? I believe the 3 series rear end was used because it was very close in size to the GT rear end. That, and the member got it from a junk yard and very cheap. What about a 5 or 7 series? I know anything is possible but, would that be dangerous?

Hello,

That was me. I'm hoping it won't be dangerous!!! I researched lots of different rear-end possibilities, and wound up with the 3 series. I believe there is an Isuzu unit ??? you may want to consider (not available up here in Canada, but is in the States) if you want to go IRS.
It's tough to find a rear axle for the GT that isn't wider than stock, as it is such a narrow car.
I believe the pumpkins for the 5 series are the same as or similar to the 3 series, so I don't think you'd want to go to a 5 or 7 series - way too wide, not much stronger!
I am running FWD offset rims to bring the track back in a little. I am also running the same rim with spacers on the front, but I have changed the front suspension. (Actually, I'm beefing it up now after finding out it is too weak currently - thanks to this site! Look for a picture soon in a different thread)

So my front track is still an inch narrower than the back (like stock), but they are both slightly wider than stock.

HTH,

jtb

[opelnut10]

IRS is much more forgiving than a live axle, you can get away with a wider track in the rear to a degree. Handling is as much tires and wheels as it is suspension along with spring rates and shock tuneing, why do you think manufactures pay engineers big bucks to design and test all these features on the automobile. The biggest improvement for the money to handling in a GT is (1) rear sway bar (2) front sway bar (3) shocks and (4) tires and wheels.

[gtopel72]

Hi jtb,

I saw the pics. of your work. Nicely done. What other rear ends did you research and what did you find as a result? I want to widen the stance of the car, but i don't want to make a Vette out of it. My concern was the difference in whidth between the front and back suspensions. It looks like I'm going to have to change the rear because the stock rear won't handle the HP but I still want to drive the car around corners (I live in the mountains). So, it looks like the front end gets changed also. I have some ideas on what I can do but, that's for another thread.

[Killer Texas GT]

Try the search engine and put in 'IMPULSE' there along with brake the upgrades is also some information on suspension mods using the Impulse and Opel by Buick (Kadett C) for wider tracks or close to stock tracks.
My GT has a 4" wider track over stock using the Impulse suspension, and is for the most part still OPEL derived at least for the rear end and parts of the front suspension.

[jeff denton]

If handling more horsepower is an issue then the Impulse rear isn't going to gain you anything but wider stance and Rodney's brake upgrades. The BMW installation is great, but what a project! You might want to study the Camaro/Saginaw ten bolt option.

[jtb]

Originally Posted by gtopel72 Hi jtb, What other rear ends did you research and what did you find as a result?

Do a search on here, you'll find lots of different possibilities! The other one I was seriously considering was a Mazda RX7 GSL SE - not IRS, but lots of ratios available, disc brakes, and goes nicely with the mazda engine I'm using. Possible to use without narrowing, using FWD rims, but would be a much nicer fit if it was narrowed.
Another one was a Mazda miata (MX5). IRS, in its own little cage. Nice, but damn near impossible to find used in my area.
There are lots of choices, but it depends how much fabrication you are willing to do (or have done).

jtb

[scrapman]

Just a question, if the rear end is kept stock, would putting wheels and tires on the back that are wider than the front create any handling or dangerous issues?

Rick

[baz]

Originally Posted by scrapman Just a question, if the rear end is kept stock, would putting wheels and tires on the back that are wider than the front create any handling or dangerous issues? Rick

reread post #3 this thread

[jeff denton]

Boy, I don't know. After I went to the wider rims all the way around, even with more backspacing on the left than on the right, it seems almost every time I drove my GT there was a problem. Either I ran into other cars or they run into me. Maybe it was the big bald tires. Or just the way I drive.
But seriously, did you have a plan on how to fit wider tires/wheels into the fenderwells?

[GT_Pheel]

For those interested......the Miata independent rear measures approx. 10 inches wider than a stock GT rear. The carrier, shafts and a-arms could be modified, but now you're getting into engineering territory. If it were to be used unmodified, the stock Mazda back spacing for the rims might make up for the difference, but would look strange with a deeper wheel in the front, unless you are going with wheel flares.

I part out Miatas fairly regularly (in SoCal) so if you want measurements, pics, seats or parts just let me know and I'll do what I can within reason to help. The seats are taller than stock, but there are "seat mods" within the Miata community to help out tall people that would give back leg and head room in a GT.

Philip

[RallyBob]

Originally Posted by GT_Pheel I part out Miatas fairly regularly (in SoCal) so if you want measurements, pics, seats or parts just let me know and I'll do what I can within reason to help.

How about a power steering rack/pump/lines? I have need for one for a project car.

Thanks,
Bob

[GT_Pheel]

I have three (complete) in the garage at the moment..........if it were to be headed toward the front end of an Opel, I'd love to get some info on the swap.

Philip

[RallyBob]

Originally Posted by GT_Pheel I have three (complete) in the garage at the moment..........if it were to be headed toward the front end of an Opel, I'd love to get some info on the swap. Philip

Yup, for my Dad's GT. Not a direct fitment however, I'm building the front suspension completely from scratch. And adding an inline steering quickener. But with 235/40-17 tires and 9" wide wheels I don't think my father will want to keep manual steering.

Oh, I'm interested in the first gen MX-5 BTW, those are narrower and will fit within my plans for my control arm lengths.

If it works out okay I will probably fit a similar setup to my Sportwagon at a later date, which would be a repeatable feat worth documenting. But I have no plans to fit one to a stock GT.

Bob

[GT_Pheel]

Originally Posted by RallyBob Yup, for my Dad's GT.....with 235/40-17 tires and 9" wide wheels I don't think my father will want to keep manual steering. Oh, I'm interested in the first gen MX-5 BTW, those are narrower and will fit within my plans for my control arm lengths. Bob

I don't think I would want to fight that in the parking lot, either.

The racks from '90-97 are all the same physically except '94-97 has slightly different valving, but both can be swaped for the other. Two of the ones I have are definitely '90-93. The other one would have to be looked at, but I believe it to be from a '95. I can do a complete set-up for $160 + shipping. By complete, I mean a rack from a car driven into the garage w/ usually less than 100k miles including: ball joint to ball joint, the rubber mounts w/ metal brackets to mount the rack, the pump with engine bracket & adjuster bracket, all lines, the reservoir and all of the hardware. I don't have a junk power plant frame or I would cut the mounting brackets/tabs off that, as well. I can get you the angle and pics if needed, though. The belt has 4 points/groves and runs the A/C off the crank, so if you want the lower pulley to play around with just let me know. They don't really sell, so I have a few kicking around, on engines or in boxes.

If there's anything needed from a Miata, I've got it or can point you to the guys who have the best price above me. I got into parting them to help out some of the SCCA guys locally, not to make a living, so I try to set prices lower than everyone else.

Hope this helps.
Philip

[RallyBob]

Originally Posted by GT_Pheel I can do a complete set-up for $160 + shipping. By complete, I mean a rack from a car driven into the garage w/ usually less than 100k miles including: ball joint to ball joint, the rubber mounts w/ metal brackets to mount the rack, the pump with engine bracket & adjuster bracket, all lines, the reservoir and all of the hardware. I don't have a junk power plant frame or I would cut the mounting brackets/tabs off that, as well. I can get you the angle and pics if needed, though. The belt has 4 points/groves and runs the A/C off the crank, so if you want the lower pulley to play around with just let me know. They don't really sell, so I have a few kicking around, on engines or in boxes.

I'm interested, so I'll PM you lest the wrath of the moderators befalls us....

I won't need the tabs off the crossmember, I can make those. The belt drive shouldn't be an issue either, I can adapt a single-groove pulley if needed, plus I have sources for generic power steering pulleys in various diameters. I need to calculate the speed of the pump though...maybe an OD dimension of the stock crank drive pulley would help me out to transfer it to an Opel engine. Stock redline of a Miata engine? I probably will never top 7000 with my dad's engine (2.6 litre...all torque). Thanks!

Bob

[WannaBOpelOwner]

i was wondering if anyone has put a posi trac rearend in an opel or a detriot locker. i was discussing this with a friend and he said that to detroit locker wouldnt work be cause there arent holes in one of the gears? just wondering......

[RallyBob]

Originally Posted by WannaBOpelOwner i was wondering if anyone has put a posi trac rearend in an opel or a detriot locker. i was discussing this with a friend and he said that to detroit locker wouldnt work be cause there arent holes in one of the gears? just wondering......

Eaton Posi-Trac and Detroit Locker do not make limited slips to fit the Opel axle. Not enough demand to warrant designing a new differential to fit the Opel's ring gear diameter, that's all.