Jag IRS

CastleMGBV8 · Post by **CastleMGBV8** » Sat Dec 01, 2007 9:56 pm

Anyone used a Jag IRS in any car and know wheher narrowing the drive shafts and lower wishbones equally by 8" has any effect on geometry?

Also any tips on installation to achieve the correct height for the diff to achieve the correct static height geometry.

The intended reciprient is an MGB GT.

Kevin

kiwicar · Post by **kiwicar** » Sun Dec 02, 2007 10:25 am

If you shorten the drive shaft and wishbone you will change the instant swing arm length because you will change the incident angle of the wishbone and the drive shaft, also you will change the rate of change of instant swing arm length.
You should draw it out as originally designed and with the shortened wishbone/ drive shaft combination, if you draw it out you should be able to work out the instant swing axel length at ride height on the original then move the inner lower mount on the shortened version until the swing axel length is as the original. The shortened version will reduce the swing axel length more quickly than the original but it's a start.
as a rough guide with about 3.5 inches of bump from ride height you want instant swing axel length to be in the range of 120 to 150 inches at ride height down to about 35 inches minimum (in fact not less than 2/3 track width) or things get twitchy

16 inches is alot to take out of this set up.
Best of luck
Mike

JC. · Post by **JC.** » Sun Dec 02, 2007 11:14 am

Why not buy yourself a hoyle IRS system instead?
Its purposly designed for the MGB rather than an adapted after thought?

The kit is £1700 plus some misc' ford parts from a granada.

stevieturbo · Post by **stevieturbo** » Sun Dec 02, 2007 6:10 pm

Some of the US bolt in live axle options might be cheaper ?

mgbv8 · Post by **mgbv8** » Sun Dec 02, 2007 6:32 pm

Yes Stevie.
The GM 10 bolt axle from D&D fabrications is a good option. And its a drop in replacement.
I'm looking to buy one in 2008 and stick a 4 link kit on it.

But while I'm saving up, I'll keep my original MGBV8 axle as I reckon they can take a lot more stick than folk believe.

I guess the telling time will be when the transbrake is fitted. I may only get one launch in on that day.

Kev
Are you going the IRS route for comfort and handling or to use more power through it?

chodjinn · Post by **chodjinn** » Mon Dec 03, 2007 10:42 am

JC the Hoyle kit is very good but its a lot more than £1700 - that's just for their fabricated kit. You have to buy the diff and a hell of a lot of other stuff like brakes etc. which puts it in the £3k-£3.5k.

I think you only have to shorten the IRS by 8" in total not 16" but not too sure. Running a Sebring wide arch kit does help a fair bit. When I looked into it I think that the shortened parts as used by the cobra boys were sufficient to fit the MGB (with the sebring arches tho), therefore would be ok in terms of geometry.

If you look around you could get a half decent irs for about £150, plus new brakes and a few other bits etc - I figured it to be about a grand all in. You can use the original diff cradle as well but needs the mountings messing around with. There is/was a very good website by some bloke that had done it all himself but i can find it . . .

chodjinn · Post by **chodjinn** » Mon Dec 03, 2007 10:55 am

Righto even better;

I hacve two word documents in my posession, one for fitting Jag IRS and one fore Jag IFS to MGB. Got basic photos and info in them.

Also have a selection of emails/posts from teh MG Enthusiasts pages discussing various rear suspension set ups, including Jag IRS, Hoyle, and Frontlines 5-link set up.

I'm afraid I can no longer find the website which showed how to modify the jag diff cage/subframe for an MGB.

If you wing me a PM with your emails I send what I have out to anyone that's interested, never know it may be useful.

BTW just scanned the Jag IRS doc and it says the wishbones and driveshaftsd need shortening by 5" in total (2.5" per side) when running with Sebring arches.

chodjinn · Post by **chodjinn** » Mon Dec 03, 2007 11:05 am

FOUND IT!!!! lol

http://www.mgbv8.co.uk/

the guy is Nick Smallwood - spoke to him a few times he's very nice. If you want info just email him (and wait a bit hehe!) and he'll help you as much as he can - well he did with me!

I also have some other info on mounting the Jag diff to an MGB and other info on diff ratios etc.

CastleMGBV8 · Post by **CastleMGBV8** » Mon Dec 03, 2007 4:48 pm

Mike,

Should have made myself clearer the 8" reduction is in total width ie. 4" per side from an XJ6/12 standard IRS and I believe not far off the E Type width. Thanks for the science but it left me a little confused. I understand that with the shorter arms and driveshaft that the camber will change more rapidly than with the full width set up

Most people I have spoken to seem to rightly or wrongly just shorten the drive shafts and wishbones equally.

I have done quite a lot of research into fitting the Jag rear end into an MGB and it's a do able proposition at about half the cost of a Hoyle IRS and thats without a limited slip diff.

Olly it was me who sent you the information and link to Nick Smallwood.

Perry, The intention was to improve ride and handling with additional reserves of strength for potential engine upgrades in the future.

I recently swapped in a narrowed SD1 axle with 3.08 ratio which came for free to give me decent gearing for the road and at the same time fitted a pair of Moss reverse spring eye 110lb springs for the GT which are double the price of the standard replacement leaf springs. The new springs have lowered the car 2" as stated by Moss and transformed the ride and handling, the suspension now works and powering through a corner the back end just sits down and grips which brought a large grin to my face, in addition the springs seem to handle the torque of the V8 under normal spirited! driving.

The question I raised above came from a guy in the states on another BBS who is in the process of fitting a jag rear end and asked me the question.

Thanks for the responses

Kevin,

chodjinn · Post by **chodjinn** » Mon Dec 03, 2007 5:29 pm

sorry for wasting your time then.

Why the hell would someone want to shorten the Jag IRS by 8" if 5" is enough for a standard 'B?? Bigger tyres??

have you done any other mods to the rear suspension other than replacing the springs with reverse eye ones? Such as panhard rod??

I'm still debating whether its worth spending the money changing things round if a panhard rod and better springs will do the job. I also run a shortened SD1 with LSD and given the abuse they can take am reluctant to spend money unneccessarily

CastleMGBV8 · Post by **CastleMGBV8** » Mon Dec 03, 2007 8:44 pm

Olly,

No problem,

Re Panhard rods, personally on a road car they are more trouble than there worth unless you are replacing the leaf springs with coil overs and 4 link suspension and then obviously you need a panhard rod to keep it all in place.

The only other mod to the rear end is the use of telscopic dampers which were already on the car as part of the original Moss-Hopkinson handling kit. I took them off to test them, and found they were Gabriel non adjustable but in good condition and with a good damping rate and all I can say is the are very compatable with the new springs.

My car was a rubber bumper model, so the reverse spring eye springs lower the car to the correct height without the need to resort to lowering blocks, which IMHO are a bodge and can increase axle tramp because of the additional leverage on the springs.

I also removed the rear anti-roll bar as it was already making the rear suspension too stiff. MGB's need to be reasonably stiff at the front and softer at the rear to make them handle. I have double wishbone and coil overs on the front with 500lb springs Gaz adjustable dampers and a 7/8" anti-roll bar and the balance of the car front to rear is very good with very neutral handling so I appear to have got it right for me anyway.

If you intend to drag race yours you will need to do something to prevent axle tramp, as that will break your axle quicker than anything. The other thing you need to be aware of is that I am told by someone drag raced his car a couple of times is that the SD1 axle halfshafts can be a weak point especilly if they have been shortened and he managed to twist the splined ends at the diff and had to replace them with some he got specially made.

There are different ways to avoid axle tramp and if your interested let me know and i pass on information and links.

Kevin.

chodjinn · Post by **chodjinn** » Mon Dec 03, 2007 9:11 pm

I'd love that info Kev if possible. Also interested in your front suspension mods - mine is completely standard at the moment! I'm liking the Hoyle front set up for the money; the rear is very expensive tho IMO altho prob the best available in this country.

I'm aware of the alleged probs with with the SD1 halfshafts. However, I have it on good authority if they can handle 4yrs of 400bhp track abuse in an SD1 then I think i'll be ok (I hope lol!) in an MGB that weighs half as much. . . time will tell i guess lol.

I have thought about 4-linking the rear of mine actually. I'd use a Watts linkage over a Panhard rod any day though (6-link vs. 5link) - esp. as I have the vitesse diff with the Watts mount already. Already running telescopic Spax dampers, but yeah tramping was a major problem when i had it running last. Fitting a 5/6link would be a good challenge involving a fair bit of fabrication . . . gotta learn to weld first

Mine is a 1972 late Mk2 and the ride height is pretty good at the mo, dunno what it'll be like when i get the 16" MOs on tho

bah, i should be getting the engine in but i'm here supping a brew watching telly and frowning at the cold outside

CastleMGBV8 · Post by **CastleMGBV8** » Fri Dec 07, 2007 11:38 pm

Olly,

Sorry to be slow replying re anti tramp solutions.

There are various solutions depending on how much you want to spend.

Cheapest is suggested by Bill Guzman of Classic Conversions in the states.
You take the second leaf from an old MGB leafspring and remove the smallest leaf from the bottom of your existing springs. Using this small plate mark the the second leaf rear part by lining up on the centre hole and cut off the rear section of the second leaf. Replace the short leaf with the shortened second leaf with the long section facing forward and thats it.
If you search the MG Experience BBS archives under anti Tramp you should find some pictures on this and the modded kit below.

The additional leaf does not form part of the spring and does not increase spring rate, what it actually does is resist axle wind up and helps traction and launch from a standing start, Bill has used this on cars that are drag raced in the US and says it is very effective.
If you can scrounge a pair of scrap springs it's also free.

I don't particularly like the standard anti tramp bar kits that are available because of the conflict of the spring that lengthens in bump and the fixed length bar, but they can be modified with a pair of bellcranks on either side of both spring hangers, modifying the tramp bar by cutting off the rear bush housing and threading the bar for a rod end to make it adjustable. You then fit a bush at the other corner of the bell crank which sits on top of the leaf spring, and then adjust the rod end to provide a small amount of preload, voila! Cost £200-£250.

More sophisticated solutions like 5 link kits £1000-1500 + fitting costs.

Jag IRS fully rebuilt and reconditioned £1800 ish + fitting. Hoyle IRS £3500 + fitting!

Personaly I like the Jag IRS but can't justify the cost at the moment, it's good value and could be done for less if you have the ability to rebuild it yourself.

Hope thats a help.

Kevin.

CastleMGBV8 · Post by **CastleMGBV8** » Sat Dec 08, 2007 3:45 pm

I should have said the MG Enthusiasts BBS

Kevin

http://www2.mgcars.org.uk/cgi-bin/gen5? ... t=&thread=

mgbv8 · Post by **mgbv8** » Sat Dec 08, 2007 8:16 pm

I have to admit Kevin
I've never had any probs with tramping on my V8. Be it on the street or on the strip.

I thought my MGB V8 would tramp, so before I let it loose after the last upgrade, I fitted new standard V8 cart springs. fitted the cheapy uprated lever arm dampers from the MGOC. I fitted blue poly bushes all round in the rear end as well.
I can rag the car in the dry or the wet. On strip or street using the Dunlop sports or the BFG Drag Radials and all she does is spin up nice and smooth.

I have fitted the cheap Phantom Grip diff locker though. Jon Savage at Cambridge Motor Sports sorted that for me. I wonder if the tramping issue is held off because I get full lock up if one rear wheel lets go.

I always think of that scene from the film Bullit when someone mentions Tramping, or Wheel hop.
Steve McQueen is giving it large during the chase and you can see the Mustangs Rear wheel hopping like mad from launch.

My MGB seems to be charmed though. I broke a half shaft in Jan 2007, but that turned out to be a faulty one. I'm running a pair of 1800cc Salisbury shafts which are the standard ones for the 1800 and the V8 on this axle.
My old 3500 put out 260 at the fly on a 50 shot on the Dyno. I have been running a 150 shot ever since with no probs. I've now stuck the 4.6 in with a 100 shot and it still wont tramp??

If I give it the beans in a straight line or when pulling out from a junction, if she lets go its just both rears smoking up nice and smooth.

Maybe its a combination of the diff locker and the auto transmission eh?

I've had loads of feedback from MGBV8 drivers who use a manual tranny with no LSD to say that they get tramp quite easily?

In fact, the only feedback I've had about wheels hopping / tramping on MGB's is on converted cars. Possibly because they are still using the 1800 springs?

I'm just glad I tried mine out first, because I came so close to spending a shed load on various upgrades to the axle to prevent tramping. When I reralised that it didnt pose a problem, I spent the money on Nitrous stuff instead. I still get folk tell me that they dont believe my car doesnt suffer from tramping. But it doesnt.
FWIW