IRS, worth the hassle?

[qwerty]

Is converting a live axled rwd car, namely a Mk5 Cortina, to IRS worth the work or is proper axle location sufficient.

In the real world would I notice a significant improvement in road holding and overall handling?

If so would a Sierra setup be sufficient or is it worth using something else?

[ettorebugatti]

What car are we talking about?

Just trying to swap the suspensions out, is probably not giving the best result. In that case I would prefer proper axle location.

However, a well fabricated IRS with well considered geometry, spring and damper rates could be a thing of dreams.

[penski]

Sierra rear end would give you datsexycambery0... I struggle to think of any benefits beyond that.

[ettorebugatti]

If it is for the Cortina, I doubt you can fit the Sierra axle without hacking the floor pan around.

Another option to consider is a deDion axle, could use the existing mounts for the suspension arms


or a double wishbone system like this:

With a subframe mounted to the original live axle points.

[ferny]

If you're going to swap a lot of stuff around keep the points system in mind?
www.gov.uk/vehicle-registration/rebuilt-vehicles

[qwerty]

This is all just idea's at the moment. Even if I was to go ahead I would want to get the front properly sorted first.

I very much like the look of the double wishbone system to attach to standard link points.

In a similar system I have been looking at what they do with Fox Body Mustangs. Mainly because they use a very very very similar rear suspension setup to the Mk3/4/5 Cortina's



Interestingly on the 99 and 01-04 Mustang Cobra's Ford actually used an IRS setup which bolts straight up to a Fox Body,



Now I haven't checked any measurements yet but I'm imagining that it will be a bit on the large side. Looks like a good setup to work from though.

However I have also been considering a more budget friendly option of beefing the control arms up. I have already got a hold of some aftermarket S13 toe arms to use as top control arms. I was thinking about finding an engineering firm to knock me up some lowers until I spotted these:

www.latemodelrestoration.com/item/SVE-5649AT/1979-04-Mustang-Tubular-Rear-Upper-And-Lower-Control-Arm-Kit
$139!!

Now I'm definitely gunna have to go and get measurements of the arms on Foxbody and Later Mustangs!!

[bortaf]

If it's for the cortina they yes it's worth it for road comfort, not so good for off the line grip, there are some pics of IRSs on cortinas on BSC, if you can't find them i'll have a search for you ?
probably under "Steve Taylor or do-it-sideways" he's converted a few
Ford even did it, they had a test mule Mk5 estate with the sierra suspension fitted

[jrevillug]

^ What Bortaf said; live axle is better for getaways, IRS is smoother. IRS tends to grip better when you hit bumps while cornering too - largely down to the lower unsprung mass, but also because the bump only affects one wheel.

To be honest, unless you're going for VAST power I wouldn't bother - just fit a decent set of bushes in place of the void bushes and maybe look into a watts linkage for lateral location. At least it's not located by leaf springs.

[qwerty]

Thanks for the replies.

bortaf I will go and have a search. I'm after improving the handling of the car. Its not as if Mk5's are renowned for their composure and road holding, but basically thats exactly what I'm after.

jrevillug I'm not aiming for vast power yet but my target is 300hp. I want to improve the chassis, brakes and axle before that though. I'm kinda going with the get it handling and stopping really really well and then up the power.

[1980escort]

I was thinking about this a few years ago for my escort and the advice i was given was "keep taxi suspension on taxis"

Sent from my GT-I9505 using proboards

[mdh]

300 ponies is a lot for a Cortina... Way more hp than a std axle is gonna handle. You gonna go for an Atlas, or maybe a narrowed Volvo axle?

I've seen 5 link conversion kits for Escorts, maybe they'd fit the 'tina?

Defo gonna be needing a build thread! :-)

[jrevillug]

FWIW, I think the main difference in live axle and IRS is in lift-off ovesteer behaviour and, as I said earlier, bumps in corners.

In the Imp, despite the rear engine, I can get away with horrbile amounts of lift-off and still have the back end under control thanks to the decent IRS. Hit a bump mid-corner and the car doesn't change it's line.

In the Rickman that I had, lifting off mid-corner was a surefire way towards the hedge. Hit a bump mid-corner and you can feel the back end of the car step out a tad - but you soon learn to compensate. That was cart springs only though.

Similarly, driving both a Triumph TR3 (955kg, 100bhp, live axle on cart springs) and a TR4A (1020kg, 104bhp, IRS) at work the IRS car gives you more leeway to do silly things, but if you drive it properly there's little in it. Again, the live axle gets a bit unsettled by a bump, but not enough to be a problem unless you get scared and lift off...

OTOH, IRS in itself isn't a universal panacea - witness the lift-off oversteer you'll encounter in an IRS Herald.

Granted, these are all massively down on power compared to your goal, but the cornering characteristics will be much the same when cornering on light throttle - the difference is that you have the option of power oversteer.

I guess what I'm trying to say is that it'll be different, but not necessarily better. Perosnally, I doubt it's worth the hassle of a conversion when it is possible to make a live axle car go round corners nicely.

Also, popuptoaster had a Cortina with lots of power (BOA IIRC) and the original live axle - it did wheelspins everywhere, but that's to be expected from a FE/R setup and skinny-ish tyres.

Jun 24, 2014 21:57:29 GMT mdh said:

300 ponies is a lot for a Cortina... Way more hp than a std axle is gonna handle. You gonna go for an Atlas, or maybe a narrowed Volvo axle?

2.0, 2.3, and estate Mk3/4/5 Cortinas had an Atlas from the factory. And there seems little point fitting an escort 4-link kit when the Cortina axle is already located by 4 links:



Granted, lateral location won't be great with that setup, hence my suggestion of adding a Watts linkage (panhard rod won't work with those arms...).

[qwerty]

Thank you for the detailed post jrevillug. You have confirmed what I was swaying towards.

My plan for rear axle is to use a fox body Mustang axle. I'm hoping to get away with a 7.5" one for weight and cost purposes. A quick read up on some american websites show it should be strong enough for what I need. It mounts in a similar way requiring the use of cranked control arms.

I'm going to read up more about watts linkages on this type of setup and how to avoid suspension bind.

[stealthstylz]

Cortina rear axle location is a curse word design as standard,and only works because of the huge void bushes in the arms. Ive never driven a live axle car that has been confidence inspiring at the speeds i like to travel. They can still corner quickly but I always seemed to need to correct the steering mid corner. The IRS cars I've driven seem much more planted, my Granada is on narrower tyres than stock and I can throw it into corners at stupid speeds and it wont unstick.

[mdh]

Jun 24, 2014 23:02:02 GMT jrevillug said:

Jun 24, 2014 21:57:29 GMT mdh said:

300 ponies is a lot for a Cortina... Way more hp than a std axle is gonna handle. You gonna go for an Atlas, or maybe a narrowed Volvo axle?

2.0, 2.3, and estate Mk3/4/5 Cortinas had an Atlas from the factory. And there seems little point fitting an escort 4-link kit when the Cortina axle is already located by 4 links:



Granted, lateral location won't be great with that setup, hence my suggestion of adding a Watts linkage (panhard rod won't work with those arms...).

The Cortina may have a 4 link but it's curse word! I had a 1300 under powered dog slow Mk4 and it handled like a broken shopping trolly!

With 300 hp it's gonna need to be done properly, or it'll be lethal. It's also pretty close to the limits for an Atlas.

Pretty pic of an axle...

[PhoenixCapri]

Not much to add really, think the balance between IRS and live has been pretty well covered already. Personally I rather like how 'alive' a well executed live axle feels, and find that on the limit they're actually more predictable and for me easier to handle - might not be going as fast as I would with IRS due to having to correct for any mid corner bumps etc, but I'll have a bigger smile. Then again my first 2 cars were live axle RWD, so it just feels natural to me I guess - the way an IRS loads up just doesn't feel quite right to me somehow.

With live axles for 'road' car, Watts linkage is the way to go really, but to be honest you'd have to push the car near it limits fairly often to really notice the difference to a good Panhard rod setup, and these are easier/cheaper.

I've built my own watts setup on the Mk1 Capri - it's easy enough if you take you're time measuring everything. There's a load of discussion in the build thread about different link setups somewhere I think. Decided to go that way because I had a nice axle setup already and because the car is 90% intended for the track.

Mumford links are always a very left field option?! This is quite an interesting read if you're bored link

[qwerty]

My favoured option currently is a Ford 7.5" with uprated lower contol arms, a watts linkage and a torque arm, removing the upper control arms completely.

Seems a popular setup on Fox Body Mustangs.

Something along the lines of this:

[bortaf]

The rear end works well enough with a panhard rod in fact the south african cortinas are all 5 linked but the top arms are parralel to the lowers unlike the Euro versions, check under the of an XR6 some time
Atlas axles come on mostly Mk4 2.3s and all Mk3s were atlas equiped from new, other than that it's hit or miss as to what axle they have, the later the car the motre likely it is to have a Koln axle (Mk5 2.3s mainly had a Koln) (anorac off, sorry about that)

The main dislike for a live axle is their tendency to patter or walk sideways on bumpy corners, something an IRS solves, that's the main differance with an IRS in cortinas, the cornering is so differant it's like driving another car, the front setup is ok just needs tweaking but sort the rear (horrable compramise) and they are a differant car BUT i'm not sure any of the above is ideal for 300BHP ?
what ever you choose please let us see you do it

[qwerty]

Its Koln axled at the moment, I'm expecting the 2 litre to pop that under some fairly enthusiastic driving.

I'm looking forward to seeing the underneath of an XR6.......not sure when though haha.

This is all just planning stage at the minute, but its helping to keep the mojo up as I try to stitch it back together.

[penski]

Thinking a bit left of field, how about bolting the whole rear end from an Impreza under there?