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IRS conversion
Gre3D - 23/8/17 at 11:00 AM

hello guys,

my car's equiped with a baby atlas axle with a ZF plated lsd.

i had about 5km on my tyres before my last trackday, tyres looking brand new. about 30 laps later the outer side of the left tire is totally worn through the rubber layer and the inside of the tyre is like new.
the right one is ok since there weren't many high speed left hand turns. ( tyre pressure was ok)

now i know the rubber in those tyres were too soft for the track and it's temperature thus getting a faster wear.
but the wear sort off tells me i could improve a lot on laptimes if i were to be able to adjust camber and toe...

so i've been looking around for converting the thing to IRS.
have any of you done this? how did this improve handling on the car? how much time did it take and how much did the weight change?

if found these plans online,
https://locost7.info/files/suspension/RortyLocostIRSAssembly.pdf

any advice from you guys?
thanks!

Neville Jones - 23/8/17 at 05:49 PM

I've built a few irs cars.

Weight of the total rear end stuff is comparable to a live axle, maybe a little less. Depends on how you do the uprights and thicknesses of metal.

Those Rorty plans have a few tubes too many, and then some missing that help.





Uprights are 1.5mm sheet, with through inserts for the hub mounting bolts.
Wishbones are 7/8 x 14g CDW(DOM)
Wishbone inner ends have Nylatron bushes, outers are M10 rodends.

[Edited on 23/8/17 by Neville Jones]

Gre3D - 23/8/17 at 05:53 PM

Nice!
How's the difference in handling? Would you recommend the dimensions of the setup used in the plans?

Neville Jones - 23/8/17 at 06:00 PM

That car above is Sierra driveline, subsequent cars used MX5 diff and driveshafts, and MX5 uprights for economy.

So, you make the wishbone lengths to suit what you are using and your geometry.

Handling of this car and a live axle car are completely different. Once you learn about setups, yyou can even go with asymmetric and fine tune to circuits.

Nevertheless, there's a good reason why all top end race cars are IRS. It is quicker around corners, and about even in a straight line compared to live axle.

BUT, a poorly setup irs can be worse than live axle. You just have to be careful with camber and toe settings.

I've got a set of MAC1 uprights here, and a Sierra drum brake back end with push in shafts if you're interested.

Cheers,
Nev.

[Edited on 23/8/17 by Neville Jones]

Angel Acevedo - 23/8/17 at 09:14 PM

If you are wearing the outside of the wheel you may have too much roll.
Your solution may be easier.
Maybe changing springs or fitting ARB.
Cheaper than changing complete Rear End.
HTH...
There are a few members here that may provide deeper insight.
Best regards
AA

procomp - 23/8/17 at 09:50 PM

Hi.

If your taking the outside edge off the tyre then your running the wrong tyre pressures.
If you put 0.2 neg camber and 1mm toe in on rear axle and set the thrust angle up correctly you'll be hard pushed to find an IRS setup that fits in the back of a Seven that will out handle a live axle.
It just needs setting up properly and controlling the Roll and pitch IE properly setup dampers.

Gre3D - 23/8/17 at 10:10 PM

So you guys think i'm better off adding a roll bar and maybe stiffer springs then to change to irs? That surprises me.
The car does tend to roll more then a bit in cornering, i was planning to look into it over winter and maybe do the irs same time

procomp - 23/8/17 at 11:33 PM

Heres a 245hp Atom V a 225hp LA Gold with live axle not much wrong with the LA nice and Stable.

https://youtu.be/wBR2zCtV1XA

mcerd1 - 24/8/17 at 08:06 AM

quote:
Originally posted by Gre3D
So you guys think i'm better off adding a roll bar and maybe stiffer springs then to change to irs? That surprises me.
The car does tend to roll more then a bit in cornering, i was planning to look into it over winter and maybe do the irs same time


^^ I think the best way to sum up the above is:

- IRS is better overall, but only if its done right (and there is lots of room for error)

- Sorting out your existing live axle setup will be much cheaper / quicker and easier and should get you pretty close to the performance of a decent IRS setup


besides a baby atlas axle with a ZF diff is a pretty good starting point
if you sort out the live axle setup and you still don't like the results, then you've still got the IRS option.....

Gre3D - 24/8/17 at 10:01 AM

quote:
Originally posted by procomp
Heres a 245hp Atom V a 225hp LA Gold with live axle not much wrong with the LA nice and Stable.

https://youtu.be/wBR2zCtV1XA


nice driving from this guy!
previous post you say 0.2deg neg camber and 1mm toe in? how do i go about doing this with the atlas axle and wouldn't this stress the driveshafts too much? or am i reading that post wrong ? :p

Bluemoon - 24/8/17 at 02:02 PM

^^ shims or running a bead of weld (!)/ Someone on here will have adjusted them so can confirm (Search around 2004 on here might find what you need).

Might be worth ringing Pro Comp for the real answer. I think shims might be the best way if the splines can cope with the misalignment.

Have you measured the current geometry? If might already be "off" causing the symptoms you have..