IRS vs Solid Live Axle?
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Thread: IRS vs Solid Live Axle?

  1. #1
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    Default IRS vs Solid Live Axle?

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    So my question is if there is any general consensus on whether a RWD rally car should have Independent Rear Suspension or a solid live axle?

    I've been around and crewed for various different race teams (open wheel circle, drag, Formula SAE and a rock crawler). While I've always enjoyed watching rally, I've never had much exposure to it in person. So, as part of my plan to get my feet wet, this year I'm hoping to volunteer at whatever races will have me in Oregon and Washington. In the mean time I want to start reading and researching what vehicle I might want to start hacking on - I'm leaning towards RWD (FWD to place, RWD for more smiles per mile?), but not necessarily a Volvo 240 .

    So this brings me to IRS or solid axle - arguments of axle strength and available traction devices aside, is one better overall? I understand a live axle is generally stronger since there are no CVs, but wouldn't the reduced unsprung weight and adjustable alignment of IRS be better over rough surfaces, or are live axles popular because there's not much alignment that can get knocked out.

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  3. #2
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    IRS on the bug. I've never broken an axle, cv, suspension part, etc... and the car has taken a beating (hitting berms, ditches, rocks, culverts, tree stumps)... knock on wood.

    I think you'd have to start putting down some serious HP to start breaking axles/cv's. And, you are on a loose surface so I'd think it would be hard to do.
    Behold the POWER of cheese! www.huebberally.com
    Organizer: www.100aw.org & perryville.100aw.org/ | Rally Car: 1970 VW Beetle

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    I have a solid front and rear axle. They are beefy, and I still managed to bend them (blew out the shocks, bump stops tore off, and then a few hard landings from jumps did the rest). Honestly, if you are new to rally, it really isn't going to make a difference which way you go. The one drawback to a solid axle are leaf springs (if you have them), and I only say it's a drawback because it takes some R&D to figure out the right arch and spring rates that work, but you can't swap springs around as easily as coils.
    Jim Perrin - #716, but no more Jeep.... - TeamHazardRally.com

  5. #4
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    I'd suggest choosing neither until you've gotten some more exposure to the sport.

    The world is chock full of half started, never finished, or barely used rally cars. I don't hear of too many people that build one, then realize they wanted something else, and actually building it.

    I've been driving the same POS car for 5+ years....

  6. #5
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    Thanks for the replies.

    Quote Originally Posted by bentmettle View Post
    I'd suggest choosing neither until you've gotten some more exposure to the sport.

    The world is chock full of half started, never finished, or barely used rally cars. I don't hear of too many people that build one, then realize they wanted something else, and actually building it.

    I've been driving the same POS car for 5+ years....
    Good advice, for sure. I'm at least 9 months away from pulling the trigger on another vehicle, so I have plenty of time to change my mind yet. In the mean time, I'm gonna keep reading so I know what I'm looking for when the time comes

  7. #6
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    Live axles can be nasty on stutter bumps when in a corner. They have a tendency to skip sideways.
    Dave Cotie
    VA3 COT

  8. #7

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    There was a great video I found once, I think at PassionFord. It was two Escort's. One that had the regular axle and one that they had put a Ford Sierra IRS setup in. BTW: Ford Sierra is a Merkur XR4ti and one of the few RWD cars you can buy in US that was raced by a manufacturer team in the WRC.
    The IRS car looked much smoother.

    But really, what it all comes down to is that the best option is whatever the car has stock! If you go with a Volvo, 4 link and a Toyota rear end and you're done. If you go with a Merkur, Supra diff swap and your done. If you go with a BMW E36 Compact, Supra diff swap and you're done.
    There's more options for cars out there, but honestly those are the ones I'd consider. Toyota Corolla or Starlet would be decent if you can find the car for cheap. You'll have more work to do to even get close to the easy and plenty of power achievable in my three preferences.
    Grant Hughes - 1985 Merkur XR$TI Group 5
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  9. #8
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    Where in the NW are you? Should be a few Rally cars out at Pacific Grand Prix this Sunday.

    I have a Merkur, 250lb ft torque, and have not had any CV/ axle issues to date.
    Build what you like. FWD/RWD does not make any compettitive difference at the club level. At Mount Hood there were 2 RWD cars in the top 5, out of 17 2wd cars. 13 of those were FWD. Just pick something you can get stuff for like suspension, something strong, and reliable, and go have fun!

    Just remember as a NEWB you most build a G2 car for Rally America unless you have much previous racing experience. NASA does not care.
    #141
    1986 Group5 Merkur XR4Ti
    Sean and Jenne Medcroft

  10. #9
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    There is a lot more to this question. Not all live axles are the same, and not all IRS are the same.

    Early Ford Escorts had a live axle and leaf springs, and a lot was done modify them to better control the axle location. Lateral location was poor as the spring twisted, and spring wrap was an issue. A panhard rod was added to fix the lateral location and links added to control spring wrap so these things made this axle behave a lot better, and it was a successful rally car despite a non-optimum rear axle and suspension from the factory. The early 70's Opels had a lot better live axle setup from the box, with coil springs, a panhard rod, and a torque tube. The first weakness in those was the rubber bushing around the torque tube that broke and moved about. But it was genenrally a better starting package than the live axle with leaf springs; these cars dominated Showroom Stock A in the early 70's (but not toally due to the rear axle; that was just a help). Both cars were improved by the mods to upgrade the locating system to a 4-link (5-link if you count the panhard.)

    A swing axle can be counted as an IRS but the Triumph Spitfires had horrible handling with this under braking. The Datsun 510's were quite good for rally out of the box, as they had an early IRS; I believe it was a semi-trailing arm type. Our Starion has a simlar RWD semi-trailing arm and does decently; never broke a half shaft with 250 or more HP. (But I don't abuse them.) But the movement is limited by the geomtery, and the car is limited more by other things than the rear geometry. I would expect a fully independent IRS would be best but again, other factors will likely dominate the car's ability as a rally car.

    Pick a good well known rally paltform with good success, and the fixes/upgrades will likely already be there to make it work as well as it can. I would expect the 'nut behind the wheel' will be the main limiting factor for 98% of ralliers anyway...

    Regards, and best of luck in rally!
    Mark B.

  11. #10
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    Quote Originally Posted by phlat65 View Post
    Where in the NW are you? Should be a few Rally cars out at Pacific Grand Prix this Sunday.
    Eugene, OR - it looks like PGP is a good 4 hr drive for me, but I didn't see anything on the calendar at their website. What's the event?
    I saw volunteer sign ups just opened for the Olympus Rally . . . I think I was #3

    Quote Originally Posted by starion887 View Post
    There is a lot more to this question. Not all live axles are the same, and not all IRS are the same.
    Yeah, I didn't want to get into which link set-up is best. I know from my circle track friends that a well sorted link setup still has alot of adjustability - I'll skip the accel hop, brake hop, rear steer, instant center equation bits for now.


    So from what I'm hearing, it sounds like IRS is generally superior to a solid axle in relatively stock form. Thanks for all the replies so far

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