To IRS or not? Disks vs drum rear end?

[wattsworth]

Locost builder newby here trying to weigh the merits of a trailing-arm independent rear suspension with few differential choices vs. a live axle with many choices for differential and gearing? For this Toyota-based Locost build, the rear axle choices are a Mercedes 240D or 300D IRS with disk brakes (LSD might be found, but they are kinda rare and very few gear choices are available) or a drum-brake 8" Toyota live axle with lots of different differentials. While I have both in hand now, are there better choices out there with a 5 on 4.5 wheel bolt pattern?

[locostv8]

... are there better choices out there with a 5 on 4.5 wheel bolt pattern?

I'm attempting to build one of each. The "better" choice is a mixture of Tbird, Cobra and MK VIII pieces which is an aluminum cased 8.8 with a wide range of gearsets available for cheap.

[WestTexasS2K]

5x4.5 bolt pattern is common. It is used on mustangs and some nissan imports 350z,300z so wheel selection is good. I would prefer the limited slip with a gear ratio that is favorable for your intended use over the IRS or straight axle. Straight axles are easier to set up and can run with an IRS all day. The down side is it can be a little heavier depending on the IRS unit used and has limited adjustability.

 

The IRS can provide a little smoother ride but is more complicated to set up correctly. Be careful to check bump steer on an IRS if it is out of whack the car will handle horribly. With the added adjustability you can chase your tail a bit trying to get the handeling sorted. So pros and Cons to each.

[eVox]

I am building a locost with a Toyota drivetrain. Currently, it is planned to be IRS, using modified toyota hubs, from a 3rd gen IRS celica. Nothing behind the rear bulkhead has been built yet.

 

 

From what I gather, the only discernable difference IRS makes is in ride quality. With less unsprung weight in an IRS, there is obviously less mass to move when you hit a bump. Remember though, what you are building. It is not exactly known for its ride comfort.

 

Simplicity wise, the solid axle wins. In my case, I need to re-engineer the existing hubs, design the rear geometry, and package it all. Then build it. Right now, I am stuck with Celica hubs that I cannot get apart. The solid axle would just need to be bought and attached.

 

 

You also have more rear end space with the solid axle (good for a fuel tank and a trunk storage area).

 

 

I would recommend using a solid axle over IRS, just to avoid the headaches. I may be switching to a solid axle plan from my IRS plans. I would use a 3rd gen Celica GT rear axle to keep my tracks the same.

 

 

 

 

If you truely are wondering about a trailing arm independent suspension, or a semi-trailing arm suspension, you might want to look here:

http://www.locostusa.com/forums/viewtopic.php?t=1949&highlight=arm

 

Basically, trailing arm suspensions arn't really worth a thought.

[Mandurath]

Unless building a light weight version, I wouldnt worry about the brakes too much either. Drums work fine, look at the Stalker guys. Not sure if there are disc conversions if you want them on the rear, but there probably are with it being Toyota. I am using a 7.5 out of a mustang on mine (+4).

[slngsht]

Theoredically IRS is a better setup, particularly for the street, but there is MUCH more to screw up in its design and tuning.

[jbanker]

Yes, you can put rear disk brakes on a Stalker. Several owners have them on their cars.

 

Jack

[wattsworth]

Guys, thanks for the ideas and feedback!

 

Perhaps I should say what I'm after. My aim is to build a fun Locost 7. I'm not planning on putting in any major horsepower. I like simple, efficient, reliable equipment that I don't have to fix very often. Becoming a slave to a toy ($$ and time) just sucks all the fun out of it for me. Hence an easy to work on, rather bomb-proof old Toyota doner to start with (1600cc 2TC or upgrade to a bigger 3TC engine) . I might never visit a track -- just enjoy some twisty roads in good company. I'm willing to go to some extra trouble if it looks to be worth the effort, but I don't want to loose sight of my intended purpose ... fun!

 

While the MBZ IRS is a trailing arm type of design, the hinge axis is not parallel to the half-shafts driving each wheel. As the suspension compresses, the top of the tire angles inward. When unloaded, the opposite also happens -- changing the camber. Have you guys looked at this? Is it worth anything? One thing I like is that the entire IRS from the MBZ is bolted to the chassis via elasnomeric mounts. Just loosen a few bolts and out drops the entire IRS with disk brakes at the wheel and parking brake cable. If the track-width is right and the weight isn't too bad, you can just bolt it to your frame and have the driveshaft altered to fit. It doesn't get much easier ... but is the complication of the MBZ IRS worth it? On bumpy roads the MBZ has some wheel-hop which I really don't appreciate. There were some MBZs with limited slips, but I suspect finding one in junkyards in my area is probably a lost cause.

 

I agree that a live axle would be simple & reliable -- which is very attractive. But having basically no experience driving performance cars (my MBZ has a nice cushy ride, but it has no pickup at all), I don't really know what I'd be giving up by choosing a live axle over IRS. I do know that I can find all sorts of diffs for a Toyota 8" if I want to spend the money. Yes, I could probably arrange a rear disk brake conversion for a Toyota 8" axle to make stopping much more dramatic. I would also do a triangulated 4-bar linkage, probably with coil-overs. I do actually like a fairly hard ride so I can feel the road.

 

So which set-up am I going to like better in the long run given my stated purpose?

 

I really appreciate your help steering me right because I don't want to waste time or money and end up with something I don't like.

[eVox]

While the MBZ IRS is a trailing arm type of design, the hinge axis is not parallel to the half-shafts driving each wheel. As the suspension compresses, the top of the tire angles inward.

 

What you are describing is a Semi-trailing arm design. While the camber DOES change like you mentioned, the toe also changes. This was also sometimes referred to as a '4-wheel-steering' mechanism (think mitsu 3000GT). This is all fine and dandy until you go into a turn or a bump and the car doesn't behave predictably. Really not much merit to this system besides it's simplicity.

 

If you are going to just drop in a premade subframe that uses a semi-trailing arm, its not the end of the world. Building one from scratch however, is a poor use of time (as you could build a much better design).

 

A premade semi-trailing subframe will add a lot of weight to the build since the parts were probably designed for a heavier car. It will save money and time.

 

 

 

In your case, I would reccomend a solid axle, 4-link design with a panhard bar, as in the book plans. Definately the easiest, good approach to the rear suspension. It will be easy to design, and handle well.

[slngsht]

In your case, I would reccomend a solid axle, 4-link design with a panhard bar, as in the book plans. Definately the easiest, good approach to the rear suspension. It will be easy to design, and handle well.

 

I agree...

 

But if you're not sure, round up some cali guys to give you a ride in theirs (solid and irs). Chances are the differences will be minimal.

[xromad]

Keep it simple but effective! Don't over engineer it. Especially don't over engineer it before you have it on the road.

 

1. The stick axle is a darned good design, especially for your intended usage.

2. If the axle comes with drum brakes, use them until you have the car on the road! Then worry about a disk conversion.

3. Don't get all funky about anti-dive, anti-squat geometry, dynamic camber.......... Just put together a solid neutral design.

4. Stick to a nice flexible motor/trany combo that will keep you happy for years to come.

5. For the front suspension, remember that safety and strength are VERY important.

 

Many of the cars you see on this forum (and in internet photos in general) are focused to meet the needs of the owner/driver. In some cases they realy are a better design but only the skill/knowledge of the builder has allowed him to pull it off. If you try it and you don't have his skill and it will end in failure. Some designs are just BAD and the owner doesn't realize it, or the owner realizes it but the context of the photo doesn't tell you it is bad. Some designs are good but only for very specific purposes. And, some designes, while not bad, merely reflect the mass hysteria of the day.

 

Find a good simple design that is within your skill level and use it. Hint, if you build by the book, you will be pretty close. If you build by the book and use your head, you will be very close. And, if you build by the book, use your head, look critically at existing cars, and ask lots of questions you will be right on target.

 

Brent

[wattsworth]

You've convinced me to do a live axle. Thanks very much for making the pros & cons clear!

 

I've found a stock axle that is perfect! It's a Toyota, lighter than an 8" diff, matches my doner well, has a triangulated 4-link suspension deisgn with rear disk brakes and an available LSD. Anybody in California have a 1985-ish AE86 Toyota Corolla GT-S parts car with a T282 or T283 rear axle they want to part with? Look on the placard in the engine compartment on in the driver's door jam for the code that looks like this: C/TR/A/TM 138 HR11 T282 T50 or C/TR GJ7/KQ41 A/TM T283/T50. These numbers are the Color (paint) code, Trim Code, Axle Code, and Transmission Code.

 

Any help is appreciated!

 

[wattsworth]

I did a recon at two local pull your part yards and found a number of Toyota live axles with 4 link suspension, panhard bars, drum brakes, and ratios from 3.4 to 3.9 from as early as 1978. Also present were two Toyota IRS limited slips: a 7.5" two-pinion from an '85 Celica Supra with the unfortunate trailing arm design and an 8" four-pinion with ventilated disk brakes from an '87 Supra with wishbone rear suspension. Seems like finding a good rear axle system for my project won't be so hard after all -- even if the elusive '85 AE86 live rear axle with disk brakes and a limited slip can't be found immediately.

 

Any opinion on the merit of four pinion diffs vs only two pinions? One available choice is a 6.38" ring gear with a 3.909 ratio and four pinions. Would this be stronger or weaker than a 4.3 ratio 6.7" ring diff with just two pinions? I presume the 6.38" diff would be slightly lighter, which could be good.

[solder_guy]

What is the bolt pattern of the 85 AE86 GTS rear? Is it 4x100 or 4x114.3?

 

Rob

 

[SkinnyG]

AE86 is 4x114.3mm (4x4.5" ). I'm using Chevette front spindles (4x100) that have Datsun 210 hubs and rotors mounted (4x114.3" ).

 

I've been waffling between IRS and solid axle for a few years now. I still haven't really made up my mind.

 

Reducing unsprung weight can be a significant advantage in motorsports, and certainly comfy in street use.

 

The original 7 chassis and the Book locost chassis is really designed to have the rear "axle" loads taken through the outer sides of the frame, not the center like you would have to to mount an IRS pumpkin. To mount the diff there properly you would have to triangulate the chassis to bring the loads back to the outside - by bracing and/or through a tunnel. Using a Sierra/Merkur diff like Caterham does makes mounting pretty easy, but available gear ratios suck (deeper gears are available, as well as LSD, but we wouldn't be building a Locost if we were willing to afford the real McCoy). deDion runs the hub forces back through the sides of the chassis, unlike a double wishbone on the rear needs.

 

For an ultimate lightweight like I want to build (for the next one), I'm not sure the IRS, with its required additional bracing, is lighter than the solid axle on a simple chassis. I'm undecided.

 

If you can build a front suspension, rear is easy.

 

IRS:

+ Better ride quality

- More complex

 

Axle:

+ Simple

- Better acceleration/braking? (no camber change on power/braking)

- Torque steer as chassis rotates (on/off power)

 

What may really sway me is I already have an axle and LSD.

 

G

[solder_guy]

AE86 is 4x114.3mm (4x4.5" ).

 

Well that being the case .. then the Supra rear solid axle on the '79-'81 Supra's will be a choice also. 4x114.3mm The '80 and '81 cars had LSD as an option. Supra '82 is the year that it began as IRS. These early Supras use shims to hold the disc calipers in place .. whereas the later AE86 must use the improved "pin sliders."

 

Rob

[gjslutz]

I had thought the live axle in the Birkin I purchased would be a problem for street driving. Everything I had for years was IRS. The live axle works very well in the car, I wouldn't go through any work to change it even if it was free. I drive on many uneven roads with a fair share of bumps, the car handles great, except for Tirestones (Firestones). The ride is also quite good, better thsn expected. I'll wait till spring for some fresh soft rubber, but looking for another set of tires & wheels now.

[slngsht]

 

Well that being the case .. then the Supra rear solid axle on the '79-'81 Supra's will be a choice also. 4x114.3mm The '80 and '81 cars had LSD as an option. Supra '82 is the year that it began as IRS. These early Supras use shims to hold the disc calipers in place .. whereas the later AE86 must use the improved "pin sliders."

 

Rob

 

I have one that's been sitting in the back of Dennis' shop for almost a year now . It has brackets welded on for a 4 link setup.

No LSD, but rear discs.