Rear suspension.

[Stubby Pete]

Hi guys.

I've got a live axle from a mk2 Escort in my kit and I'm limited on the user freindliness of the car due to a limited number of diff ratios available and the problems that a live axle (without LSD or ATB) gives when near the limit. (Purchasing an LSD will eat into the funds for the possible remedy below . .read on)

What I'm looking to do is to utilise a BMW E36 Diff (they come in several ratios which suit my needs) and sierra hubs, if I can't use the same ones.
What I'm looking to do, to make the conversion easier is to use existing rear suspension assembly from another kit manufacturer and adapt it to suit.

Now the strange request: If you don't mind, it would be very useful for me to see the alternative set ups that are out there and therefore, can I ask you to post pictures of your rear suspension set ups so that I can then contact the manufacturer for the relevant bits I need.

The most suitable I've seen on surfing is the MK Engineering Midi set up but if there is a car out there that has a rear subframe that I can use to assist the fabrication, it would make my life a lot easier.

Many thanks.

[Sam_68]

My advice would be to forget it: competently modifying an existing spaceframe to accept IRS is a far from straightforward task.

If you're really intent on such a conversion, then maybe think about a de Dion set-up, so that you can keep the existing longitudinal and lateral location and only need to worry about mounting the diff.

[singlecoil]

You haven't put the make/model of car in your post, but looking at your profile I'm guessing it's a Sylva car. There was a 'discussion' here recently which delved into that particular design and considered many aspects of its performance.

I'm familiar with the live axle design, and most of the IRS designs, and I personally think you would be better off to keep it as is, make sure every aspect of it is working properly and to choose the diff ratios as the problem to deal with rather than the problems of changing to IRS or De Dion.

[Stubby Pete]

I don't see how that would be that much easier to modify to, especially as it's the diff that I need to change and that has to be mounted to the chassis.

I will look into it though, thans for your reply.

[Stubby Pete]

singlecoil said:

You haven't put the make/model of car in your post, but looking at your profile I'm guessing it's a Sylva car. There was a 'discussion' here recently which delved into that particular design and considered many aspects of its performance.

I'm familiar with the live axle design, and most of the IRS designs, and I personally think you would be better off to keep it as is, make sure every aspect of it is working properly and to choose the diff ratios as the problem to deal with rather than the problems of changing to IRS or De Dion.

I absolutely agree in principal, the prblem is that to change the diff ratio alone is not possible. I need a ratio of around 2.9:1 from the English Axle (not Atlas, guess I got the donor car wrong?) The only widely available diff with close to this ratio comes from the E36 BMW or I can get the gears made by Elite Racing but not for this style of diff, also there would be no LSD or ATB.
Burton, uaife etc only shorten the ratios, no-one raises them, the longest available is the 3.54:1 which is what I have. The only other option is to put much bigger wheels on which would ruin the look of the car and yes, you're correct in your assumption that it's a Sylva Phoenix.

[singlecoil]

With a wider car, one approach would be to fabricate a Haynes Roadster set up, where the book (Build You Own Sportscar on a Budget) will give you all the wishbone dimensions, and the mounting points, and details of the how to fabricate the uprights to use Sierra components, then to find a way of mounting the chosen differential by supporting it in position and welding in tubes and brackets to suit.

Trouble is, the Escort axle is quite a bit narrower- but how about a big-arched Phoenix? Or swap the chassis for a ready made IRS version, as the body will have to come off anyway?

[Sam_68]

Stubby Pete said:

I don't see how that would be that much easier to modify to, especially as it's the diff that I need to change and that has to be mounted to the chassis.

Because you'd only need to work out mountings for the diff (relatively straightforward and not sensitive to geometry - it can be supported off the back of the transmission tunnel tubes; it's just a matter of making sure teh mountings/bracings are strong enough), not the pick up points for the wishbones.

Bear in mind that a competently designed IRS will have been specifically designed to work in conjunction with the front suspension of the car - the front and rear roll centres need to work with each other, apart from anything else, so even ignoring the physical complexity of locating the pick-ups in positions that the original chassis wasn't designed to accomodate, grafting on the rear suspension geometry from just any 'Seven' type car is potentially opening a can of worms on the handling and set-up front.

I too was assuming you weree talking about a Sylva/Fisher derived car, from you profile, in which case the (apparently) obvious solution would be to graft on the rear suspension from one of the IRS models. If you talk to the factory, though, I'm sure they'll tell you that by the time you've stripped everything down to facilitate access and accurate jigging of the wishbone pickups and worked out how to add the necessary structure, you might as well buy yourself a new chassis to swap the parts into.

Edited by Sam_68 on Sunday 4th April 10:14

[Kevp]

Stubby Pete said:

need a ratio of around 2.9:1 from the English Axle (not Atlas, guess I got the donor car wrong?) The only widely available diff with close to this ratio comes from the E36 BMW or I can get the gears made by Elite Racing but not for this style of diff, also there would be no LSD or ATB.
Burton, uaife etc only shorten the ratios, no-one raises them, the longest available is the 3.54:1 which is what I have. The only other option is to put much bigger wheels on which would ruin the look of the car and yes, you're correct in your assumption that it's a Sylva Phoenix.

What engine to push that kind of ratio? Needs to be a screamer or a stump puller. Also as I believe you are looking for cruising not acceleration or lively performance, is a LSD required?

[Sam_68]

Kevp said:

What engine to push that kind of ratio?

Yamaha R1 bike engine, judging by profile. So yes, it's a screamer...

[Stubby Pete]

Sam_68 said:

Kevp said:

What engine to push that kind of ratio?

Yamaha R1 bike engine, judging by profile. So yes, it's a screamer...

Correct-a-mundo!

With the diff ratio I have now of 3.54:1, the car tops out at 120mph (track only obviously) but has absolutely no trouble getting there. In 6th gear RPM/100 = mph.
Therefore 120mph at 12,000rpm. Unfortunately this also equates to 6,500 rpm at a steady 65mph on the road which doesn't make for relaxed driving on a trip or longevity for the engine.

[Sam_68]

Always a problem on BECs.

And if you fit an ultra-tall diff ratio, the tall first gear and closely stacked ratios aren't always ideal, either.

Not that it would make a huge difference, but are there any larger diameter wheel/tyre combinations you could use? Or anyone who makes an alternative set of ratios for the R1 box?

Failing that, I still think a de Dion would be your best bet.

[antnicuk]

I considered this on my stylus for different reasons but glad i didnt. I have to say though re the comments of the rear suspension is designed to match the front set up. I think you maybe giving kit car manufacturers too much credit. When westfield, caterham, slyva moved from Live to IRS, did they modify the front? I suspect not.

It would be nice if there was a complete IRS that could be just welded/bolted in so that you just need to attach the diff cage to the car and the wishbones all mounted to it so the geomitry was set up.

[Sam_68]

antnicuk said:

When westfield, caterham, slyva moved from Live to IRS, did they modify the front? I suspect not.

No, but they will have considered where the front roll centre of their existing suspension was when they designed the geometry of the new, independent rear end.

Admittedly, the roll centre locations/roll axis of a typical 'Seven' type car don't vary dramatically between manufacturers, so you might get away with it - but I was thinking also of things like spring rates and physical positions of mounting points, which do vary quite a bit, so if you simply pull one manufacturer's IRS components 'off the shelf' you may find that you have a lot of structural complications to resolve in before you can bolt a set of wishbones on whose mounting points are in thin air on your current chassis, then resolve a significant mis-match in front-rear spring & damping rates before you can get it to handle properly... it's definitely not a simple solution to a gearing problem.

[anonymous-user]

My Sylva runs a Lotus Sunbeam axil with an LSD, nearly the same size as the Escort, my race mechanic wants to change ot to a Toyota LSD axil as he knows these better, and as fitted to my Locost, the Toyota is almost a straight swap size wize, and all we need to do is fabricate the axil mounts, So toyota is acheap way to get an LSD, if you'd like to know what model axil he used let me know and I'll ask him

[Sam_68]

Berw said:

My Sylva runs a Lotus Sunbeam axil with an LSD

Big problem isn't going to be the LSD; it'll be getting a CWP ratio that's tall enough to work with the gearing on the bike engine.