Rear Suspension and wheels and gaps and stuff

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ericholm
Joined: Fri 15 May, 2015 14:24
Posts: 32

  Z3 roadster 3.0i
Location: Milton Keynes

Rear Suspension and wheels and gaps and stuff

Post by ericholm »

Hi All,

I am converting my Z3 into a Ferrari 250 swb Convertible hybrid

I have changed the back end of the car and need some guidance on fitting the very large rear wheels and tyres.

The current setup on the rear is standard Z3 2002 3.0i suspension (recently fitted new springs and top damper mounts). The wheel and tyre combo is:

Rear - current dunlop 245/40/17 91W
Wheel rear 17" x 8.5 ET41 Type 78

Which looks like this:
Image
Image

I have new alloys/tyre combo to go on and they are:

Rear 17" x 8 ET35
Tyre 235/65/17 104W dunlop

Which looks like this:
Image
Image

So is there a mod that can give me more top gap, i.e. raise the suspension? maybe pad/shim out the rear spring? or a mod that can limit the suspension travel upwards so that the tyre will never touch the wing section? Or if nothing can be done I will have to come down to a 235/55/17. How far does the suspension travel before bottoming out I wonder?

Any comments/suggestions no matter how wacky are welcome.

Thanks in advance.

P.S - if you are interested in my build thread please find it here: http://www.madabout-kitcars.com/forum/s ... php?t=5535
siwilson
Joined: Fri 19 Jun, 2009 09:54
Posts: 790

  M roadster S54
Location: Horley

Re: Rear Suspension and wheels and gaps and stuff

Post by siwilson »

You have a few options.

1. New wheels with a higher offset to bring the outer edge inboard.
2. Adjust the rear camber with an eccentric bush or a weld in adjuster so the top of the tyre tucks inside the arch.
3. Fit a narrower rear end (trailing arms and drive shafts from a 1.9 or an M to bring the wheels inboard.

Options 2 and 3 will help you retain the wide lip alloy, but a bit of work. Mind you you're already in for a penny eh!
2001 M roadster S54 Laguna Seca Blue
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Southernboy
Joined: Thu 07 Oct, 2010 12:39
Posts: 6436

  Z3 roadster 3.0i
Location: Johannesburg

Re: Rear Suspension and wheels and gaps and stuff

Post by Southernboy »

Load up the boot and include your passenger and then see how much weight it will take to bring the body down to the tyre. If it's a considerable loaded weight, then simply go to 55 tyres which should allow sufficient suspension travel...?
You can also look at "sport" shocks which are harder than OEM. This will restrict the amount of travel too.
"Normal is overrated"
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c_w
Joined: Thu 19 Aug, 2004 16:50
Posts: 4032

  M roadster S50

Re: Rear Suspension and wheels and gaps and stuff

Post by c_w »

Iwould try andkeep those tyres as they look period.

You can double the spring pads (And get thicker pads from BMW also), at top and bottom of springs so that would yield 10mm-20mm. You need to mdify the bottom with the useless bump stop (the bumpstops of the Z3 don't come in to force until the car belly is touching the ground in my experience!). But you don't want it looking too high. Can see the wheel from a side position, nbut you can have them fairly flush without rubbing due to the suspension going quite negative camber under load.

To limit travel add bump stops to the rear dampers (also use top mount reinforcement plates if you don't already). I think the ones on my car are from a mk2 golf front bump stop. They are ribbed so can be cut down to size. As a base point mark the damper at the bottom of the dust shield whilst it's on the car at normal rideheight so you know off the car how compressed he damper is static, then fit the bump stop so that it's about 1/2" or so off this point. The reason you can go so close/limit travel is the bump stops are progressive so even if it rides on them a lot you won't feel it. I use them on mine to increase rear spring rate.

I would recommend SPAX rear adjustable dampers (E30 short damper), don't get regular e30 they are too long. You can go from soft as stock to stiff to reduce squatting a bit.
ericholm
Joined: Fri 15 May, 2015 14:24
Posts: 32

  Z3 roadster 3.0i
Location: Milton Keynes

Re: Rear Suspension and wheels and gaps and stuff

Post by ericholm »

Thanks all for the brilliant suggestions, some which would require more work than others.

Ideally I would want the tyre inside the arch, I dont like the idea of a camber as it would need to be significant, Changing the rear drive shaft to an M series is well beyond my skill set and tools.

The 2 ideas that are attracting me currently are going down to a 55 tyre, I'll gain another 20mm clearance. This is a simple and relatively cheap way forward,

Or,

Fitting the bumpstops and trying to stick with the current setup, again a cheap and simple solution, one which I feel I can undertake with my limit experience/skill set.

I have changed the rear springs recently and they were really difficult to pry in and out of place, I cant imagine it will be an easy job (with my limited tool set) to easily fit thicker pads under them or add extra's compared with the work required on the 2 options above.

Thanks once again it has been very helpful. :sunny
therealdb1
Joined: Tue 25 Jun, 2013 21:47
Posts: 263

  Z3 roadster 2.0

Re: Rear Suspension and wheels and gaps and stuff

Post by therealdb1 »

"pry in and out"!
Did you not use spring compressors? Changing springs without them is a very dangerous game!
There also appears to be an emphasis on lack of tools which considering the enormity of the task of building a kit car is surprising.
I hope the car turns out well, certainly what you have done so far looks good, so keep us informed of progress but the importance of the correct tools for the job cannot be stressed highly enough to avoid unnecessary damage to the car but more importantly injury to yourself or anyone helping you.
gookah
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Joined: Thu 07 Aug, 2008 09:51
Posts: 2737

  Z3 roadster 2.8

Re: Rear Suspension and wheels and gaps and stuff

Post by gookah »

ericholm wrote:
I have changed the rear springs recently and they were really difficult to pry in and out of place, I cant imagine it will be an easy job (with my limited tool set) to easily fit thicker pads under them or add extra's compared with the work required on the 2 options above.
Did you disconnect the shock absorber and anti roll bar links, this makes it much easier
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Hairyscreech
Joined: Thu 16 Apr, 2015 12:08
Posts: 69

  Z3 roadster 2.8

Re: Rear Suspension and wheels and gaps and stuff

Post by Hairyscreech »

To be fair the tyres do look period and cars of that time would have been on 16" or 17" wire wheels so the size is not really far off, trouble is the 250 had 15" wheels and 185/80 profile tyres for a diameter of 675mm. (147mm sidewall).

With a 235 width tyre at 65% profile you have 153mm of side wall which is close but because of the 17" wheels you are up to 738mm diameter.

To really work you would want to go down to a 15" wheel with a 215/70, 225/60 or 245/55 tyre. This would look the same as a 250.

250s also came with 16" wheels which would give a 215/60 and 245/55 set up.

If you stick with the 17" wheels your down to 215/55 and 235/50.

Classic crossply tyres or semi radials tended to be 80 profile, if you see a size that is just 185R15 then it will be an 80% profile, often referred to as "full size", however classic tyres tended to be a lot narrower than the radials we use now.
Getting a classic look really comes down to matching rim size and sidewall height rather than profile.

One final important thing, Despite the large profile the old tyres had much stiffer sidewalls and lower grip levels, modern radial tyres in large profiles can be awful to drive as they flex too much, my personal experience has been anything over a 50 profile tends to be too floppy to give good feedback when pushing on.

Your 3l Z3 had equal power, better brakes and a much more competent chassis than the 250 GTO so it might be wise to consider sacrificing a little of the classic look for something that does not drive like a jelly.
Extra load tyres will help in this as they often have dual ply sidewalls, this helps maintain the shape of the tyre under duress and stops the "falling over the wheels" feeling you get from larger sidewalls.
ericholm
Joined: Fri 15 May, 2015 14:24
Posts: 32

  Z3 roadster 3.0i
Location: Milton Keynes

Re: Rear Suspension and wheels and gaps and stuff

Post by ericholm »

Thanks again all,

I am just a hobbyist changing panels on my car and taking on some of the work with basic tools, no risks and if it looks too hard then I am not going to tackle it :D

I am going for the look of the 1961 LeMan car which had huge rear tyres so not stock, however they went under the arches ok. So still want large rears and will try to keep them as big as possible but need to fix it so I don't destroy the arches, back end when driving.

Image

So I will go for bumpstops as a suggestion as easy to fit, cheap and with the size of tyre profile should give me some comfort in the ride. Worst case I reduce to a 55 profile tyre.
littlefeller
Joined: Sun 28 Apr, 2013 18:06
Posts: 683

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Re: Rear Suspension and wheels and gaps and stuff

Post by littlefeller »

what did you do with the old panels? same colour as mine, I need a drivers side rear, not desperately but at some point it has gotten a car park ding (was like it before I brought it)
ericholm
Joined: Fri 15 May, 2015 14:24
Posts: 32

  Z3 roadster 3.0i
Location: Milton Keynes

Re: Rear Suspension and wheels and gaps and stuff

Post by ericholm »

littlefeller wrote:what did you do with the old panels? same colour as mine, I need a drivers side rear, not desperately but at some point it has gotten a car park ding (was like it before I brought it)
Sent you a PM.
Mugs
Joined: Wed 07 Aug, 2013 01:26
Posts: 341

  Z3 roadster 1.9

Re: Rear Suspension and wheels and gaps and stuff

Post by Mugs »

another suggestion, the rear springs from eurocarparts make the back end sit much higher. i fitted them to my 1.9 and the back end went up by at least an inch. i soon took them off and fitted lowering springs as it was way too high for a standard Z3 but it might work for your build?
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Brian H
Joined: Tue 16 Dec, 2008 19:55
Posts: 2505

  Z3 roadster 3.0i

Re: Rear Suspension and wheels and gaps and stuff

Post by Brian H »

Mugs wrote:another suggestion, the rear springs from eurocarparts make the back end sit much higher. i fitted them to my 1.9 and the back end went up by at least an inch. i soon took them off and fitted lowering springs as it was way too high for a standard Z3 but it might work for your build?
Good point, I've seen E36 rear springs fitted on a Zed and this raised the rear by quite a bit.
ericholm
Joined: Fri 15 May, 2015 14:24
Posts: 32

  Z3 roadster 3.0i
Location: Milton Keynes

Re: Rear Suspension and wheels and gaps and stuff

Post by ericholm »

Mugs wrote:another suggestion, the rear springs from eurocarparts make the back end sit much higher. i fitted them to my 1.9 and the back end went up by at least an inch. i soon took them off and fitted lowering springs as it was way too high for a standard Z3 but it might work for your build?
The new springs I fitted were from eurocarparts :lol:
Hairyscreech
Joined: Thu 16 Apr, 2015 12:08
Posts: 69

  Z3 roadster 2.8

Re: Rear Suspension and wheels and gaps and stuff

Post by Hairyscreech »

The race cars had 225/70s I believe.
ericholm
Joined: Fri 15 May, 2015 14:24
Posts: 32

  Z3 roadster 3.0i
Location: Milton Keynes

Re: Rear Suspension and wheels and gaps and stuff

Post by ericholm »

Hairyscreech wrote:The race cars had 225/70s I believe.
Hi this would make them 696mm in dia

My ones are 737mm dia. not much difference. However there is a huge difference in the wheel arch from the original to the kit car :-(
Last edited by ericholm on Fri 21 Aug, 2015 22:41, edited 1 time in total.
Mugs
Joined: Wed 07 Aug, 2013 01:26
Posts: 341

  Z3 roadster 1.9

Re: Rear Suspension and wheels and gaps and stuff

Post by Mugs »

ericholm wrote:
Mugs wrote:another suggestion, the rear springs from eurocarparts make the back end sit much higher. i fitted them to my 1.9 and the back end went up by at least an inch. i soon took them off and fitted lowering springs as it was way too high for a standard Z3 but it might work for your build?
The new springs I fitted were from eurocarparts :lol:
try 15mm spring pads, that should raise it a bit. there is a formula somewhere to work out how much rise you get from the different spring pads. if i remember rightly, 15mm raise it approx 25mm
ericholm
Joined: Fri 15 May, 2015 14:24
Posts: 32

  Z3 roadster 3.0i
Location: Milton Keynes

Re: Rear Suspension and wheels and gaps and stuff

Post by ericholm »

Ok, so having thought long and hard about it, I am going with thicker rear pads to increase ride height a bit 15mm or so giving me twice the gap I have now, I will also add damper bumpstops so that the movement (in an upwards direction) is limited so it wont smash the arches. I may also trim the inside of the rear arches to give a little more room if needed. My fall back position is as above but with lower profile tyre's 55's maybe whish will give me around 20mm more gap.

Thanks all for your help and advice :D :D :D :D
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c_w
Joined: Thu 19 Aug, 2004 16:50
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  M roadster S50

Re: Rear Suspension and wheels and gaps and stuff

Post by c_w »

As mentioned the rear springs are a lot easier when you have disconnected both dampers and both anti roll bar drop links (the 13mm nuts on the trailing arms), they will drop loads and the springs will practically fall out.
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