Rear Spring Rates

[lil_red_roadster]

A question/invitation to discuss etc...

Has anyone considered whether the spring rate of the standard Marina 2-leaf (113lb/in iirc) is excessive for a Roadster?

If so what other conclusions did you reach..?!

My thinking is that firstly I need to get the car weighed and then do some proper calculations, but the facts that stick in my mind are that the Roadster is obviously lighter than a Marina, and earlier MGB Roadsters (still heavier than a Roadster) used c93lb rated springs...later were a bit higher to offset the weight of rubber bumpers and middle-aged spread.

So...hence my question...backed up by the fact that whatever I do with tyre pressures and damper settings the back still feels unsettled, especially on the back roads (keeping up with some crash-helmeted bike-engined Se7en clone the other day had me bouncing all over the place).

Conventional wisdom (for live-axled RWD) is soft rear springs and firmer at the front.

Over to you guys...!

[daniels917]

Re: Rear Spring Rates

Iinteresting thoughts Jez, mine now seems a bit hard, as the back is on 195x50x15 rubber so less spring in the side walls but I can get it on to the bump stops if I push hard on the back roads round here! so do we need softer springs and better damping, and a longer travel? I am playing with damping and only running 18psi in the tyres any other ideas are welcome.

[stevejgreen]

Re: Rear Spring Rates

Hmmm.
The Marina is around 950Kg a Marlin around 720Kg it's only 25% less.
Part of the history of a Marina was its bad handling IMHO quite likely due to 200Kg of engine and gearbox primarily in front of or at least over the front wheels.
A Marlin, complete with driver, certainly one of my size, is almost a mid engined car, I am tempted to stick it on a weigh bridge front then back to see what the real balance is like, but my guess is that it will be pretty even. Difficult to tell from a stock 1.8 as I have a Ford gearbox and a Dolly back axle but it will be near enough for others to judge.
I have standard Herald shocks on the front and Spax adjustables on the rear. if I set the Spax to their minimum settings I can easily bottom the rear suspension on my local country roads, not comfortable) so they are set quite stiff.
I am on 185/70/13 rubber with 18/22psi (F/R) and am in process of fitting MGF 205/50/15 wheels and tyres.
I suspect that the current 185/70 tyres deflect quite a lot both vertically and laterally and am anticipating a stiffer ride on the 205/50's and will soon be following Merv's experience and lowering the pressures even more. I am hoping though that the wider lower profile tyres will reduce further the tendancy to rear steer. Incidentally I have a full set of Superflex bushes front and rear.

My feeling is that with a stock Marina wallowing down the road (I used to drive one in '74) that the spring rates are on the soft side for a Saloon car but probably OK for a lighter Marlin.

[lil_red_roadster]

Re: Rear Spring Rates

I'm running 185/70/13 at about 18psi rear/20psi front. Front bars are Estate-spec so a little stiffer than standard.

Damping is by Gaz adjustables at the front (which handles well apart from the perennial absence of self-centering). I was running about 3 clicks and at high speed there was a degree of "wander" especially when overtaking/on camber changes...simply increasing the damping by a couple of notches made a world of difference to that. I may try increasing the tyre pressures a little more at the front...it might make the steering a little more responsive.

Rears are Spax on about 5 clicks...lower than 3 and I get too much wallowing and some grounding any higher and there isn't any spring movement..! They do seem very sensitive...I guess it's just trial and error to find the sweet-spot...perhaps knocking it back by 1 click may just do it..!

I think before the tax runs out I'll find a weighbridge and do some calculations. I certainly need to Superflex the rear over the winter and was going to treat myself to a new set of springs as the existing set are showing their age.

[b_caswell]

Re: Rear Spring Rates

One problem is if the tyre pressure is too low the car bounces around on the tyres instead of the shocks working properly.
One intrepid owner who broke a spring while on holiday in Italy(quite a few years ago) had to resort to a local mechanic. He managed to make a pair of Fiat panda springs fit and that worked perfectly. Its in a Pit Stop somewhere!

[stevejgreen]

Re: Rear Spring Rates

That was a different experince...
Went to my local(ish) public weighbridge and VOSA were there. Apparently this particular weighbridge is 24Hrs self weigh but they cannot print me an official ticket.

The results were as follows, with me in the car.
Front Axle 375Kg Rear Axle 460Kg Total Gross weight 835Kg
That was with half a tank of fuel (by a Marlin gauge) and 85Kg of me and a few other odds and ends.
That would give a dry weight of the car at around 720Kg.
If you assume that most of my weight and the fuel was over the back axle it suggests that the dry balance is pretty much 50/50.
It would be interesting to weigh a Marina in the same way and to compare the results.
My guess is that the Marina has more weight on the front axle.

Ben is right about tyre pressures and side wall deflection. Certainly the early Marinas were on crossply tyres that would react very differently to a radial.
I think any setup will be a compromise between tyre profile, pressures, spring rates and shock absorber settings and ultimately will come down to personal preference.

But none of this answered Jez's first questions.

[donsayers]

Re: Rear Spring Rates

I suspect that the front/rear balance on Marinas differed depending on the engine. I never remembered my old 1.3 under steering, but the 1,8 I drove recently was simply dangerous. I did wonder whether I was going to make it round the roundabout.

The rear spring rates varied too, the estate had much stiffer springs. Never having been much of a fan of leaf springs as such I did find a company that would make a custom single leaf parabolic, but I am blowed if I can find the link now.

[stevejgreen]

Re: Rear Spring Rates

The weight of my car is based on all the various mods I have made.
It started life as a 74'1.8 Tc based model but now has a Ford Type 9 gearbox (heavier) and a Dolomite Sprint back axle (heavier) as well as several other mods.

As the weighbridge was being used for enforcement by VOSA I did not get the chance to weigh the car without a driver, perhaps next time....
There are many 'Free' public weighbridges around the country some of which are 24Hr self operating.

So far as solving the understeer problem, Lee Hunter has posted that he has re-engineered his Roadster front end.
I for one would be interested if he has drawings of the relevant mods.

Custom road springs are available from...

Midland Motor Springs - http://www.springs.me.uk/

Jones Springs - http://www.jones-springs.co.uk/home.html

ML (UK) Ltd - http://www.mluk.co.uk/leaf-spring-manufacture.php

Other alternative suppliers exist but these are ones I have had some dealings with over the years though never on a Marina/Marlin.

[lil_red_roadster]

Re: Rear Spring Rates

Thanks all...especially Steve...you may have saved me a trip to the weighbridge.! I think I have enough figures to play with...just need to find the right formuale...

[lil_red_roadster]

Re: Rear Spring Rates

A quick play with the numbers does suggest a reason for the rear handling issues...the front and rear wheel rates are almost identical...there should be (ideally) a difference of about 10 biased to the rear. With even rates you apparently get a porpoising effect over undulating surfaces...which seems to be what I'm experiencing.

Cures would be to increase the rear spring rates (overkill on a Roadster), decrease the front spring rate (i.e. move to standard-spec torsion bars) or increase the rear unsprung weight (finally fit the Sprint axle).

Lots more to think about and research/analyze/refine. Plus I need a definitive (rather than guesstimate) unsprung weight figure for the front. I have a firm number for the rear.

I feel an article coming on...

[stevejgreen]

Re: Rear Spring Rates

And Finally.
Kerb weight of Marina 1.8 TC Coupe 957Kg
Marina 1.8 TC Saloon 965Kg
Assuming a 50/50 weight distribution but I am sure that the front end will be heavier, that puts Dry weight 482Kg on the rear axle.
I have asked on the Marina Forum for more accurate front and rear weights. It might help for comparison.

[daniels917]

Re: Rear Spring Rates

All that seems to explain the horrible porpoising i had recently on the concrete section of the A52 near Bingham, I thought that the beast had broken again, but I think the expansion joints are at just the wrong spacing for a short roadster, avoid in the future.