Brake servo on Marina based cars

[FranklinLeicester]

I am always interested in discussions about Marina master cylinders and servos but I didn't want to hijack Meverett's recent thread on this subject so I thought I'd start my own.
If I was going to do a major re-work of my brake system I would consider doing away with the servo for 2 reasons.
Firstly I find my 1275cc car too eager to lock the front wheels on a damp road.
Secondly, if the servo was not there it would be possible to alter the angle of the steering column to get the wheel at a more attractive position, i.e. angle of about 20 degrees as opposed to the nearly vertical "motor boat" angle.
I have discussed this with Ben and he says that his 6 cylinder car doesn't have a servo, neither did its donor Vittesse.
I maintain that the only reason Paul Moorhouse used them in the Marina based cars was because it was easier to leave it alone as fitted to the donor car than to mess around with a simpler alternative.
As regards the locking up in the wet problem, I have had 3 sets of tyres in 17 years and they all suffer from it.
Any comments?
Franklin.

[andyf]

Re: Brake servo on Marina based cars

I've been having the same problem. I was out last weekend and was locking up all over the place. I'm thinking that there's not enough braking effort on the rear axle. I fitted new brake linings on the rear because the old ones had been contaminated with oil but it still refuses to lock the rears at all.

[b_caswell]

Re: Brake servo on Marina based cars

The rear wheel cylinders come in at least two and possibly more bore sizes 5/8th and 3/4 being the most common so if you can increase the rear cylinder size it will improve the effort to the rear brakes.

Making sure the rear brakes are working efficiently and are adjusted correctly also helps.
Another problem is most replacement linings are now asbestos free and seem to be harder compound that doesn't grab as well.
Its interesting to note that my car is Vitesse front end and engine and the original Vitesse had a reputation for going straight on when braking.

Also softening the front suspension can help as when you brake the load of the cars centre of gravity will tend to move forward loading the front wheels more.

As far as tyres go they do get harder and less efficient as they get older. But these days it is possible to get softer compounds that have more grip.
Tyres that are too big may look good but there is an optimum foot print size that works best for the weight.

[andyf]

Re: Brake servo on Marina based cars

That's true Ben. I have the 0.75 cylinders on the back. I have considered Steve's disc conversion as I suspect it would have improved balance.

[FranklinLeicester]

Re: Brake servo on Marina based cars

Thanks for those comments. I had often thought that the relatively light weight of the 1275cc engine and set well back compared to the donor car leads to poor flexing/grip of the front tyres. I didn't build my car but I don't think the manual says anything about altering the brake balance front and back. Anyway, how about the need for the servo, has anyone tried removing one from the Marina set-up?

[lil_red_roadster]

Re: Brake servo on Marina based cars

Thanks for those comments. I had often thought that the relatively light weight of the 1275cc engine and set well back compared to the donor car leads to poor flexing/grip of the front tyres. I didn't build my car but I don't think the manual says anything about altering the brake balance front and back. Anyway, how about the need for the servo, has anyone tried removing one from the Marina set-up?

Easiest way is to track down a Marina non-servo pedal box along with a correct master cylinder...then swap your already shortened pedals onto that. They sometimes come up on eBay or post an ad on FastMarina.
Failing which you could fabricate an MC carrier to fit on the existing pedal box, bolted down using the 4 servo mounting holes.

[stevejgreen]

Re: Brake servo on Marina based cars

A Marina based Roadster has a very different weight distribution to a standard Marina car where the engine is mounted directly over the front wheels. A SWB 1800 Marina based Roadster is virtually 50/50 weight distribution with a driver and passenger, there is not much difference with a 1300 especially with variable passenger weight! Perhaps Ben has numbers that differ for Triumph based Roadster but I guess the engine position is similar enough to make no significant difference (any thoughts?)

I would far prefer that my car locked its front wheels before the rears, which sounds positively dangerous. The presence or absence of a servo would be largely irrelevant as the same relative force is applied to both front and rear, but without a servo the relative leg effort will be higher, why should that prevent the fronts from locking?

Ultimately, car manufacturers spend hours if not days and weeks, testing different combinations of suspension spring and shock absorber combinations, against brake disk size (generally limited by the wheel dimensions) and the energy dissapation through heat, to try to find a best compromise. Marlin Roadsters take that compromise, change the weight distribution and overall weight in the hope that it all works out. There are bound to be numerous deficiencies.

[andyf]

Re: Brake servo on Marina based cars

I have also considered fitting one of the peddle boxes with a balance bar to be able to adjust the bias... I had a proportioning valve on the rally car and it made it easy to fine tune the balance based on the conditions. So long as you brake in a straight line you're ok ;-)

[FranklinLeicester]

Re: Brake servo on Marina based cars

Thanks for all those comments. Steve, you are quite right about the servo not making any difference to the brake balance but my thought was that if there wasn't a servo, it would get rid of the sudden (panic) braking that the servo tends to give rise to. This is just a layman's thought, not a scientific thesis. My main reason for getting rid of it would be to give more room for lowering the column to give a better (in my opinion) steering wheel angle.
Ben, I take your point about softening the front suspension, maybe I ought to look at my shockers, I think they are adjustable but couldn't say sitting here what setting they are on.
Franklin.

[jhaines]

Re: Brake servo on Marina based cars

My old Marina Roadster Q621CGY had a brake bias valve to adjust the pressure front and rear, you can still get them from the likes of Demon Tweaks and the like. Current Vitesse roadster doesn't have a servo and seems to stop ok despite the considerable lump of cast iron up front.

[stevejgreen]

Re: Brake servo on Marina based cars

Brake bias adjustment is illegal to use in a road car! It does not however make them illegal to fit.

[Tony Stott]

Re: Brake servo on Marina based cars

^ ok, accepted so what should they be set at/to? I have one fitted to my Berly , the valve being installed to better balance the lsd/ disc brake configuration mods up the back!

[milliemarlin]

Re: Brake servo on Marina based cars

Brake bias adjustment is illegal to use in a road car! It does not however make them illegal to fit.

My understanding is that you can fit a device to change front to rear brake bias to a road car but it must not be adjustable. Unlike a racing car where the trained driver can adjust the bias as conditions change.

I could well be wrong though, so check before fitting.
Paul

[stevejgreen]

Re: Brake servo on Marina based cars

By definition a Brake Bias Adjuster, is adjustable. But if it cannot be adjusted from the cockpit of the car, then it could be viewed as permissible in a road car, it's a matter of interpretation.
As for settings, there are too many variables and even comparing to a donor car with different weight disribution, no comparisons can be drawn.
Vehicle designers spend a long time setting up suspension and brakes depending on the feel that they want on the finished car, it's close to being mathematically calculated, but variables like road surface and conditions cannot be ruled out of the equation.
Race drivers are constantly adjusting the bias on a circuit but they are skilled drivers and generally have a lot off run off areas and the budget to repair their cars, and no insurance to worry about.

[scott h]

Re: Brake servo on Marina based cars

^ ok, accepted so what should they be set at/to? I have one fitted to my Berly , the valve being installed to better balance the lsd/ disc brake configuration mods up the back!

During the rebuild of my Berlinetta I changed from rear drums to discs. I thought it would be a good idea to fit a limiter in the rear brake line as it would be very dangerous to have the rear wheels lock before the fronts. So far in short testing with a driving chassis the valve is set to have no effect and the front still locks first. I have standard brakes from an XR 4x4 Sierra all round with Mintex pads. See Berlinetta build and restoration for the full story.