Wombat 33 story number 1 - My 16v boxer

[the wombat]

Front brake braided hoses turned up today. Just need to get my University work done and I can go and get these big brakes sorted!!

Not sure if anyone has experience of them before, but www.speedflow.co.uk are the company who got these made up.

Very impressed with their service. I emailed them to ask if it was possible, they gave me a good set of instructions as to what they needed to quote, I sent them a 33 flexible and a Vectra caliper hose fitting, and 50 quid later I have a set of braided stainless hoses the right length with the right fittings. Took a week from my initial enquiring to having the hoses in my grubby mits.

Very impressed.

[Steve Webb]

Front brake braided hoses turned up today. Just need to get my University work done and I can go and get these big brakes sorted!!

Not sure if anyone has experience of them before, but www.speedflow.co.uk are the company who got these made up.

Very impressed with their service. I emailed them to ask if it was possible, they gave me a good set of instructions as to what they needed to quote, I sent them a 33 flexible and a Vectra caliper hose fitting, and 50 quid later I have a set of braided stainless hoses the right length with the right fittings. Took a week from my initial enquiring to having the hoses in my grubby mits.

Very impressed.

Nice to hear there are still some places out there that provide a good service.

So what are the specs of the Vectra calipers, and what size disks are you gonna be able to run with?

[the wombat]

The Vectra calipers are ATE single piston and slider off any Vectra 'B'.

They are a straight bolt on replacement but are huge in comparison to the tiny 33 originals. I will measure up the piston area and do some hydraulics calculations (I am allowed, I am a Physics Teacher). I can probably get a part number off the Vectra calipers as well.

Although they are not the most sophisticated pistons in the world, the have a much bigger piston than the originals, and much bigger brake pad area. In theory, I can work out roughly what force you apply on a brake pedal, calculate the moment through the leaver, calculate the force on the master cylinder piston, work out the hydraulic pressure created, work out the force at the bigger piston caliper piston, and use that force to work out the pressure applied at the brake pads to give an idea of the difference (or get a clever student to do it for me).

Disc wise, I am using a set of standard new 155 V6 front discs which are 284mm diameter compared to the 239mm 33 discs.

These will not fit under 14 inch wheels, but do fit nicely under 15s (already done a mock assembly).

I might have mentioned I will be using a 75 V6 master cylinder. It is identical externally, but the bore diameter is 22mm instead of 19mm.

Hydraulically, if I were to use the 75 master with the original set-up, I would end up with less pedal travel (good), but would need to apply more brake pedal effort (bad, as it is already pretty high). I need to check the maths, but using the 75 master with the Vectra Calipers is primarily to shift more fluid with the same travel (bigger calipers = more volume of fluid needed to move the pistons, so more pedal travel with original master), but I think this should keep the brake pedal effort about the same as well for much more force available at the caliper.

The interesting thing (for some) is that pressure = force/area, so there is a play off between more force at the piston, but that force is acting on bigger brake pads. Bigger brake pads/discs are better generally (better heat dissipation etc), but not if the pressure you can exert on the disc becomes less than the original pads and discs.

Will be starting this later this week hopefully, so pics to follow and some technical info and calculations.

[wrinx]

The interesting thing (for some) is that pressure = force/area, so there is a play off between more force at the piston, but that force is acting on bigger brake pads. Bigger brake pads/discs are better generally (better heat dissipation etc), but not if the pressure you can exert on the disc becomes less than the original pads and discs.

I was having a similar discussion elsewhere about brake efficiency and area, it concluded that a larger surface area did not improve the braking ability as such, but did so by improving the heat dissipation ability of the set up via larger discs and pads.

I still can't get my head round it (a much smaller pad just wouldn't work, but is it heat or friction by area?), but the others produced tech articles which agreed with their POV so who am I to argue...

wrinx

[the wombat]

It is an interesting thing to explore (again, for some ).

I think the pragmatic story goes something like:

Effort on the brake pedal needs to be light and with some feel, so as a hydraulic system can be a force magnifier (with the trade off being pedal travel), a small master piston size is good.

Feel can be built in based on the pedal travel versus how far the pads at the other end have to travel, and how the friction material starts to work (heat comes into this as well)

Just to finish the pedal end of things, it is a little more complicated as we are not just applying a force straight to a piston, there is also a lever system (pivot, force, and distance from pivot that the force acts) which also allows some pedal travel to be fine tuned. And there is a servo!

So at the caliper end of things, the bigger the piston, the more force can be applied. The trade off again is piston travel. For a given amount of piston travel and fluid movement at the master end, the larger the caliper piston area, the less movement.

Ideally, if you have a whacking great piston at the caliper end, and the brake pad is the same size as piston, all is well.

I will do the calculations for the original front brakes/master, and the new ones, but from an initial simple view:

Original master (19mm dia) for a force of 100 Newtons, pressure exerted on the fluid = 35.2 N/cm squared

New master cylinder (23mm dia), pressure = 24N/cm squared

So straight away you can see why you have to push the pedal harder on the larger master, as the increased piston area means less pressure exerted on the fluid. There will be less pedal travel though.

At the caliper end, I don't have the measurements yet, but just as an example, lets say the original pistons are 3cm dia, and the Vectra pistons are 5.5cm dia

Area or originals = 7.1 cm sq.
Area of Vectra = 23.8cm sq.

If the pressure of the original master is 35.2, Force at the caliper = 35.2 x 7.1 = 250N

If the pressure of the new master is 24, Force at the Vectra caliper = 24 x 23.8 = 571N

So we have more than doubled the force at the Vectra caliper for the same effort at the pedal (and hopefully balanced how much fluid to move).

So on to pad size.

No measurements again, but roughly, lets say the originals are 8cm x 3cm, and the Vectras are 10 by 5.5

So, original area is 23cm sq. Vectra is 55cm sq. Double this for 2 pads (both sliding calipers so force is still only coming from 1 piston - now that is why multi-piston calipers are better)

Original pressure = 250 / 46 = 5.4N/cm sq.
Vectra pressure = 571 / 110 = 5.2N/cm sq

So there is your dilemma. Pressure at the larger pad is less than the pressure at the original pad (based on my estimates) even though the force at the caliper is much greater.

Amonton's laws of friction state that it is proportional to force, and independent of contact area, so in the calculations above we have produced a less effective braking system, which is counter-intuitive.

When I get the actual measurements of everything, I bet we will find that actually the larger calipers and changed master cylinder make a slightly more effective braking system, but perhaps only by 10% or so.

It is worth seeing if the original master cylinder is adequate for the new calipers at the expense of more pedal travel to move them, and I might try that as it will move things more in the right direction.

So, interesting right (don't you just love Physics!!!)?

There are many more factors or course, and the larger disc and pad to spread the resulting heat and dissipate it is an important factor in brake performance when time applied is taken into account, and the impact heat has on the material in contact, the brake fluid etc.

More on this when I get the actual measurements together later this week. As mentioned, I predict an increase in braking effectiveness of around 10% - so is it worth it for the cost and extra weight, or could the same be achieved just by new original discs, better pads, a good brake service, or even a better vacuum source for the servo!!!

Oh, can you tell I am bored with the Uni assignment I am working on???

[the wombat]

Did a few bits and pieces over the weekend.

- Seat bolster repaired and seat back in
- Wanted to measure the vacuum on each throttle body, but my vacuum gauge was swinging all over the place. So did the butterfly movement by eye and adjusted the linkages accordingly. Ticks over more smoothly now and pick up seems a little more rapid
- I wondered if bleeding the clutch might help with the gear shift, as the fluid looks quite old (grey rather than beer colour). What a PITA. Had to take dizzy cap off, intake hose off, and several other bits and pieces just so I could get a bleed pipe onto the slave cylinder. Even then had to use long nose pliers to get into the space to push the pipe on!!!. Luckily had a long 7mm spanner to reach the bleed nipple. Fresh fluid in the reservoir and clutch, and the gearshift is quite acceptable now. So not planning to change the linkage bushes any more. Will replace the box oil and should be good.

Car already drove well, but every little tweak seems to get more out of it.

[the wombat]

Got a spare set of front hubs sorted this week - new bearings and a splash of paint after a clean up. These are now ready to go on with the new brakes which have also been cleaned up and painted.

Looking through the various bags and boxes of spares I have acquired with each car, I found a full set of superflex bushes for the front suspension, so plan to pop these in when I have the front end to bits.

Apart from that, been spending a few hours working on this:



I bought it because the underside and most of the body are in extremely good condition. No holes, no major rust, just a few dings. Because I have now got the 16v, decided we need to sell something to make sure everything has got dry storage over the winter. This was the one - hard decision as it is a really nice drive (late 8v 1.7 with cat - very torque and laid back).

I have been offered another 16v SW as a swap which is of huge interest, so may not reduce the fleet per plan. Now need to get on and create more garage space!! See where the white door and bin are on the right of the pic. I can fit another 33 in there!!!

[Steve Webb]

I am allowed, I am a Physics Teacher

Is it wrong that my mental picture of you just changed.

Anyone remember Denzil Dexter from the Fast Show?