Imp rear disc brakes?
Chris Morriss
I have a Clan Crusader (Imp suspension etc) to which I've already fitted
front discs using Vauxhall Viva discs and calipers. These work very
well, but to eliminate having to adjust the rear drums, I'm looking to
put discs on the rear as well.
These would require a separate handbrake mechinism of course. I have
heard that Vauxhall Astra disc bells can be machined to fit on the rear
hubs, with Ford Sierra rear calipers used to match. I think the Astra
has a PCD of 100mm as against the 4 inches of the Imp, but the studs are
thicker so AFAIK only the centre hole has to be enlarged to get them to
fit on the hub. (Has anyone got experience of this or alternative
methods?
Thanks,
--
Chris Morriss
Andy Macfadyen
Chris Morriss <cr...@oroboros.demon.co.uk> wrote in article
<ot79+DAd...@oroboros.demon.co.uk>...
To fit rear discs to a Clan you MUST also fit a formula one style
adjustable balance bar and second brake master cylinder to the pedal box.
This might cause problems in a road going Clan as if memory serves me
correctly Clans use the standard Imp petrol tank and the most commonly used
balance bars fitted the master cylinders above the pedal box -- as
fabricated by Davrian. Many years ago Ray Paine at Hartewell used to make
a balance bar that fitted below the bulkhead but these were at bit of a
fiddle and not really popular.
Use 0.7" "Sport" brake master cylinders and no servo. The most
successful racing Clan (the Kenny Allen/Stan Share car) used 10.4 inch
discs all round, this was also the size used by Davrians (including Harry
Simpsons) and Maguire space frame Imps.
The Caliper and disc size should as far as possible be the same front and
rear as under braking there download is slpit 50/50 front to rear. The
parts needed to fabricate a balance bar are available off the shelf from
most competition brake suppliers.
If you are using 13" wheels on the front 10.25 inch 2litre Viva HC/Victor
FD discs work quite well but increase the front track by an excessive
amount (it is already much wider than the rear). Avoid the smaller 8.4 inch
standard Viva HB discs as they tend to warp if the car is raced. 8.4 inch
HC discs are better as they are thicker. All these discs will fit the
standard Viva hubs.
Davrian and Maguire both used identical discs front and rear, the Davrian
pattern brakes fitted over the existing hub like a brake drum in a similar
way to that used on most modern cars, I think they were based on 70's Alfa
units redrilled to fit a 4inch PCD. The exact origin of the discs was a
well kept seceret known only the makers of Davrian (Adrian Evans and Tim
Duffee). The calipers supplied by Davrian circa 1980 were Formula Ford all
round (no hand brake) The rear caliper brackets were very simple chunks of
steel arc welded to the trailing arm. Tim Duffee lives somewhere deep in
Wales is still building rear engined Darren rally cars which are descended
from the Davrian and I am sure will be able to supply a road legal rear
disc conversion.
If you want go the DIY route try the front discs from Ford Sierra ? or
Rover 216?, Austin Montego?, or the front or rears from Rover 820?
re-drilled to 4inch PCD. A lot of people tried using rwd Opel based disc
in the past (1980s) and found it impossible to fit them over the Imp hubs.
Any largish rear caliper should work Sierra 2.8 or Rover 820 Rear calipers
should be fine, don't fit overly large calipers to the front
Viva/Avenger/Mk1 Escort calipers will be more than enough remember 60% of
the static weight is on the rear wheels.
Import points ---
The design of the pedal box is critical get the leverage exactly right or
you will get excessive pedal travel and spongy brakes.
Make sure that the "tilt" of the balance beam is limited so that in the
event of one brake circuit failing the other will still operate.
Make sure all your welds are crack free if in doubt grind it out and
re-weld.
Make sure the calipers sit absolutely square to the discs or a spongy pedal
will result.
Make sure the 4 inch PCD on the discs is accurately drilled and that the
stud holes are close fit for the 7/16" wheel studs --- better to drill 4
fresh holes than to try and adapt ones the almost fit.
Andy MacFadyen
Chris Morriss
In article <01bd13c8$175e87e0$2de8b094@am018>, Andy Macfadyen
<am...@post.almac.co.uk> writes
Viva HB ones of 8.4 inches and seem adequate for road use. The calipers
are also Viva, with a fitting adaptor obtained from the owners club. I
would prefer to find a suitable size rear disc/caliper that doesn't
require a balance bar, (perhaps a pressure limiting valve instead). My
Renault Alpine GTA is also rear engined with a 40/60 weight distribution
and uses equal sized discs front and rear, though it is easy to lock up
the fronts on a wet road all too easily.
My Clan has 165-65/13 tyres all round on 5.5 inch wheels, with different
offsets front and rear to allow for the increase in front track caused
by the disc conversion. The disc conversion totally cured the wheel
unbalance that the old drums always had.
If there was room I'd love to fit a dual master cylinder with adjustable
balance bar, but the fuel tank won't let me. The car has a front
radiator which helps the weight distribution a bit. I ought to get a
grp battery box from Dave Weedon (the joint owner of the Clan moulds) to
mount the battery centrally and a bit lower in the car.
I'll let people know how I get on, there seems to be a number of
Imp/Clan enthusiasts around.
Bye,
--
Chris Morriss
Andy Macfadyen
> >
> thanks for the advice. It's only for road use; the front discs are the
> Viva HB ones of 8.4 inches and seem adequate for road use. The calipers
> are also Viva, with a fitting adaptor obtained from the owners club. I
> would prefer to find a suitable size rear disc/caliper that doesn't
> require a balance bar, (perhaps a pressure limiting valve instead). My
> Renault Alpine GTA is also rear engined with a 40/60 weight distribution
> and uses equal sized discs front and rear, though it is easy to lock up
> the fronts on a wet road all too easily.
>
> My Clan has 165-65/13 tyres all round on 5.5 inch wheels, with different
> offsets front and rear to allow for the increase in front track caused
> by the disc conversion. The disc conversion totally cured the wheel
> unbalance that the old drums always had.
>
> If there was room I'd love to fit a dual master cylinder with adjustable
> balance bar, but the fuel tank won't let me. The car has a front
> radiator which helps the weight distribution a bit. I ought to get a
> grp battery box from Dave Weedon (the joint owner of the Clan moulds) to
> mount the battery centrally and a bit lower in the car.
>
> I'll let people know how I get on, there seems to be a number of
> Imp/Clan enthusiasts around.
>
> Bye,
> --
> Chris Morriss
>
Pressure limiting valves are not a good idea on a rear engined car I have
had a couple really bad experiences with them causing sudden rear wheel
locking in front engined cars -- once in Chrysler 180 and once in Talbot
Alpine and that was bad enough in a Clan it would be truely horrible. Also
you need the extra master cylinder because you need to pump a lot more
fluid to to the wheels with discs all round due to pad "knock back"or you
will get really excessive pedal travel. Standard Imps used 0.625 inch bore
master cylinders, the sports used 0.7 bore, cylinder most cars with disc
brakes use a 0.825 or larger cylinders.
It is possible to make a balance bar that fits inside that car but it is a
fiddle. I did one race with discs on the front in a Davrian Imp saloon
before I swore to fit rear discs, I foud drums all round were much better
than the hybrid setup. But 10 inch disc all round setup is fantastic, if
properly setup nothing out brakes an Imp (except a Davrian because of the
its longer wheelbase).
The other problem you will get with a disc/drum combination is the rear
drums tend to "bellmouth" ie. distort under the force of the brake cylinder
and as a result the rear shoes only rub on the outer edges.
I am pretty sure a lot of special saloon racing Imps are lying in lock-ups
up and down the country so try advertising for discs. If you do fit discs
on the rear it is essential they are as close as possible in size to the
front discs, with small Viva discs this is just about impossible as on the
rear the disc must mount over the rear hub. A Chrysler 180 rear disc would
fit if re-drilled but finding a suitable caliper would be just about
impossible. The complete rear brake set up from a Rover 820 might be ideal
as the discs aren't too big and the calipers are a decent size, and the
scrap yards are full of them !!
Another good tip is make sure you use two adjustable track rods and that
they are set to equal length as elimating unequal bump steer makes a
MASSIVE improvement to Imp based cars under braking.
Andy
Chris Morriss
In article <01bd13dd$70f8dde0$LocalHost@am018>, Andy Macfadyen
<am...@post.almac.co.uk> writes
>
>(A lot of useful info deleted)
>
>Another good tip is make sure you use two adjustable track rods and that
>they are set to equal length as elimating unequal bump steer makes a
>MASSIVE improvement to Imp based cars under braking.
>
>
>Andy
Thanks again Andy, the use of the adjustable track rods each side is one
of the first things I did!
--
Chris Morriss
Andy Macfadyen
Chris Morriss <cr...@oroboros.demon.co.uk> wrote in article
<wc3ryAAr...@oroboros.demon.co.uk>...
> In article <01bd13dd$70f8dde0$LocalHost@am018>, Andy Macfadyen
> <am...@post.almac.co.uk> writes
I checked Rover 820 rear discs are 260 mm (10.25") dia and 10 mm thick not
sure what the pcd is but they have enough material to allow drilling a 4
inch pcd at 45deg to the original and they should fit easily over an Imp
rear hub. The calipers are of floating single piston type with integral
hand brake working on the main pads. The pistons are a slightly smaller
bore than your Viva HB fronts (Girling P14 ?) which should give a safe bias
towards the front.
The brackets required to mount the rear calipers are much simpler than the
front ones. The Rover uses a separate handbrake cable for each wheel which
then connects on to a short cable to the lever.