Fitting sidescreens
This technical tip is reproduced by kind permission of Kit Car magazine as part
of Gerrys' Tech Tips series. We shall be including
one 289 relevant tip from Kit Car each issue. If you are impatient, back issues
can be obtained at 25% discount by mentioning that you are a member of the 289
Register.
Cobra-replica sidescreens are available to fit many
makes of kit, but because there are dimensional differences between cars, some
work on the screens is necessary to achieve a neat finish.
To trim the frame to the correct length, open the door and offer up the sidescreen to the opening. A sensible precaution would be
to protect the adjacent paintwork to prevent inadvertant
scratching.
Position the sidescreen fore and aft, such that the
rearmost part of the U-section will align with the rear edge of the door when
it is shut. Then mark top and bottom rails, so that the top rail is about 5mm
short and the lower one is 10mm too long. This is because when the sidescreen is finally fitted the lower rail is positioned
inside and behind the screen pillars.
Now here's the clever bit. Cut a long thin piece of wood or plastic strip about
20mm wide by 3mm thick, and carefully use it to make a template, by drilling two
holes as accurately as possible to match the pins in the bottom rail.
With the door still open, reposition the frame on the car, but with the
template set under the frame. Make sure that the lower rail is positioned
correctly inside the screen pillar (using a scrap of hardboard or plywood) and
check that you have a gap of about 4mm between the rail and the inside face of
the pillar.
Align the rear of the sidescreen frame correctly, and
then draw around the template where it rests on the body at the front and rear
- or draw onto masking tape if the body is painted. Remove both the frame and
the template.
Shut the door. Accurately position the template to the marks
just made on the bodywork, and tape it in position.
Drill 8mm holes down through the template into the door skin. Remove the
template, open the door and check the sidescreens in
position. They should be spot on.
If you are
happy with the fit, open up the holes to take the ferrules (usually 1/2"
UNF) and complete the final assembly. We always use Sikaflex
for final fixing. The sidescreens should be trimmed
in material to match the hood and then glazed with Perspex.
For SVA inspection, if the ferrules protrude inside the cockpit, cover the ends
with rubber or plastic. Covers machined to a domed thimble shape from nylon or
other plastic material give a particularly neat finish.
Fitting a hood to a Hawk 289
It is strongly recommended that you first fit your side screens. In fact, this
is absolutely necessary if you want to fit a hood at a later date and expect
them both to provide anything close to a water resistant fit! Also, if you
intend fitting a hard top, then set the screen to suit the hard top first, then
fit the side screens to suit the hard top. This means that you will be able to
swap from soft top to hard top without any problems. The same side screens fit
both soft and hard tops..
Make sure that your workshop is nice and warm, or that it is a nice warm day.
The hood will be easier to fit if the material is warm. It stretches better.
This means that it is easier to tension.
The hood stick ferrules (two mushroom shaped threaded finishing tubes) fit into
the two small tubes behind the doors. The ferrules can be bonded in place using
black bathroom sealant or Sikaflex but you probably
don't want to do this if the body shell hasn't been painted yet! The two small
tubes into which the ferrules fit are bonded to the body shell when the
manufacturer pre-fitted your door latches.
Assemble the hood frame and insert the ends into the ferrules, so that the
section that hinges is to the rear. Check that it fits nicely and looks
straight. Relieve the ferrules, or frame pins, if necessary, and make sure the
two parts separate easily. They may need a little leaking. When you're happy
with the fit, remove the sticks and fit the two material straps from your hood
kit to the two sticks and re-fit to the car.
Position the two front screen bows in place and fit the over-centre catches. It's a good idea at this stage to have some
scraps of the foam strip placed inside the front bows. This will ensure that
when the front bows are finally fitted the hood tension is still correct. You
may like to cover the ends of these bows in some of the material used for the
hood. This is easily done using impact adhesive and some scrap hood material
that you can normally beg from the hood supplier. Next, fit the side screens
onto the doors and then drape the hood over the frame. Align the hood with the
side screens. It helps to have an extra pair of hands at this stage, but it is
amazing what you can do with masking tape!
Make sure that the sides are aligned correctly behind the door shut lines, centralise the rear, and gently pull the material to shape
so that it follows the line neatly and evenly without any folds around the
back.
With tape holding the front in position centrally and evenly to the side
screens, check that you're happy with the fit, and then mark the positions of
the lift-a-dots. Remove the hood and fit the lift-a-dots. Actually this is best
done incrementally, by fitting the lift-a-dots first of all at the centre rear and at the extreme front, just behind the
doors. It is then a lot easier to work round each side tensioning the hood as
you go.
When completed, re-fit the hood which is now held securely by
all the lift-a-dots and tension the front over the top of the screen.
Check, and then apply Evostick, or similar good
quality trimmers impact adhesive, to the front edge of
the screen bows. Pull the hood forward over them, and when correctly aligned
and tensioned, pull the hood down onto the adhesive and press down evenly. When
it is bonded, unclip the front bows, trim off surplus hood material, leaving
enough to wrap around and stick inside the frames. The hood material only wraps
around two thirds of the inner section of the bows. It should not be stuck to
the rear of the u-section. A strip of adhesive rubber seal can be fitted inside
the bows to avoid scratching the windscreen.
Brakes - David Pilbeam
I suppose the first time I really thought about brakes is when I purchased my
Hawk kit. Most Hawks were using a modified or new MGB braking system. I did not
see the need for the Tilton dual brake cylinder system and sought advice as to
whether a servo was necessary.
My perception at that stage was that a system with two master cylinders and a
brake balance bar was unnecessarily complex and expensive, but a servo was
desirable to limit the pedal effort. After all, if you mostly drive one of the
euro clones you can be sure that it will come with a servo as standard. Of
course, a kit car is lighter, so I don't need a servo do I? However, I did want
a dual circuit system for safety reasons, but, never mind, I could easily
retrofit a dual circuit single brake master cylinder.
Well, as always, my perceptions were wrong. The Hawk 289 doesn't weigh much
different to an MGB roadster. A properly designed braking system does not need
servo assistance and it isn't possible to easily retro fit a dual master
cylinder, unless you hack a hole in the wheel arch. Two single brake cylinders
are shorter than one dual cylinder!
Of course, it took me some while to come to these conclusions. Looking back,
having bought the kit, my main aim was to get it on the road. I could have faffed about attempting perfection but back in '97 the SVA
was lurking ominously round each monthly corner. Fortunately, the deadline was
ever disappearing into the future, but if I had not had a deadline, I would
still have been building the most perfect Hawk 289 this side of the millenium, and it still wouldn't be on the road! No kit is
ever finished, but at least it was legal at the end of June '97.
Now it was driveable, but summer had arrived and I
still had to trim it, paint it and generally fart about with various bits of
mechanicals. In the end, I compromised by promising to address such issues in
the winter and enjoy the driving. Well, the driving was enjoyable, the summer
was crap, and I fitted anti-tramp bars as a sop to my conscience.
I also worried about the brakes.
I had fitted Triumph 2000 front calipers with MGB V8 disks. The rear brakes
were standard MGB GT items. All parts were new or reconditioned, with standard
pads both ends. The master cylinder was standard B single circuit with no
warning light and all worked fine-ish. Gerry was
right, a servo is unnecessary, but two things worried me - the single circuit
nature of the braking system and how to improve the braking performance. The
performance wasn't bad, but you know how us kit car bods
always try to improve things.
It seemed to me that MGBs have been around since the
year dot, so improving the brakes without resorting to servo assistance should
be a doddle. Somebody surely has already done it - it
should be just a matter of plugging into the right people. Well, I did ask
around, queried the Web and read some books. The distillation of all this wet
summer wet activity was:
1. Front brakes can improve with different pads and calipers.
2. Nobody mucks about with the rear brakes coz they are drums.
3. If you bugger about with either end, you can upset the brake balance with
dangerous results.
4. Calculating the braking characteristics of drum brakes is impossible.
5. Fred Puhn is the greatest living writer of
readable books about braking systems.
In the end, I devoured Fred Puhn's book on braking
systems, and concluded that to improve the brakes in a controlled way, I had to
convert the rears to disks. Anything done to the front would change the braking
balance, and as it was impossible to calculate the effect of any changes to the
car as a whole, I had to have disks at the rear. After all, the calculations
would be simple AND they would look really sexy through the wire wheels.
Just like the originals.
Motivation to do something about it came sooner rather than later, when my rear
axle had to be rebuilt due to a knackered diff bearing
and leaking rear oil seals.
To cut a long story short, the last several months have been spent designing
and building a disk conversion kit for the MGB rear axle. The Hawk prototype
has been fitted to my 289 since early October, and both versions are being
piloted by third parties prior to being marketed.
It is now 18 months-ish since my 289 became driveable. I still haven't fitted dual circuit brakes and I
most definitely do not need a servo. Although the braking is not quite up to
the effortlessness standard of current euro clones, it is full of feeling and
predictable. The back end does not lock up, and I can change the brake pads all
round in less than an hour. I've also fitted uprated
pads all round, and I now KNOW the theoretical braking characteristics of the
car.
After converting to disks and proving their validity, I borrowed a set of
corner weights and refined the calculations according to Fred Puhn. I had previously guessed the crucial measurements of CofG height and weight distribution.
Interestingly enough, the CofG height of a Rover engined Hawk is just under 8
inches above the axle centre line. I had thought it
would be lower - must be due to the thicknesss of
glass fibre Gerry uses in the body!
The weight distribution with a fatty driver and 2/3rds full petrol
tank is 46:54 front to rear. I had expected closer to
50:50 but, the MGB rear axle is heavier than it looks.
Given these characteristics, the braking system is almost perfectly balanced.
Without any compensation, the back end is calculated to lock up only slightly
before the front. Since it is never desirable for the back to lock up first, I
have fitted a proportioning valve to decrease the rear line pressure by 6%.
For those of you interested in the exercise, I have listed here the car weights
for both MGB and Hawk.
The full spreadsheet of calculations is available in Excel, should anyone want
a copy.
Car Weights |
Hawk *
MGB |
Front
Roadster models |
Rear |
TOTAL |
DISTRIBUTION MGB weights as per
British Automotive Technical Information (MGB 4) - 8/25/98 |
Car Handling -
The Hawk Cobra looks great and sounds wonderful but, ultimately, it is how it
drives that makes the difference between having a 'posemobile'
and having a car that is a pleasure to use. How should it handle is an
interesting starting point. How closely should one try to replicate the
original's road holding - having, as it were, tried to replicate everything
else a real Cobra has.
Well, there are limits. The original Cobra is, after all, a fifties design and
those of us who can remember cars from this period when new, know that
generally they left much to be desired. Most so called sports cars had about as
much grip as a dog on lino, and brakes that faded
like a holiday tan.
A certain amount of period feel all adds to the enjoyment of driving the Hawk
Cobra. But, the car is addictively fast and if it is to be used with
confidence, rather than white knuckles, then a well balanced feel when
cornering and braking is a must.
Having finished KWP 429D, first impressions on the road weren't good. The front
and back ends weren't talking to each other, a certain
vagueness in the steering over 70, undulating roads hinted at bumpsteer, and lots of pitter patter in the front
suspension on rippled surfaces. The brakes weren't anything to write a postcard
home about either, but that's another story.
An early track day on the full grand prix circuit at Silverstone
(bags of room to fall off; which we did), allowed us to play with the rear ride
height, shocker settings and tyre pressures, but not
much else. Thus began a long year of experiment, the results of which I'll
share with you.
If you intend attempting to change anything on your Cobra, then take it one
step at a time and record everything before and after you've tried it. Be
prepared to waste a lot of time, a 10% return on effort is good going - there
are no short cuts down this road.
The key factors which influence handling can be summarised
as follows;
- weight distribution,
- tyre choice and pressures,
- suspension design,
- ride height,
- camber and castor stuff,
- steering.
Weight distribution
The original Cobra had a near enough 50:50 weight split fore and aft - though
whether this was laden or unladen is unclear from
everything I've read. As we're not going to spend time legging it down the road
outside our cars, it's what it weighs avec the driver that is important.
What your actual car will weigh in action, and how its distributed, will depend
on whether you have a Ford or Rover lump up front (the latter tips the scales
around 100 kg less), and to a lesser degree if you have a conventional axle
with cart springs or the Jaguar IRS set up. Plus, I suppose, whether you favour chips and Mars bars over the local fitness centre.
The standard MGB front suspension tends towards understeer,
so the Rover set up won't help this and turn-in will lack sharpness. If you
then add a Jaguar back end plus sticky,
I moved as much weight forward under the bonnet as possible, the bottle jack, the wheel hammer and so on - everything helps. I also added
an angled plate between the rad and the front cross
tube made from 20mm steelplate, which added another
40 lbs or so ahead of the front wheels. I also have the vented disc kit up
front which adds avoir du pois - those Princess 4 pot calipers could have come off a
Chieftain tank. £7 spent at my local weighbridge furnished me with the
following printout :
Gross weight 1140 kg (2280 lbs) including yours truly at 82 kgs,
and 2/3rds full petrol tank,
Front weight 520 kgs (1040 lbs), rear 620 kgs (1364 lbs) - roughly 46:54 front:rear.
(cf. Autocar 12.11.65 289 Cobra gross laden 1202 kgs, 2611 lbs - incl.2 persons?)
(cf. MGB unladen 54:46 front:rear, according to MGB buff Lindsay Porter)
If you have a Ford engine, then you'll be closer to 50:50 including a driver,
though the gross will be up by a 100kgs or so, and the steering heavier.
Tyres
The original Cobra had narrow tyres by current
standards (7 - 7.35 inch x 15), and if you enjoy looking forwards via the side
windows then 185x70x15 should suit. Having bought 215x70's (which only fit the
spare wheel well when deflated), I think 205x70's are probably the best
compromise. But they don't fill out the wheel arch so convincingly.
Yokohama's (my choice) are sticky (except at Silverstone),
and combined with the Jaguar rear end grip like the proverbial - which is okay
once you understand how to drive to suit them. Only a violent clutch drop will
smoke them. A harder tyre (
I haven't found tyre pressures make a big difference.
About 20lbs front and 23lbs rear seems okay - any more at the back tends to
make the ride choppier, more up front increases understeer.
Did you know that there is right and wrong way to balance centre
lock wire wheels? No, nor did I or any of the tyre
fitters I spoke to - Motor Wheel Services will explain if you ask.
Suspension design
The original had all round independant suspension
(transverse spring then full coil spring IRS) so I plumped for the Jaguar rear
end. You also get rear discs thrown in and a fully adjustable set up plus a
useful range of axle ratios.
It is a heavyweight in all respects, the lower wishbones wouldn't disgrace a roadroller, and the subframe adds
more lbs. The diff' has to be good for at least 300bhp, and having manhandled
both into the back of my trusty Polo, I wouldn't say there's much difference on
the scales between the whole rear end and a Rover engine.
The flaw is, in my view, the B front end. Never particularly good even on a
fast tourer like the B. The Armstrong damper is a
pre-war design, and has to act as the top wishbone as well. A fair amount of understeer and hippity hop over
ripples was the norm.
For me a change to a fully independent, adjustable front layout had to be worth
a try - but, in late 97, from where? Moss
Size is, as they say, important when it applies to telescopic dampers. The
maximum available springing length for any coil spring is the difference
between its static length and its compressed, coilbound
length - on an 8 inch spring not much more than about 3.5". As you lose
roughly half of this taking up the static weight of the car, this leaves only
about 2" max for downward movement - which is about the minimum for a
tolerable ride.
An 8" spring probably means a telescopic of around 12-13" centre to centre - which is about
the maximum the MGB geometry can accept without excessively raising or lowering
the telescopic eyelet mounting points. Nothing wrong with this, Colin Chapman's
rear strut design on his early race cars probably measured about 24"
between centres, but one golden rule has to be
"change original geometry at your peril". And, Chapman was a
technical genius.
After 8 months of delivery promises, I decided to fabricate my own version. It
took about 3 months to make the first corner, and only 3 weeks to make the
other. I took the opportunity to widen the track by about 5/8" each side
to bring it in line with the original (which looks much better), and
incorporate castor and camber adjustment. The sketch illustrates the general
fabrication. If you are feeling bold I still have the drawings, templates, data and so on. Or, if you have £700 or so to spare, then
buy the NG/Hawk kit which looks good - funny that, I spoke to both of them 18
months back, when waiting for Moss to sort themselves
out!
After road trials and several swops, I've settled for
350 lbs x 8" springs with the damper adjuster dialled
in at about 50% clicks. Soft springs and hard shockers isn't a bad rule of
thumb. If you have a Ford lump then you'll probably have to go to 400lbs or
maybe more. The standard MGB coil spring is 370lbs x 9" if the ones I had
tested are anything to go by.
I made up some extension pieces for the track rods to cope with the wider track
and fitted urethane bushes through out (which though strong in compression are
poor in shear and I found can tear easily in trial assembly).
The rear end was also widened, by lengthening the lower wishbone and drive
shaft with camber adjusted via the shims on the inboard end .
This solved a problem of the ally' hub carriers clashing with the chassis tube
on full bump. I had already tried to cure this by locally scalloping and
welding the tubes and adding a scrap built bump stop using Triumph TR5 rubbers.
The new hub carriers were offered up whole, marked out and cut in situ with my trusty Makita jigsaw.
If you want to learn about springs (which are much more
complex dynamically than one might think), call Peter Faulkner of Faulkner
Springs, 01243 543049. He supplies all the top race teams as well as
Demon Tweaks, and he's cheaper as well as being helpful.
Ride Height
The fake snake 427 fraternity often set their ride height so low I suspect that
many of these cars are running virtually coilbound.
289 ride height is a compromise between aesthetics and
functionality. Gerry seems to set his cars up very high at the back which I
think looks odd, and also pushes even more weight forward onto the front
wheels. Maybe it is a result of using the standard cartspring
axle.
Two to three fingers width between the tyre top and
the wheel arch suits my car - it looks authentic and gives adequate suspension
travel. But, remember I do have fully adjustable layout and those of you who
haven't will have to solve it another way. A slight nose down attitude also
helps the weight balance with a lump up front.
A rough guide is that when static the lower wishbone should be parallel'ish with the ground. Always check ride heights by
measurement between identical fixed points on the chassis and a level garage
floor - my fingers thing is only to illustrate the visual component.
Lowering the ride height tends to reduce the pre-load
compression of the coilovers, and raising it the
reverse. So a certain amount of trial and error will be needed. At the
height above my rear coilover springs aren't quite
compressed enough on full extension. You can hear a clack' noise when the nose
dips coming off the power sharply, for instance. Longer springs would cure it,
or adding helper springs, but I can't be bothered to take them off yet again,
at present.
Camber and castor stuff
By and large, car designers have thought hard about their suspension geometry.
So, if you do decide to tweak it, first sit down with a cup of tea and a
Garibaldi and contemplate the future before touching anything. Remember
everything is inter-related, and a tweak 'here' to solve one problem is quite
likely to make something over 'there' actually worse.
Camber angle varies a lot, eg. 71 MGB front +1deg, 68
Jag E front +1/4deg, and -2deg gives most BMW's their knock-kneed look at the
back. On KWP 429D I've set the fronts at -1/4deg to help turn-in,
and -3/4 to -1deg at the rear. Any more at the back would produce severe
negative on full bump compression due to the wishbone geometry.
I true'd up the front castor to spot on 7deg. to
improve the self-centreing which had been poor. But,
remember more castor means heavier steering. Whatever you do, don't alter
the Ackerman angle as it affects so many things - as people who fit
wheel spacers or wider wheels often learn the hard way.
Steering
Inevitably, the big steering wheel with its skinny rim will feel quite lively
compared with the well damped, cushioned jobs in a modern car. Also, wide tyres mean more tramlining and
kick-back. Mine still has a bit of slop in it, and I don't know why - maybe it
is an accumulation of the two uj's and the rack. It
is offputting at higher speeds. If anyone has the
answer, please let me know.
Toe in has been set at +1/8". Bumpsteer has diminished
probably helped by the wider track which has reduced the arc of the track rods.
Remember, lowering or raising the suspension will change the relationship
between track rod and steering arm, and create/reduce bumpsteer.
Summary
Those of you who are still following the plot, should be wondering what all
this effort and cursing has amounted to. Well, it is better, much more chuckable and less likely to be thrown off line by poor
surfaces. I've learnt to use the power available to shove the sharp end around
the corners, but, slow in and fast out is still the
general rule. Though, the slow is getting faster - or maybe, I'm just getting
bolder.
If any of you know a reliable, knowledgable gent
(preferably in the SE), who professionally knows what he is doing , and could
do the final tweaking of KWP429D then, again, I'd be pleased to hear from
you.
Lastly, I'll endorse David's plaudits for Fred Puhn
in this case for his book ' How To Make Your Car
Handle', which lucidly explains many of life's technical mysteries.
Product Update - Replacement front end for Hawks, MGBs and NGs
NG and Hawk Cars both use MGB components and naturally they have come up with
various ideas to improve things over the years.Their
latest offering is a replacement front suspension system that fits the Hawk
Cobra's 'B' based system.
The double wishbone set-up with a coil over shocker unit is basically a direct
bolt on replacement for the original, old fashioned lever arm system.
The wishbones are 1 1/4 " steel tube and will accept rubber V8 or
polyurethane bushes. The lower wishbone features a camber adjustment using a
snail cam. The top wishbone has castor adjustment as well. The coil over
shocker units using 2 1/4" ID springs are fully
adjustable for ride height and damper rate.
The new wishbone set-up increases the track by 5/8 inch on each side and
enables the tyres to fill out the front wheel arches
to more accurately replicate the original.
The anti-roll bar link arm has a rose joint in place of the standard rubber
bush which should give more positive location and greater adjustment.
The end of the cross member will need modifying to clear the coil over shocker
unit. This you can do yourself or a ready modified exchange unit can be
supplied. Price is expected to be around £645 + VAT.
Contact either Hawk Cars on 01892 750341 or NG Cars on 01372 748666 (if you
have an NG)
Incidentally, if you order a Hawk kit with one of these front suspension units
Hawk will make the necessary modifications before powder coating the chassis
all for free!
Product Update - Hawk/MGB rear disc brake conversion
Two versions of the disc kit are available, one for the Hawk and one for the
MGB. The two versions are necessary due to the differences in wheel diameters
and width, although the Hawk version with larger discs can be fitted to MGBs which have larger and wider than standard wheels. Both
use standard braking components, albeit with slight modifications and no
changes are required to the MGB axle or handbrake system. The drum brakes can
easily be retro-fitted, if required, to retain originality.
Both versions are undergoing pilot testing and will be available for purchase
early in 1999. Further details can be obtained from the Editor or direct from:
Shaun Bateman,
Batemans Vehicle Performance and Restoration,
Ashington,
West Sussex
Telephone: +44-1903-893704