Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Fitting Brake reaction bars

Diolch Sewin

Never see a torque figure for the top oiler bolts, I've always done them up spanner tight and then just a bit more, but but not much more lest they shear off due to the 3mm hole down the middle.

Had a local 4/4 in for a kingpin change with both oiler bolts sheared and had been sheared for some time and owner was a bit surprised when I told him.

A watch point for fitting the top bolts is that it's too easy to get them cross threaded into the kingpin so you need some sort of lining up rod - mines an old screwdriver

Crosshead braces date back to the three wheeler days, Clarrie Coombes covers it in his book on three wheelers.

Personally, I make up my own braces to make sure they fit properly. Rubbing on the sidewall of the tyre is not a good idea. I also use -3 tubing with a clear plastic coating make up my brake hoses to prevent rubbing - I think Compbrake sell it

Arwyn

Sorry..I do try to explain the dynamics more than rushing to an unexplained punchline. Blame Luddite! I used to castigate him for overlong answers, though they are always great.
Once you get hooked on the fascinating things about mechanics, you act like one newly come to religion

The punchline:

1. You want brake reaction bars to steady the front end and brace the cross-frame in hard braking.

2. BUT NOT SO MUCH AS TO PREVENT THE CHASSIS FROM BENDING/PLEATING/ABSORBING IMPACT FORCES before they reach you or your passenger

3. As for slotted tops for brake reaction bars, they are silly. How does one expected to steady the front end with a slotted brace at the most important point?!!

The Morgan flexing chassis/frames, front and back, save lives, just as Formula I cars do. They were not designed with that in mind..just a lucky feature. Brake reaction bars
that do not that have chassis fittings that shear is unwise in a dangerous impact. The brake reaction bar goal is to steady the front end when braking, not to become a
structural feature that transfers impact forces to occupants. That can be managed by merely using less robust bottom fittings.

Here are two examples, one of a trad I pulled off the internet just now and another of the Aero crash test. This is what you want these cars to look like after a frontal
impact. Not hard to find other images the same. You will notice that the occupant areas are untouched.

Struggling with this theory on how a small tube structurally pinned at each end can transfer impact forces to occupants.

When subject to a frontal impact of any significance the tube will buckle under compressive loading if the lower location bolt doesn’t shear. This will effectively render its contribution to the stiffness of the car to zero.

Occupants in an impact experience a rapid deceleration, the extent of which influences the energy released. The stiffer the car the more rapid the deceleration the more energy is expended. Crumple zones are designed to reduce deceleration which reduces energy. The stiffness of a small brake reaction bar will be several orders of magnitude less than that of the chassis and consequently will have minimal influence on energy transferred to occupants in a frontal collision.

Thanks Arwyn,

I'm sure potts will be grateful for that info! I too use a modified screwdriver as an alignment tool when inserting the pins. My latest version is an old cut down electrical screwdriver with the extended plastic shielding but if fitting the new hardened chrome pins I'm not sure of the benefit that will give, mine are still stainless and I like to think it helps them as I jiggle it to align them!

Lorne that's exactly why I have been experimenting with brass rear fastenings to the chassis rail./BRB I want to know for sure that I'll get a failure point there should I be unlucky!

That's easy to answer. They should NOT transfer impact in a happy world. Their lower fittings should shear or they can crack where they are pleated and bent,
top and bottom or both. That is the minor thing that must be understood. I realize it is counter-intuitive. Most make their own and they unwittingly be made
added to so they are too strong and then prevent the frame from absorbing impact forces and transfer them down the line to the cockpit. That is what seems to
be happening with the CXs. They are apparently transferring frontal impact forces to the driver. But Morgan trad and Aero frames bend and pleat at the
bulkhead, a happy result for occupants.

Brake reaction bars are smart and well-chosen if minor precautions are taken to assure their lower fittings will shear or the bars themselves will crack in extremis.

Physics is fascinating. It is all in the small details, As we have a chassis frame that performs well, even miraculously, for us why make these bars "structural"? Do
we want them to negate the frame's known safety feature. The "small tubes" have hopefully we weakened enough by flattening and sharply bending their edges, top
and bottom. But no one has tested that. Best to add some redundancy with the weak shear bolts customarily supplied from the best sources?

BTW, have you ever examined the very thin flimsy fringe on the bottom of the chassis' hefty vertical section. Are you aware that if this thin thingie it is broken or cracked,
the hefty frame above will follow suit? One does not have to look important enough to change physics.

gomog

Wise man! We need more of your sort. When you find something in brass..give me a head's up. I am using cheap pot metal fittings but I like brass.

Lorne

P.S. What is a fuse but a predetermined failure point that prevents burned up components and wires melting willynilly? Amateurs often negate their usefulness and protection
merely by putting in too strong a fuse..

I don't have reaction bars on my MOG, but a rough structural calculation of the energy to destroy the reaction bar.
I take as hypothesis  for the reaction bar :
diameter 25mm
thickness 2mm
length 500 mm
angle : 45°
force at chassis level : 5000 N (~500 kgf)
mechanical stress : 53 daN/mm² (over elastic limit)
Energy : 40 J (Joules)

Energy of a 900 kg car at 36 km/h (10 m/s) : 45 000 J

ratio : ~1/1 000 (40 /45 000)

If you have the dimensions of the reaction bar I can update the calculation, but I think it cannot change the conclusion.

Hi CBY,

Hope this finds you well. Thank you for your post.

Forgive me, as I love theory but experience, actually being there, so often trumps theory with trads. For example, their flexing chassis frame is unknowable as EACH
is different and become more so as they are used. JHS2 used to say that "I never speak of something I haven't seen or experienced myself".

Below is what racers and some moggers used to (and many still) use as brake reaction bars.Can you estimate whether THEY will affect the absorption of force in a
frontal trad impact at speed? Do you think they might inhibit absorption? One learns LOTS with trad mileage and race crewing. Some here have shared
that with me. As one example, I carefully read everything Arwyn writes. I love anecdotal learning.

Whenever one has a doubt about a harmless non-invasive hint, that costs nothing, DO IT. There is no downside. Can't say the same about ignoring it.
Best to you, (as Sheally used to say)

Lorne

During an impact at 36 km/h (23 mph), the energy is equivalent to lift your car to a height of 5 m (~5 yard). You can believe the brake reaction bar could not collapse during the impact.
Figures are stubborn.

Lorne,

I too have tried other materials, alloy, pot etc. and all had some merit so One afternoon I thought that I'd shock test them and all but the brass instantly sheared, This was however in the vice at the bench and as you say theory is fine but practice is something else! brass had the greatest ability to distort and still hold before failure and this showed me that I was unlikely to have a loose end of a BRB waving about after failure from general fatigue or a minor suspension incident which might be unknown to me! Brass also allowed me to tighten the BRB to the chassis with adequate force to prevent the BRB being able to move around causing wear fatigue to the fastening along with no corrosion or galvanic degradation to any degree.
I also tried stainless steel and the latest offerings are not as soap like as they used to be so on the lathe I cut a shear waist at the through chassis point but on test I found that the stretch factor was unpredictable and I could not find a consistent level of tightening. If it failed at this point then that was obviously too weak yet if not, under testing further again results were varied so I moved on again!

I have to agree with you though that the BRB's on general offer at this time are probably about the best we could use to gain maximum benefit with minimum problems. I really can't see that in a severe crash the BRB could give such increased rigidity to the chassis so as to threaten the occupants and any one who has had experience of fitting them would quickly see jus how weak and foldable the crushed/folded ends are and it would simply deform from these points regardless of impact type.
My interest in this was just to eliminate any chassis damage to the longitudinal members in the event of what in effect would be a minor impact and not transfer the damage to an area further to the rear.