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Twin rear brakes on an open "D"

vince998

Well Known and Active Website User
VOC Member
Hi All,

Has anybody ever modified a series D RFM for twin rear brakes.

Looking at my RFM, and comparing it with my comets, i see that the lug for the torque arm is present, and so are the bosses for the cross shaft. They´re just in "unbored" condition.

Does anybody know if the factory took the same lower RFM castings for C´s & D´s, but left the D´s unbored/un drilled.

My biggest issue is of course weakening the front RFM casting in some way.:confused:

The plan is to take the VOC spares companies new 8" setup for the front, and then use the existing left hand brake and drum (water excluder and all) for the left hand rear.
I´ve done some initial calculations, and it should all fit.

Any help/experience appreciated.

Thanks.
 

Vic Youel

Well Known and Active Website User
Non-VOC Member
I have never found the twin rear brakes on series C's very effective. Both my machines alwasy give good readings at MOT test time but they will never lock.

I have read somewhere that the design leverage exerted was limited in order that they would not lock on a normal dry good road surface. I wish I could remember the source of this comment though.

So I therefore doubt the efficacy of such a modification.

Vic
 

Tom Gaynor

Well Known and Active Website User
VOC Member
Manx Nortons have single (early) or double (late) 2ls brakes at the front, and single 1ls at the back. This is because they knew that sls brakes are pretty ineffective, regardless of diameter, AND that the very last thing one wants is a rear brake that will lock the wheel. Simple Norton solution: use effective 2ls at the front, and ineffective sls at the back. (The Norton rear brake is the same as the (standard) Vincent front.)
I can speak from personal experience, having had a rear brake jam on in a race when the operating rod broke at the front and dropped on to the track. The rear wheel locked, down went the bike, and I got dumped on my arse.
I might also say that if your front brake is any good the rear wheel will be only just on the deck, so if your back brake makes a significant improvement to your stopping, then your front brake isn't working.
 

deejay499

Well Known and Active Website User
VOC Member
You could also consider the set up like the Python, using a special cable that goes through the right hand brake arm and onto the left by going around the back. The first brake is pushed on and the second, pulled on. Have seen it done on a `D`.
Cheers, Dave
 
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vince998

Well Known and Active Website User
VOC Member
Thanks for the answers guys,
Back wheel just of the deck!!!!, i can do this on my yamaha, but on the Vin it´s out of the question.
By fitting the second drum to the rear the worst i can achieve is a 100% increase in braking area (plus 10Kg in weight judging by the list of parts i need)
Maybe the new VOC spares Co brakes will give me the backside in the air braking forces i´m looking for?? ;-)

Comparing the D RFM to the C, it looks as though Vincents used the same front casting on both, leaving the D unmachined for the cross shaft.
Once i´ve worked out a way of drilling the 2 cross shaft holes in line, i´ll give it a go.
Should everything all go horribly wrong, i can always replug the holes i suppose?
 

Hugo Myatt

Well Known and Active Website User
VOC Member
I have never found the twin rear brakes on series C's very effective. Both my machines alwasy give good readings at MOT test time but they will never lock.

I have read somewhere that the design leverage exerted was limited in order that they would not lock on a normal dry good road surface. I wish I could remember the source of this comment though.

So I therefore doubt the efficacy of such a modification.

Vic

Vic,
You're right. It is in the literature somewhere. The previous owner of my Rapide had used it with a sidecar. In order to increase the leverage on the rear brake he had modified the brake pedal so that the brake cable anchor point was closer to the pedal pivot point. There is a limit to how much it can be moved back without the lock nut on the torpedo nut fouling the footrest. Mine has been relieved at this point to accomodate this. Obviously this results in greater pedal movement for the desired effect. I suppose if you had size 14 boots you could increase the pedal length for the same effect but the movement would be much greater.
Hugo
 

Alan J

Well Known and Active Website User
VOC Member
For us with sidecars it's a poor brake!-the leverage is only about 3 to 1! my A.J.S. is about 6 to 1. I have adjusted the brake pedal high up in order to get a good leverage with my size 10 boots! I really need to put a disc on the back wheel-but how to overcome the idleness inertia? Managed 10 rides so far on my "twin" this year, but not very long rides!
 

chankly bore

Well Known and Active Website User
Non-VOC Member
Rear brakes

"The rear brake leverage has been purposefully reduced to prevent lock and it is not permissable to lengthen the cam arms." Richardson Page 40,1955 edition.
 
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