Topic: Speedo drive on marzocchi fork

[s4r3]

Hi there,

I've been reading up and searching for info for my '70s racer, based on a k6 frame and k0 cobra engine (in NOS later cases). Have collected many parts already and now in the process of the mock-up to see what is good and what needs revision or more work.

In this proces I've wrenched on my Marzocchi front end, the one with the unicorn top plate. It has 38mm upper legs and a 20mm axle. I have already changed out the wheel bearings for the 20mm axle. I would like to use the original k0 speedometer and tachometer, for the speedometer I need a mechanical speedo drive that works on the recessed parts of the hub and accepts a 20mm axle. Doing research on this forum has led me to a topic on a R6 fork swap with similar demands but no answer. There is a list on DoTheTon providing information on axle diameters.

http://forums.sohc4.net/index.php?topic=161166.25

https://www.dotheton.com/index.php?threads/axle-diameter-size-list.20952/

I've already discussed changing the original speedo drive to accept the axle with a technician that is working on a CR750, however it will not have enough flesh on the steel bushing and changing the bushing would result in even more work with the internals of the drive.

Before I start my search and research on my own with regard to different speedo drives for 20mm's I am wondering if anyone else has a Marzocchi front end with a mechanical speedo on it or if any of you has some good advice.

Thanks in advance for thinking with me.

Here just a few pics of my project:

[bwaller]

Sorry man i have no answer, but I wonder why you want a speedo on a racebike?

[simon#42]

that was my thought as well .

[Leino]

If you are racing the bike forget the speedo.

[turboguzzi]

well, with rubber footsteps and a side-stand, looks to me more like a cafe racer rather than an AHRMA racer

if i was you would search ebay for a used speedo gearbox for something that has a 20mm ID, maybe off a heavy custom or something, say an old Magna or XRV750 Africa Twin... guess those had thicker axles

Plan B. - When i fitted to my KZ750 a top end that did not have a mechanical tacho drive, i managed to transplant the electronic workings of a digital rev-counter into the body of a mechanical one by opening the crimping.....

Plan C - if originality is not an issue, there are now "vintage" speedos that run on GPS signal, no cables needed other than +12V

[s4r3]

Don't mock me yet, just started with the build and am far from ready. And hey, it needs to stand on something while I am working on my other projects

However, this fork came with an aluminium unicorn style upper plate, that is intended to run both the speedo and the tacho. Although I don't see the benefit of the speedo for race purposes this is a period correct race fork so I just assumed it is period correct to attach the original gauges. And that is the aim, period correct racer. If it doesn't work out I'll just run the tacho and fill the axle with a spacer.

[simon#42]

there  is no such thing as a period correct racer with a speedo . what do you intend to use the bike for ?

[turboguzzi]

there  is no such thing as a period correct racer with a speedo . what do you intend to use the bike for ?

might be true in UK/europe, but in early AMA superbike racing, speedos were often left in place.... working or not is another question, most likely not....

http://vintagesuperbike.com/photos/own/1980Honda.html

[simon#42]

i wonder just how many sohc hondas raced in ama superbike in the early years . someone will know !

[bwaller]

I don't know Simon. However in the pro ranks, my guess would be not many. Amateur classes would be certainly different.

Early on remember Reg Pridmore on a BMW, Cook Neilson on his Ducati. The "big four" were embarrassed that their high horsepower streetbikes wobbled around and burned clutches etc.

This very quickly became a factory effort transforming streetbikes into racebikes. There wasn't room for a 60's designed engine in that mix.

EDIT; Ok so the above sounds negative. This is not to poo poo guys efforts to have fun racing and building their SOHC4 projects. I do that as well and I think it's interesting work after all.

[simon#42]

i found one in 76 , a couple in 78 and in 79  . it all happened for honda in 1980 when spencer won on the new twin cam bike .

[gschuld]

https://ironandair.com/blogs/news/pops-yoshimura

https://paulritterblog.wordpress.com/superbike-pioneers-1976-1979/

https://www.motorcyclespecs.co.za/model/Honda/honda_rcb_0000.htm

Honda created the RCB1000 in 76 which was (engine wise) sohc4 750 based with a DOHC 4v head and near 1000cc, but it was designed for endurance racing not Superbike.

Kawasaki and Suzuki seemed to get much of the early superbike glory for the Japanese manufacturers

I imagine the sohc4 heads were simply outgunned in valve size capacity/flow compared to the KZ and GS offerings for 1000cc.

FWIW, in the US, the AHRMA Superbike class requires both factory tach and speedo gauge shells mounted in the factory location at a minimum, along with factory fuel tanks and side covers to mimic the genuine period superbikes of the era.

George

[turboguzzi]

nice links george, but the question was just if it's "period correct" for a racing CB750 to have a speedo....

the CR750 types sure didn't, but i tend to believe that in the more "street based" racing categories, people kept them.... im locked away from my library so cant check my old magazines..

back in the library i have also a pretty big book on production bike racing in Italy in the early 70's and many of the sohc4s shown in that book deffo speedos. luck wants that the CB400four in the cover doesn't though!

https://www.giorgionadaeditore.it/en/special_offers/le-derivate-tutte-le-gare-per-moto-di-serie-negli-anni-70.html







 

[bwaller]

Well there you go, speedo look needed to be class legal. I'm sure it doesn't need to work, nor do you want all that extra weight or the distraction. 

At the very least get a 85mph face from an 80's DOHC model to stick on it! 

[turboguzzi]

or do this...

1976' McLoughlin's R90S....

[simon#42]

the actual question was about speedo drives i think ...... and none of us has been much use !

this question would be best asked in the sohc 4 bikes section , im sure it will have been done before but not everyone reads every section .

[VonDutch]

Ducati Cagiva Alazuzrra Pantah style Speedometer Drive, I've seen them in different ratios.
cable is on a angle to clear the disk.

[s4r3]

Ducati Cagiva Alazuzrra Pantah style Speedometer Drive, I've seen them in different ratios.
cable is on a angle to clear the disk.

Thanks will definitely try that one out.

Further, with regard to answering the actual question, although Simon is right I do want to point out that I value each of your responses so far. The overall aim of this bike is the classic cup, starting with a few demo races and eventually taking it up a notch. The information provided by, amongst others in this topic, George is therefore highly appreciated.

[gschuld]

Ducati Cagiva Alazuzrra Pantah style Speedometer Drive, I've seen them in different ratios.
cable is on a angle to clear the disk.

Thanks will definitely try that one out.

Further, with regard to answering the actual question, although Simon is right I do want to point out that I value each of your responses so far. The overall aim of this bike is the classic cup, starting with a few demo races and eventually taking it up a notch. The information provided by, amongst others in this topic, George is therefore highly appreciated.

http://www.fim-europe.com/fim-europe-presents-endurance-classic-cup-2019/

By Classic Cup, are you referring to the 4 event European vintage endurance series?

George

[s4r3]

Yes that would be a great cup to participate in :-), I will certainly join the CMRCH (classic motorcycle race club holland) http://cmrch.nl and go on tour with an oldtimer that has been racing a cb550 for as long as I know him all over Europe to learn and get acquainted with the world of classic racing.

[simon#42]

well you can forget that speedo then .

[gschuld]

Wow.  The superbike class they race under would be a real (fun👍) challenge for a Cb750 based machine.  I’ve been fascinated by the endurance series run over there. 

Endurance racing for vintage bikes narrows down your needs a bit.  Especially considering there is night racing on fast tracks like SPA in total darkness.😬

So you will have to contend with a few things most road race bikes don’t need to consider if you want to be as competitive as you can.

Such as a functional charging system to keep up with powerful lighting(most adapt modern motorcycle lights to the required vintage style lenses, usually in a side by side arrangement)

I see you have a ARD magnetic self generating ignition.  Classic caveman era solution for ignition draw.

You would ideally want a maximum allowable capacity fuel tank (24 liters?, I forget) with if possible a dry break filler for fast pitstops, plus special dry break type gas cans.  Saves precious time in the multiple fueling pit stops.

And nearly all those bikes run onboard electric start systems(highly encouraged by race organizers in that series)

You will need a sizable crew on hand, full safety gear for dedicated refueling crew, spare parts by the pile, spare wheels/tires, and a poop ton of fuel.

It would be a large undertaking to build a cb750 based bike(or any bike) and compete in that series.  MotoGuzzi bikes seem to be the dominant bikes lately.  Kawasaki KZ bikes do well, Suzuki GS, etc.  There is nothing IMHO chassis wise that makes any of those options superior to a well sorted Cb750. 

The challenge is likely more engine related.  These guys are running 1100+ cc engines with big valve heads.  They seem to be highly developed and putting out a lot of power at an endurance setting(measurably less than max power).  It may be a real challenge to equal their power output with 4hr endurance reliability, but you may be able to get away with a bit less as your bike should be able to be a bit lighter that the competitors.

I’d think you will have a much easier time if you are not shooting for podium finishes but rather a strong showing in the field.  But with a serious and dedicated effort, it may well be possible to surprise some people with what a top level Cb750 could do😉

I wish you the best of luck and please keep us informed.  There are some fantastic resources available around here and beyond so don’t be shy asking questions.  I’m personally better in helping you find the information than ...knowing...  the information.  But I’ll do what I can.

George

[simon#42]

i have been to quite a few of these endurance races and i dont think its a viable option for a standard framed sohc honda . in fact i think you would struggle to even get an entry at all . the class is very over subscribed and the organisers are really after interesting period machines  .  also it would not exactly be the best place to learn to race .  i would recommend you have a word with voxonda he has an interesting bike he uses in a dutch series . its not racing as such but is a demonstration of older bikes . this could be a great place to start and wont cost a couple of thousand euro a race like the endurance series will .

[turboguzzi]

i have been to quite a few of these endurance races and i dont think its a viable option for a standard framed sohc honda . in fact i think you would struggle to even get an entry at all . the class is very over subscribed and the organisers are really after interesting period machines  .  also it would not exactly be the best place to learn to race .  i would recommend you have a word with voxonda he has an interesting bike he uses in a dutch series . its not racing as such but is a demonstration of older bikes . this could be a great place to start and wont cost a couple of thousand euro a race like the endurance series will .

+1... in the classic european series, there is no class where a CB750 could be even semi competitive. The eligibility window is constantly shifted fwd in the years and and even kawasaki fours have been pushed aside in this progress.
Even at basic level i believe that youd be spending way more than a couple thousand per round....do your homework, visit a classic endurance race just to see before you jump into it.

[s4r3]

The overall aim of this bike is the classic cup, starting with a few demo races and eventually taking it up a notch.

Based on the last few post I felt the need to quote myself,  but other than that I want to thank you all for the advice so far. Thanks Simon for the advice on contacting Rob, and I will in in the near future, but priority is first get the bike rolling and up to the standards. And till then I've been and will keep doing my research. And yes, the moment I see a Rickman frame for cb750sohc up for grabs it will be mine. 

I have contacted an acquaintance of mine that works with (older) ducati's. George is also looking into the possibility of a Magna front end speedo conversion. Will report back if I have some new info on this!

Thanks again all so far!