Topic: Can a cb750 be competitive in novice production heavyweight road racing?

[gschuld]

Watching the races on Sunday was a bunch of fun, though clearly Kickstart had more !  I spoke for a bit with Andrew, the other guy with the cb750.  He has been racing for about a year and a half.  Not a veteran racer nor did he have a bike nearly as well prepared as Brent Waller's machine, but he did state that he felt the twin cylinder bikes were more ideal for this class of racing(novice production).  The weight difference appears to be a significant factor.

I have not been able to locate and study the fine details of the limitations of this class, but this is what I learned based on our conversation.

-original head pipes must remain.  A factory 4 into 4 exhaust bike must retain the 4 into 4 exhaust.  Different mufflers may be swapped in, or OEM mufflers may be gutted(as his was).  No 4 into 1 exhausts allowed as it was not available OEM by the 1972 cutoff.

-engines must remain stock bore and stroke(up to .020 overbore is allowed)

-OEM appearance must be maintained to a "reasonable degree".  As in factory seat pan, OEM fuel tank, brakes, etc.

-he indicated something to the effect that "wild cams" were not allowed, but aftermarket cams are OK.  Not sure where the grey area is there...
 
-not sure about head porting, lightened cranks, rods, etc.

Seems like dropping every ounce of weight, going as wild with the motor, chassis, and tires as the tech guy will allow, and having a very light rider would be necessary to attempt to be competitive with the leaders.

As I'm 6'3" 225lbs, I wouldn't be off to a good start would I  ...... ?

George

[gschuld]

Some details from the AHMRA rule book:

10.6 NOVICE HISTORIC PRODUCTION Novice Historic Production is open to production street machines built up to December 31, 1972 (the build date is determined by the date of the frame and major component parts) and like design. Machines must use original unmodified, OEM frame, swingarm, forks and handle- bar mounts. Un-needed tabs and brackets may be removed, but gusseting (no strengthing) can be added, no geometry can be altered. Gas tank, seat pan and fender must be OEM equivalent or replica replacements. Fenders must be untrimmed. Engines must be original OEM bore and stroke regardless of displacement (plus allowable overbore, as described in rule 9.7.2c). Classes are Lightweight (four-strokes up to 500cc, 250cc two-strokes and specific 350cc two-strokes listed with an asterisk (*) below) and Heavyweight (four-strokes up to 750cc and 350-500cc two- strokes).

10.6.3 Among the eligible NOVICE Historic Production Heavyweight motorcycles are:
BMW R75/5 BSA, all 650 and 750
two- or three-cylinder Ducati 750 round-case Sport or GT (no Desmos)
Honda 750 four-cylinder
Honda 500T
two-cylinder Kawasaki 500 two-stroke
Laverda SF750 (SFC must compete in Formula 750)
Moto Guzzi 750 (V7 Sport must compete in Formula 750 or Sportsman 750)
Norton, all 650/750 (except short stroke)
Suzuki 500 two-stroke
Triumph, all 650/750 two-valve head, two- or three-cylinder
Yamaha RD series 350/400 air-cooled two-stroke Yamaha R5

10.6.4 REQUIREMENTS AND MODIFICATIONS FOR HISTORIC PRODUCTION

a) CARBURETORS: Must be OEM and same type (e.g., if CV, must use CV), no larger than the original OEM bore size. (Restrictors may be introduced to ensure par- ity of performance.) Air intake: Airboxes and/or air filters may be removed. No per- formance increasing devices may be added (velocity stack, scoops, etc.) Aftermarket foam or mesh type air filter(s) may be added.
b) EXHAUST SYSTEMS: Four stroke engines must have the same number of pipes as provided on that motorcycle, i.e., 2 into 2, 4 into 4, or 3 into 2 for three-cylinder engines. Exhaust pipes may be aftermarket, may be tucked or raised for ground clear- ance and must have an effective silencer. Two stroke engines must use production street exhausts from the time period, any manufacturer. No expansion chambers.
c) BRAKES: Front and rear may be upgraded to Sportsman 750 specifications in the  Heavyweight class only. Lightwieght-class mahines must use OEM brakes as fitted on that make and model year.
d) RIM SIZES: Replacement rims must be the same diameter and material as OEM. Maximum allowable rim widths are  WM3 (2.15 inches) front and WM4 (2.5 inch- es) rear.
e) Sidestands and centerstands must be removed.
f) CONTROLS & FOOTRESTS: Controls: Handlebars must bolt to the stock loca- tions. Clubman bars are allowed. Footrests may be trimmed down or raised for ground clearance. NO rearsets.
g) Lighting equipment must be removed or totally taped over to the tech inspector’s requirements.
h) FAIRINGS AND BODYWORK: Fairings of any type are NOT allowed. Gas tank, seat and fenders must be OEM equipment or period replacements. NO modern “GSXR style” bodywork will be allowed.
i) SUSPENSION: Front end must be period OEM of the same dimensions as stock. Springs may be replaced. Shocks must be straight body, no reservoirs, mounted in the stock location(s).
j) NUMBER PLATES: 10-inch by 12-inch rectangular. White background with red numbers. Also see rule 9.5.
k) Specialty machines such as Rickman, Dunstall are not eligible.
l) Historic Production machines are NOT allowed to bump up into any other AHRMA class.
m) Except as noted above, rules 9.3, 9.4 and 9.7 apply

[Tintop]

What the rules allow is going to vary somewhat from vintage club to club.  Certainly in a 'novice only' production class a lighter bike is likely to be an advantage.

There's no 'Novice' class in the VRRA.  Novices run with the experienced guys.  They wear a 'bib' so the others in the class know who they are.  A CB750 would run in P2 Heavy.  The only restrictions on the engine are that it use original main castings (internals free), period aftermarket (eg Japauto), or reproduction pieces that are period in look, and material type.  You can use any style exhaust with the exception of under tail, and no TI or carbon cans.  The CycleX CB750 would have been a VRRA eligible P2 Heavy, and likely a handful for a novice.

[gschuld]

I'll have a look at the VRRA specs.  It's kind of a shame that the clubs vary with their allowances, but I can understand why.

So it would take a pretty heavily modified machine to be capable of competing in VRRA P2 with a cb750 based bike?

George

[bwaller]

VRRA rules state engines are limited to 5% overbore, and there are other restrictions, "period" is the magic word re exhausts, etc. The CycleX bike ran 17" wheels which wouldn't be allowed either. However it's true our P2 Heavy class does not compare to the Novice Production class.

The AHMRA idea here is to offer an inexpensive class for people to get racing...... so some bikes fit the bill better than the CB750.....maybe....unless you own a CB750 like Kickstart.... then it's perfect! The only sticky deal for me is suspension. Hopefully at least they allow good quality non reservoir aftermarket shocks, like Works Performance just as an example. I understand with forks they have to draw the line at minimum upgrades, but if the units are in reasonable shape matching spring rates, and tuning to rider weight goes a long way. I think every street based race bike should have a fork brace, silly if not allowed.

I raced production in the seventies and found the 750 to be a handful at speed. I was 20, more full of piss & vinegar than brains. Stock CB750 shocks and forks untouched is crazy. We had to grind off sidestand mounts for clearance, Dunlop K81's were good but Dunlop & Avon race tires these days are even better which stress handling and suspension components even more.

SO after all this, for a production class, my advice is do whatever you can to improve suspension, everything the rules will allow. The bike will be way more fun to ride.

[gschuld]

Thank for the insight!  Your experience is very helpful.  Andrew did mention that his bike felt rather heavy, especially in the turns.  He has taken to running wide M bars (for leverage) and leaning hard off the bike in the turns to keep from dragging.   I believe that he said that his bike was just over 500lbs with half a tank of fuel.  So his bike only dropped perhaps 20-25lbs from stock.  He looked to be about 190 lbs as a rider.  That nearly 700lbs to push around those corners     He sure was having fun though...                 

 I'm curious what the other guys in his class were weighing all up.

George

[bwaller]

It's not that the 750 was evil handling, there were others far worse. They are just a "handful". Certainly a better sprint racebike than an endurance weapon in stock form.

This may not be fair or a scientific analysis but I found the 750 tended to loosen the rear & highside when pushed more than most. Better longer shocks likely helps.

[Lostboy Steve]

Thank for the insight!  Your experience is very helpful.  Andrew did mention that his bike felt rather heavy, especially in the turns.  He has taken to running wide M bars (for leverage) and leaning hard off the bike in the turns to keep from dragging.   I believe that he said that his bike was just over 500lbs with half a tank of fuel.  So his bike only dropped perhaps 20-25lbs from stock.  He looked to be about 190 lbs as a rider.  That nearly 700lbs to push around those corners     He sure was having fun though...                 

 I'm curious what the other guys in his class were weighing all up.

George

Where are you racing NJMP? I would love to get my 550 on the track just to see how far I could push it through the corners.

[gschuld]

Twistedengineering, Kickstart ran his cb750 there for the first time this past weekend, I only went to watch(but WAS jealous).  It looked like a blast!

BWaller,

Good info, thanks

Perhaps it would be an interesting exercise to put together a list of the ideal modifications for the different options available for the cb750 for vintage road racing.  I have a feeling that it may inspire more people(like me perhaps) to give it a try if we felt that we had a good idea what to do to the bike to prepare it properly the first time (for the most part anyway).  For those of us who have little experience with how to make a cb750 behave properly in a track setting, it would be a huge help.

So, if you and others wouldn't mind giving it a go... Perhaps we could start with Novice Historic Production

It would probably make sense to have a range, from a minimum "just trying to get my feet wet using my otherwise daily driver"
to "everything I can do within the rules to be competitive in AHMRA Novice Historic Production: 750 class (pre 1972)

Say we had two bikes to works on, one for the first category and one for the second.  Example rider size and weight (6'3" 225lbs )

1st bike:  1970 K0, low mileage, non modified factory engine(factory pistons), presumably mechanically sound(no reason yet to think otherwise), no original factory paint bodywork to worry about(have spare tanks, etc. anyway).   What would be the logical basic things to do to it to prepare it for a weekend of novice vintage racing to "try it out"?  Further improvements could be made if enthusiasm continues.


2nd bike  1971 K1  bare frame/swing arm/front end/ a PILE of factory parts/second 1971 parts bike with hurt motor .  Ready for a full out effort to make a competitive racer in Novice production from scratch.  Cutting, tig welding, bracing, modifying all easily available in house.  Ideal Engine specs?  Suspension?  Frame improvements? Rims/tires?

I hope this will come off as a fun mental exercise rather than a homework assignment   If it's the latter, my apologies!

George




 

[Kickstart]

I was just going to post the same question... and then I saw this thread.

George, too bad we didn't bump into each other, I was hoping to find a fellow SOHC4 member there.

Twisted - yeah, it was at the NJMP, you should have signed up... it was a blast, and everyone I met was laid back and really cool.  I can't wait to go to my next AHRMA event.  AHRMA should really do a better job advertising, I think I stumbled across the event 4 months ago when I was actively searching for vintage racing events... they should have had advertisements on message boards like these.

So back to George's question... I was also wondering the same thing.  I was thinking the CB750's would rule the track in the Novice Production Heavyweight class.  The year is limited to 1972 or older (or like year), up to 750 CC... I was thinking the CB750 was the best production bike at that time, so shouldn't it be the best in this class at the track?  I guess not.... I did feel like I had more HP than the other bikes in the straight, but my turning skills are at the bare minimum novice level, so I braked really early before turns.

Andrew also told me he did the following to his CB750K6:

• One size overbore
• Raised the pegs up two inches (this is absolutely essential on our bikes... there were some turns where I was dragging my peg through the whole turn!)
• Slightly taller rear shocks (I forgot how much taller he said)

Yeah, Andrew was really hanging off his bike.  My buddy mentioned that it was amazing how much he moved back and forth on the bike.  Even with that Andrew mentioned to me he managed to scrape his alternator cover in one turn.

Regarding how much you can push the mods, I think it's more a gentleman's code here to stay within the spirit of the rules and to be cool about it.  The tech inspection was really just looking for safety issues.  My bike technically did not meet the class regulations, but I was a brand new racer so no one cared.  I'm planning on putting together a CB750 that's to spec as I get better.  I don't want to start beating people and feel like I didn't deserve it because I had an unfair advantage.

I think this is a great idea:

... Perhaps it would be an interesting exercise to put together a list of the ideal modifications for the different options available for the cb750 for vintage road racing.  I have a feeling that it may inspire more people(like me perhaps) to give it a try if we felt that we had a good idea what to do to the bike to prepare it properly the first time (for the most part anyway).  For those of us who have little experience with how to make a cb750 behave properly in a track setting, it would be a huge help.

So, if you and others wouldn't mind giving it a go... Perhaps we could start with Novice Historic Production

It would probably make sense to have a range, from a minimum "just trying to get my feet wet using my otherwise daily driver"
to "everything I can do within the rules to be competitive in AHMRA Novice Historic Production: 750 class (pre 1972)

I've got Andrew's email, and I was planning on asking him similar questions.  George, I'll copy you on the email when I send it, and maybe we can keep this thread going for others.

I can also post pictures of the safety wiring I did that might help others, and some of the logistics questions about a typical day at the event and what sort of things to bring.

One thing I would like to add... I'm really happy I'm an AHRMA member now, but I've got to say I was not impressed with their race class.  Maybe it was my instructor (who was a nice guy), but we hardly covered any technical training (throttle control, grip, body position, etc.)  It was really more about race procedures and knowing the flags.  I think to some extent, they just want to make sure you've got a head on your shoulders, so they just monitor on the first day's practice sessions.  I highly recommend taking a track day course before going to your first AHRMA event... I think you'll enjoy yourself more.

[turboguzzi]

george, have a simple answer to you, even if here in italy theres no such thing as production. set up your suspension, get out on the track and stop worrying about being competitive. . your key learnings will start there. unless you are immediately at the sharp end, riding skills count so much more than level of tunning 

[voxonda]

+1 TG,

George get out there and see what is happening on track. We do race in some demo's over here and compete with much faster (theory) bikes. Us only being 736 (for now) but with a lot of rider talent and experience. We carry so much speed in corners which most cannot make up on the straights. Make sure your bike is safe and sound. Look at engine mounts to be sturdy and change that tiny front axle!! Have fun!

[fenianmoto]

Howdy folks! Andrew here.

A friend mentioned this post, so I rejoined the group after being off of the forum for a couple years.

I'm more than happy to share any insight, opinions & info I have from running the 750 this season in AHRMA Production Heavyweight.

I think the 750's can be competitive in this class, but it's there's more to it than just the bike. Any fellow racers here can attest to that.

I guess, to start, I'll answer a couple questions I saw above. The shocks I'm running are Hagons, 2" longer than stock.
Any decent aftermarket standard shock will improve the rear. Up front you can run with RaceTech emulators & better springs, fork oil's up to the rider. I've got 15w in the bike & like it.

When we were putting the bike together, I had clubmans on it.....for about 5 minutes, LOL! I put the Norman Hyde M bars on upside down for more leverage. I've ran them on several bikes over the years & just really dig the bend. IMHO, better leverage on a big heavy racebike like the 750 is an improvement.

A few other things on the bike: K&N pods, open stock pipes w/ a minimum baffle for backpressure, choke plates removed from the carbs, stock on / off for the killswitch & minimum wiring harness, BT45's for tires, pegs raised a few inches, etc etc.

Btw, nice to meet Chris & George at Jersey. Good chatting with you fellas & welcome to the AHRMA addiction Chris : )

Andrew McCarthy
AHRMA #075

[Jerry Rxman Griffin aka MuthaF'er]

Will I be seeing any of you racers at the Bonneville GP next weekend?

[fenianmoto]

We'll be at Barber Jerry, going to a good friend's wedding in Chicago next weekend.

[Roach Carver]

Marked.