Topic: CB750 AHRMA Sportsman 750 Racer

[slikwilli420]

I just got back from Road America watching a few family friends dominate the weekend on XS650 Yamahas in the Sportsman 750 class. I have the majority of the parts to put together a bike. The SOHC/4 is on probational acceptance into this class for the next couple of seasons.

Rules are pretty simple and don't restrict a whole lot. One thing is that I need to run the original round top carbs, so nothing fancy like CR carbs. Frame can be strengthened or of a dirt-track configuration but no road race frames. Brakes need to be single disc up front, whatever in the rear, and there is the option to use a period AP/Grimeca caliper, so I can ditch the swinging stock unit.

The family friends XS (punched to 750) is just under 300 pounds with 2qt oil and no gas, pushing around 70hp.

Do you think a /4 could be competitive in this class? Here are a few of the mods I was thinking based on rules:

12.5:1 comp pistons, but keeping stock displacement (or an overbore or two)
lightened crank
JMR head (whatever he thinks would work best)
Road race cam
Aluminum wheel hoops front/rear
Single disc up front w/ AP caliper and good pads
Forks (37mm from whatever, likely GL1000)
Front hub in rear with small disc/caliper (just enough to shed speed, but not lock tire)
Stock frame, detabbed and gusseted
Fiberglass tank and seat
Custom wiring harness
Total loss ignition

Without a custom frame/swingarm, how light do you think I could go? Assuming no exotic parts like Ti bolts or axles but with the above listed and any other reasonable weight saving ideas (like aluminum wheel spacers, engine mounts, etc)

How much power do you think the above package could make?

I would think I would need to shoot to get under 400 pounds with no gas to have a shot at braking as well as the XS guys.

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Here are some links to other conversations I started with different questions about my race bike:

Custom pistons:
http://forums.sohc4.net/index.php/topic,169671.0.html

Crank case venting:
http://forums.sohc4.net/index.php/topic,170117.0.html

Isotropic surface finishing:
http://forums.sohc4.net/index.php/topic,169780.0.html

Calfab swing arm questions:
http://forums.sohc4.net/index.php/topic,171774.msg1999685.html#msg1999685

Road racing final drive gearing:
http://forums.sohc4.net/index.php/topic,171364.msg1993745.html#msg1993745

Swing arm pivot stabilization:
http://forums.sohc4.net/index.php/topic,169354.msg1964990.html#msg1964990

Primary drive rubbers:
http://forums.sohc4.net/index.php/topic,163720.msg1886597.html#msg1886597

Oil cooler line routing:
http://forums.sohc4.net/index.php/topic,160711.msg1845156.html#msg1845156

[gschuld]

Good morning,

I think I can help some.  More from research than track experience.  A carefully built road racer like you are describing can be certainly be built for under 400lbs if you are dedicated enough to make all the minor weight saving changes, especially without fairing and a second front disc. 

Whether your bike will be competitive against 70hp sub 300lb SX machines is another matter entirely.

I believe the probationary status of cb750s in AHRMA sportsman class was about the carbs.  The probation mention in the same line as the carb requirements.  Personally, I'd like to know why.  I assume someone with a cb750s got in trouble for a non compliant set of carbs.

The rule is not very detailed on this.  What about using factory round top carbs that have been bored out and smoothed to flow better? 

George

[slikwilli420]

Yeah, I saw mention of the carbs. What could you do to the original round tops that doesn't include boring them out? Typically people just slap on CR and go from there. I would think that with CR carbs a /4 would be pretty competitive, but I am hesitant about the original rounds tops, since they can't be bored out.

If you have any specific weight-saving measures please list them here. Even if this never materializes it would make a good discussion for others.

[gschuld]

Well,

Searching through the sohc4 forums will give you lots of info regarding dropping weight.  A Google search general look y works best, just add "sohc4" with your subject.

The rules are ambiguous regarding allowed modifications to round top carbs.  Boring them could be OK for all I know.  Or standard practice among some racers considering it might not be the kind of thing that tech inspectors look for. 

With an very vaguely written carb rule, who knows.

A set of bored factory round top carbs are capable of supporting over 90RWHP.  How well they mix the fuel and their adjustability compared to CR carbs is another question, and unanswered to my satisfaction.

There were some fast cb750 racers back then who used round top carbs.  Unless your engine is REALLY strong at near stock displacement, round tops might not hold you back too much.

George

[turboguzzi]

300 lbs. for an xs650? lighter than a ducati TT2? not saying that it cant be done, but....

[slikwilli420]

That's kind of what I thought but that's what my buddy said. Chromoly dirt track frame and arm, single disc up front, tiny single disc rear. Front hub is stock, rear is a spool with no cush drive. Glass seat, glass 2.3 gallon tank. Running a dirt bike-style total loss ignition, so no battery, stator, alternator, etc. Sounds lofty but I'm in no position to call BS. If he isn't lying, then even at 400lbs, my /4 would still be 25% heavier, but with a much higher output potential.

[754]

I know XR 750 stock is around 315, Trackmaster Triumphs range from about 290-310.ish.. So yes its possible.. 310-315. Should be easy to reach with a twin.
 Getting below 390 on the 750 is gonna get expensive and tough..

[bwaller]

Wiseco made 62mm pistons with a finished or unfinished dome so an engine builder could go nuts. Well built at 1mm oversize 85HP is possible. Most assn's want that "period" look so bored carbs might not raise eyebrows, but better to ask the tech chairman. Don't overdo the frame & add weight, thin saddle type gussets stratigically placed is the answer. John & Alan are two guys to talk to.

[gschuld]

Cycle Rider April 1974.  Here is some food for thought regarding one bike that was brought down to 399 lbs in 1974.  Factory frame, glass tank and seat, single front disc.  Pretty much what you were describing.

George

[gschuld]

2nd round.

George

[gschuld]

Last two. 

George

[slikwilli420]

Honestly not a lot of magic there, and I see a lot left on the bone. They made parts in steel I would have made out of aluminum and they also ran a full light package for the street. Items like the Hurst rear brake are heavy by todays standards where a Nissin master weighs ounces instead of pounds. A smaller rear disc would also help a bunch. If they did 399, I could see 380-ish without going to space-age materials and unreasonable lengths.

[gschuld]

Oh sure, that was NOT meant for competition.  Just a fast(for the time) fully street legal bike.

It is merely a reference for what you can do for a track only bike.

George

[slikwilli420]

Yes, that is promising and proves that a sub-400lb /4 is possible, and without extreme measures. Every ounce counts since I would need to be able to brake similarly to lighter machines. What would be the ideal period brake setup? I would think an AP caliper with good pads and cast iron rotor would push the upper limit of braking ability.

The other piece is extracting reliable power from a motor that has stock carbs and nearly stock displacement. I would think good head work (maybe a ported F2 head?) and lightened rotating assembly with 12.5:1 compression would be extremely powerful.

[JohnN]

Slikwilli420:
 Don't forget you can enter AHRMA's Formula 750 class as well. Then you can run CR's, but you'll still have to compete with XS650 based machines anyway. You're on the right track with the front hub for rear wheel use; the rear hub is where you can make the huge weight reduction over stock. I have not weighed mine since making a few more changes but estimate I am around 405 lbs with full oil but no gas. That is with a lightweight hub from a later model MX with disc. Net reduction was about 17lbs and the rear wheel with disk is no lighter than my dual disk front wheel.
John

[slikwilli420]

Formula would be a natural progression of events. The only substantial differences are second disc up front, fairing (full or half) and no restriction on carbs. Besides those points, a Sportsman is the same bike.

I may start pulling parts together just to see what I really have, what I need, and what I can make. I want to know the eternal question - how low can I go?

[gschuld]

http://forums.sohc4.net/index.php?topic=150321.0

This thread could be another reference. 

George

[slikwilli420]

I remember that thread when it was going on. Nice to see references to actual pre/post weight of components.

[turboguzzi]

ricardo has brought the weight of his cb750 racer down to 360-370, good work but nothing too fancy other than crankshaft weight reduction. rest was well chosen components. that said, the fastest 750 class sohcs around here are big bored 550/650s, to 720-740cc, they can be lighter still. similar to Dr. Tom's big bore CB650 in ahrma a few years ago, he was beating everybody AFAIK

[scottly]

George, what happened to that Trackmaster frame that was for sale last year? I recall someone here got the Morris wheels...

[gschuld]

I didn't buy it.  I contacted the owner.  Exchanged photos.  Got the critical measurements I was looking for to do some comparative math on my end.

Didn't like the swingarm pivot area on the frame and wasn't happy with the high feather bed style upper frame rails that were much higher than a factory frame.  This requires the custom fuel tank to sit higher, and the rails were spread fairly wide at the top aft end of the loop of the frame not giving much chance of tucking your knees in.  It was over 2 inches wider than a factory or Rickman frame at the "knees area" 

I simply wasn't impressed enough with the design to consider spending a bunch of money risking it being a total failure.  Too much risk and commitment to an untried design, with little to gain.  Plus, using a custom frame would restrict me away from certain racing classes that require factory based frames.

In the end, I ended up heading down a much more conservative path.

George

[scottly]

George, not sure what you mean about the width of the frame in the knee area?

[gschuld]

Scottly,

Sorry, I got mixed up with the odd "Redline" road race frame someone was selling last year.  Might be fine for a seriously bow legged rider.

George

[slikwilli420]

I wonder how well a dirt track frame would hold up in road racing conditions? The family friend with the XS650 is using a Framecrafters dirt track frame and says its pretty flimsy on asphalt. I would be worried about trying to wrangle the bike as much as I would racing. Maybe for now a stock frame, properly gusseted would be best. Both frames posted are very interesting though.

[slikwilli420]

Another thought, and a bit of a tangent. I picked up a -410 head at a local swap meet over the weekend and would consider using it for this endeavor if it ever got off the ground. I can find +1mm oversize 12.5:1 pistons for the K and early F heads, but nothing for F2/F3 heads. Would this just be a matter of cutting the valve pockets larger for the bigger valves, or would it be better to get custom pistons? The advantages of larger valves is enticing and allows a head to be better right off the bat vs a K head. I would think that an F2/3 head with some port work, fresh valvetrain, a good road race cam and 12.5:1 compression would be a rocketship.