Topic: CB750 AHRMA Sportsman 750 Racer

[gschuld]

I agree.  The biggest benefit of using a factory frame is that there is a wealth of knowledge regarding the geometry and setup for road racing.  Starting with an untested frame leaves you in the position of chief designer, suspension, geometry specialist, and head test pilot.  Not good.

I have recently spent some time weighing the pros and cons of different setups, and I fought off my natural tendency to build a road racer out of rare, semi unobtanium vintage parts.

George

[754]

Was it a Trackmaster frame? Usually have oil in the spine.
The frame is cut off behind the shocks....no hoop..like most of the others... Which may be stiffer.
 Just tell him to slide it around the corners..

[bwaller]

Mike has suggested before that a K head was his choice. You can use larger valves, thinner stems with less combustion chamber. Every time you add more piston dome you need to add ignition lead.

[livefast_dieold]

Hi there!

I can say I have some experience racing a sohc 750 against lighter bikes!

You can easily outpower the XS with the proposed configuration, but it will be very hard to have a lighter bike. Mine at the moment is 350 pounds, of which 150 just for the engine! I haven't used anything fancy (and expensive) like magnesium wheels or titanium bolts, just a good old grinder :-)

I'm quite concern about the single disc on front, it depends on what components are allowed but with it will be hard to have enough brake force with vintage components...

I strongly suggest using a CB650, and bore it up to 750. At the end of the day the 650 engine has been developed almost 10 years later than the 750: smaller, lighter with hivo chains on both primary and distribution, no external oil tanks and much more.  Power output will be similiar (if not better) than your proposed configuration, and the engine would weight 20 kg less!

Don't waste time bracing the frame, on both 650 or 750, but actually try to reduce the weight as much as possible.
My frame weights 12 kg (26 pounds).

Ciao!
R.

[slikwilli420]

Livefast_dieold, would you be willing to detail all of the weight-saving steps you have taken? I think it would be helpful to others as well to have as comprehensive of a list as possible.

[gschuld]

Mine at the moment is 350 pounds.

Ciao!
R.

I assume that 350lbs is without fluids?  Pretty impressive weight reduction program.

George

[754]

Live fast, what did do you take off the motor to total around 25 lbs
Starter, dyno rotor and coils, plus field, still probably around 20 lbs... Hard to take much weight off elsewhere on engine.
 Now one thing I am not sure of, is the listed motorweight at around 178.....with or without carbs..?

[slikwilli420]

Can get another 7-10 off the crank. Modern pistons are lighter as well. Upgrade to aluminum sleeves with Nikasil coating is pretty serious.

[turboguzzi]

7-10 off the crank is way optimistic... 6max more like it.
and not that you'd want to chop off so much either, too light a crank has drawbacks too.

[754]

Yes running a totally lightened crank is not always desirable...

[Roach Carver]

very interesting.

[livefast_dieold]

Well, it's a long list :-) 350 without fluids of course.

But first, pictures:


(Before painting)


Engine:
> removed alternator and starter (here is where you really reduce engine weight)
> lightened crankshaft, now at 8 kg/ 17 pounds
> lightened clutch basket
> lightened engine cases
> lightened engine covers
> remove rev counter shaft (I'm using an electronic rev counter)
> modified arrow 4-2-1 exhaust from an honda cbr900
> aluminium oil tank

Frame:


> removed everything that was of no use
> modified to allow removing the head with the engine still in the frame
> relocated rear shock mountings, rest cutted
> after market rear swingarm
> bitubo front fork 41 mm (taken from a big guzzi)
> steering plates from an honda cbr900
> alloy front wheel from a ducati pantah - by using tubeless tyre you save 0.8 kg (rotating masses!)
> alloy rear wheel from a kawasaki zephyr 550 (did a lot of research for this, it's almost bolt on! and allows to save 3 kg! + 0.8 kg of tube)
> lithium gel battery (it's unbeliavable how light they are!)

[slikwilli420]

Nice bit of kit you have there. A few things there I can't get away with in the AHRMA class I want to race in but its a good start! Can you share more about the ignition you used? Going to a total loss style will help me save a ton of weight.

[RAFster122s]

There is another fellow who is here on forums and he was able to easily get their racer down to 401 pounds without lots of mods. They also race a cb160.

[livefast_dieold]

Nice bit of kit you have there. A few things there I can't get away with in the AHRMA class I want to race in but its a good start! Can you share more about the ignition you used? Going to a total loss style will help me save a ton of weight.

Dyna green coils + Dyna S

If you are looking for some spoke wheels I have my old wheels up for sale (18' front with alloy rim and lightened hub, rear 3" wide with lightened hub)

[Roach Carver]

That bike is incredible. I LOVE IT.

[slikwilli420]

Just running a small battery to keep it going?  Definitely interested in the wheels. Please send pics, price and details. Slikwilli420 at yahoo dot com.

[livefast_dieold]

Just running a small battery to keep it going?  Definitely interested in the wheels. Please send pics, price and details. Slikwilli420 at yahoo dot com.

yep it's enough for a regular race, not for endurance of course

I'll send email tomorrow!

Ciao!
R.

[turboguzzi]

willi, just one question that surprisingly nobody included me thought to ask.... how competitive you think you are? i am asking this because nine years ago i was a bit like you, thinking that all what stands between me and podiums was a competitive bike, but after nine years of racing i was obviously wrong...  once you have a basically good running machine, it's mostly the rider who makes the difference... cesare/zaizer for instance has a quite basic 650 big bore but already in his second season was racking up podiums, fast lap times, and got a third in championship, simply gifted... so unless you are such an exceptionally gifted rider (rare, but could be...) build a decent starting point, work from race to race, season to season to improve it, and then see if it's really having the ultimate sportsman cb750 what stands between you and a championship.  last year ricardo was almost going to jump ship but instead worked on the weak points of his cb750 and now he is starting to reap the results. 

[slikwilli420]

Legitimate question turboguzzi. I really have no idea. I have never swung a leg over a bike competitively. This is really twofold. The first part being a challenge to see how light I can go on a sohc/4 and be within class rules. The secondary is being competitive. It sounds silly but at this point is only competition with myself over time. I will try to do well of course but that's not my primary goal.

[scottly]

 Now one thing I am not sure of, is the listed motorweight at around 178.....with or without carbs..?

Without carbs, at least the last time I weighed one.

[gschuld]

My dead stock k2 engine weighs 184lbs without carbs, carb boots, exhaust.  It has both oil lines still attached and has whatever oil is still in the engine when pulled from the frame (+/- 1/2 gallon?).

It weighed 184lbs in that condition.  Take off 3 lbs for the remaining oil and another 2 for the oil lines, and that leaves about 179 lbs dry, minus carbs.  Sounds pretty close to 178 to me.

A set of factory K0 4 pull carbs with throttle linkage and carb boots (no boot clamps or velocity stacks) weigh 8.25lbs. FWIW, a K1 and up set of carbs are a little heavier with the big throttle linkage rack.

So I'd say a dead stock cb750 motor weighs about 187lbs with carbs (no air box) DRY.

Removing the starter, starter gear, rotor, stator, lightened crank, lightened cases/covers/clutch basket, certainly will help reduce the weight of the engine some.

George

[Haybus]

Getting much below John's weight in our class would be a big, read costly, task. I thing that also applies to the AHRMA Formula and Sportsman classes. For sure it's good to plan for components that will net you the best weight savings though. For example, I chose 40 hole rims front and back so it's not that easy now to swap in a rear disc hub from a dirt bike without ditching about $500 in the current setup. If I shaved off another 20 pounds from the bike (or another 80 from me!!) would I be able to stay with John for more than a lap? Not likely yet, I just don't have the experience or confidence to drive the corners like he does. Maybe next season I'll feel it's worth spending the money to shave off that 20 pounds. It's quite a bit of work to build a bike for racing, and it's pretty easy to get discouraged, lose interest, or simply run out of money. I agree 100% with TG, get on the track with a decent setup and let your inevitable passion for racing fuel your drive to shave weight, build power and improve handling between races. It certainly drives me. Will be really interesting to see what you come up with.

Alan

[turboguzzi]

OK, willi, good to keep that in mind and not drive yourself into an alley i think the only guy in sohc who went crazy building from day one and then when straight to winning is brent. But he was a hot shot in his youth, so doesn't count! liked the "inevitable" bit Alan, that's how it works!

[livefast_dieold]

I will also strongly recommend taking track riding courses with someone with a good racing experience. I did one last month and it helped me correcting mistakes I was absolutely unaware, i.e. I was doing things thinking they would help me go faster, but they were actually slowing me down.

If I could go back in time I would have a completely different approach to racing: to have fun, especially at the beginning, it's essential to keep complexity as low as possible and change one thing at the time. Instead of focusing on the engine, I should have started by improving handling by using good shocks, good tyres and complete set of sprockets to adjust gearing.