Author Topic: Dry weights for cb750 based road racers (Read 14117 times)

gschuld · « **on:** August 04, 2015, 04:54:53 PM »

I'm curious what others have gotten their weights down to on road racers. I was a little taken back by Mitch Boehm's article(below) stating that his Dresda framed AHRMA legal Formula 750 cb750 was "just 368 pounds without fuel". He didn't mention if it was 368lbs without oil as well, but either way, that's pretty light. I know 400lbs cb750 street bikes exist, I just wasn't expecting much lower for a full fairing road racer.

FWIW(from recent personal experience), a bare dead stock early cb750 frame (without swingarm) is almost dead on 37lbs, stripped of all the unnecessary items, it's right around 30(depending on how far you go). Gusseting brings back up to a few pounds(also depending on how far you go). So lets say 34lbs for a race ready heavily stiffened factory frame minus swingarm. Even the lightest custom (cb750) road race frames would likely weight in the low to mid 20s minus swingarm. So roughly a 10-15lb savings including a lighter swingarm.

George

http://www.motorcyclistonline.com/flashback-cardiac-kids-strike-again

Our Dresda-framed CB750, sort of an English-framed--and thoroughly modern--version of Honda's famed CR750 production racer, had handled Grattan's twisty circuit surprisingly well. It was light (just 368 pounds without fuel), flickable, stable, had killer brakes, was surprisingly fast (nearly 82 rear-wheel horsepower), and was amazingly easy to ride at the limit, especially with the sticky Dunlop KRs mounted. Considering its vintage, the bike worked a lot better than I'd expected.

bwaller · « **Reply #1 on:** August 04, 2015, 06:00:19 PM »

I'd forgotten about that article, enjoyed that one. Mitch had a hand in a KZ650 at some point later too. Nothing ever seemed to stick though! That's a lightweight heavyweight.

scottly · « **Reply #2 on:** August 04, 2015, 06:56:29 PM »

I'd never seen that article; thanks, George.
According to the satanic mechanic, the Seeley frame with swing-arm weighs 26.45 lbs. I've had my bike as low as about 400 lbs in street "legal" form (meaning headlight, tail/brake light, and tiny mirror), with oil but no fuel.

gschuld · « **Reply #3 on:** August 04, 2015, 07:42:02 PM »

Scottly,

I enjoyed that article as well. Good story...

I recalled your lightweight Seeley framed bike as I started the thread. Thanks for the Seeley frame/swingarm weight reference, pretty light (about 15lb savings). There was a nice series of articles in Cycle Rider magazine in 1973 where they did lots of experimenting with a k1 over 7 articles. In one edition they put it on a major diet and managed to get it down to 399lbs minus fuel using a stock frame and remaining street legal. Pretty impressive. It was not in road race trim, single disc front and stock tire sizes and all. But, like your Seeley, it gives a good example of weight reduction possibilities. 368lbs though, wow...

Brent,

Mitch seems to be a real character and competitor.

I'm wondering what the practical weight reduction limits are on a factory frame based cb750 road racer. I am thinking that anything close to 400 lbs in proper race trim would be a real accomplishment without resorting to modern components.

George

bwaller · « **Reply #4 on:** August 04, 2015, 08:02:23 PM »

I am thinking that anything close to 400 lbs in proper race trim would be a real accomplishment without resorting to modern components.

I agree. Some of the guys here will surely offer some numbers.

scottly · « **Reply #5 on:** August 04, 2015, 08:05:46 PM »

Wheels lighter than Morris mags, maybe 10-15 pounds. Lightened crank, 3-4 pounds. Lighter seat (high density foam is heavy) 3-4 pounds. Total loss ignition, no rotor, no field, no stator, 9 pounds, plus the weight of a battery that can last a race. Alloy oil tank with light weight fittings, maybe 2-4 pounds. Without the fairing, I could see 368, but with the fairing it might be hard to do, unless it's ultra-light carbon fiber? If anyone knows an easy way to shed weight off the motor, other than the crank and rotor, I'd like to know about it.

754 · « **Reply #6 on:** August 04, 2015, 09:19:40 PM »

Yes Scottly, the not using charging system takes off 25 percent of their difference between yours and the racerC
I think your bike is mostly period components... A Ceriani or Betor fork from back in the day may save another 5-8 lbs..
Is your shifter cover cut down? Do you run alloy handlebars..
A Grimeca mc may be lighter than stock..

scottly · « **Reply #7 on:** August 04, 2015, 09:41:45 PM »

Stock shifter cover without seal 590 grams. Cut down cover 480 grams.

scottly · « **Reply #8 on:** August 04, 2015, 09:59:32 PM »

Newest weight savings are from changing stock K7 42.8mm caliper pistons to phenolic pistons; 215 grams each vs 85 grams each.

Sam Green Racing · « **Reply #9 on:** August 05, 2015, 01:54:58 AM »

John Davies is in the process of building a 750 race bike for a top runner in England, after seeing the 500s that he built, the 750 should be quite hot.
I'm not 100% sure but I think he had the 500 frame down to 15 pounds.

Sam.

Retro Rocket · « **Reply #10 on:** August 05, 2015, 02:27:46 AM »

You've got some interesting threads going lately George...

gschuld · « **Reply #11 on:** August 05, 2015, 03:49:08 AM »

I like to keep everyone thinking...

George

jaguar · « **Reply #12 on:** August 05, 2015, 05:22:03 AM »

Here is a fairly good list of frames.
Not all of them have weights, but might help in the research.

Seeley frame is 26-27 pounds.

http://www.satanicmechanic.org/egli.shtml

gschuld · « **Reply #13 on:** August 05, 2015, 06:04:38 AM »

From a post made by Voxonda a few years ago:

"Well maybe not a '70 CR but, at least for me, close enough. I knew I had it somewhere, but managed to find it in that stack of old magazines. An article about the 1974 production racers from Honda. The Dutch importer managed to get two of these complete bikes, build at the factory. In the article it is mentioned that although Honda rather used megga's, these bikes were equipped with a, open, 4 into 1. The bikes were build with 'senior-set' parts. They gave 90 hp at 9.500 rpm, c.r between 1:10,5 and 1:10,9 and ignition advance pre-set at 40° BTDC. Weight 173 kg dry. Topspeed 260 km/hour. (162.5 mph)
Looking closely at the pictures some thing catches the eye, like the lower shock mount which is 'normal' and not like the CR750."

Me: 173KG is 381lbs for a Honda factory hand built CR750 racer with a 4 into 1 exhaust. Another good weight reference with clearly "period parts"

George

SOHC Digger · « **Reply #14 on:** August 05, 2015, 06:34:05 AM »

The CycleX race bike from the 2012 AHRMA Vintage Classic weighed at 400 even.

gschuld · « **Reply #15 on:** August 05, 2015, 07:10:42 AM »

Thanks. I was,wondering what the cyclex racer weighted.

George

bwaller · « **Reply #16 on:** August 05, 2015, 07:34:26 AM »

When I built my bike I took the approach that everything I touched I wanted it lighter. I mostly limited that to some of the parts I had here and machining etc. I also had a couple years...you have less than two months.

Do what you can in the time you have. Exchanging original components for lighter can happen in the off season and forward. Things like unsprung goodies are important, wheels, brake rotors, alloy sprockets, hubs etc. The bonus to this is improved handling. I fully planned on saving 18lbs with a different frame but you know that story so sometimes we have to settle.

Check PM.

SOHC Digger · « **Reply #17 on:** August 05, 2015, 07:48:36 AM »

One of the best places to shed weight is often the most overlooked: yourself. I don't know what your physique is, but this might be a good motivator for shedding unwanted pounds off yourself and building lean muscle. You and your bike are a single unit on race day, you both want to be strong and lean. Losing pounds of unwanted fat is more beneficial and sometimes easier than losing ounces of metal.

livefast_dieold · « **Reply #18 on:** August 05, 2015, 08:05:45 AM »

Hi there,

I weighted my bike just yesterday, 180 kg in full working condition (oil + fuel). I'm aiming to remove at least another 10 kg during the winter... let's see...

gschuld · « **Reply #19 on:** August 05, 2015, 08:53:45 AM »

Brent,

I am always looking for information... it's a disease. I couldn't possibly incorporate a full on weight loss program in such a short time frame. I have already bit off enough to choke on...

. It's in my DNA I guess.

SOHC Digger,

I fully agree, rider weight is a significant factor in overall all up weight an performance. Plus rider weight, by nature, is very high center of gravity weight. I am 6' 3" tall and built like a football player at 225 lbs. I could drop 20 lbs, and should, but getting under 200lb for my frame would not be fun. A 150lb rider would have a nice advantage that I could never make up. My reasons for bring the subject up are for personal information gathering, and by doing that, gather the information for other who are or may be looking for this info in the future through the search function.

I should be able to get the bike down to 425lbs dry fairly easily, which would give me an all up weight of around 675lbs with fuel, leathers, helmet, etc.

Mitch Boehm's 368lbs dry weight bike with a 150lbs rider would come in around 555lbs total on the track ...or 120lbs lighter than I can fairly easily manage. Both I and the bike can drop another 20lbs each, or more if taken very seriously.

I'm just starting out with this road racing deal. My primary concerns are solid and reliable handling, braking, safety equipment(riding gear), and engine. Everything else is secondary, but on my mind.

Livefast_dieold,

180kg(400lbs) is pretty impressive wet(or dry for that matter). Curious, how much fuel are you counting here, 4 gallons? Beautiful bike by the way...

George

Old Scrambler · « **Reply #20 on:** August 05, 2015, 10:09:54 AM »

G...........you can help your weight situation by getting your legs in shape

When you put added pressure on the footpegs your center of gravity is lowered. When your outside footpeg is loaded with extra weight in a turn, the weight transfer helps the tires to stay centered on the rims and provide more traction. The light-weight riders struggle to get enough weight over the front wheel when braking. Your total weight will still be adverse to a lighter rider on the same bike..............but don't worry about it.........use it wisely

gschuld · « **Reply #21 on:** August 05, 2015, 10:15:52 AM »

My understand is that in high level racing such as Moto GP, heavier riders tend to have the advantage(traction wise) in the turns while the lighter riders have the advantage on acceleration and top end speed going down the straights. I guess that I'll have to learn to handle the turns, or at least be prepared to make excuses...

George

simon#42 · « **Reply #22 on:** August 05, 2015, 10:46:50 AM »

there are no advantages in being heavy really , all mass has to be accelerated , decelerated and turned so the more mass you have the slower this happens ,
as for traction , theoretically there is an advantage in so much as you can move more mass about i.e. the rider but in practice this does not make up for the stuff you loose .
moto gp teams take rider size and weight very seriously , loris baz was hired by aspar and then fired when he turned up to the first test and they realised how big he was .

calj737 · « **Reply #23 on:** August 05, 2015, 11:04:21 AM »

You just want to select tracks that are all downhill, George. Then those puny little riders can see your backend! (Big as it is)

SOHC Digger · « **Reply #24 on:** August 05, 2015, 11:09:00 AM »

Quote from: Old Scrambler on August 05, 2015, 10:09:54 AM

When you put added pressure on the footpegs your center of gravity is lowered.

You're not actually lowering your center of gravity. Center of mass is still center of mass, whether the weight is transferred through the seat or the foot pegs.

What you ARE doing though is improving your ability to shift your center of gravity laterally, allowing your entire body to move instead of from your hips up.

Old Scrambler's right though... Get your legs in shape.

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Author Topic: Dry weights for cb750 based road racers (Read 14117 times)