Author Topic: CB750 Racing (Read 32903 times)

Haybus · « **on:** January 20, 2015, 06:44:04 PM »

I'm building this bike to race the WMRRA vintage series this summer. Donor bike is a '75. First time ever racing. Been mostly working frame stuff, belly pan, fairing and seat mounts. I plan to keep a six pack in the belly pan, you know in case I need to refuel in the middle of the race

. I know, it's huge!! Holds a gallon and I think it only needs to hold a couple quarts. It's a building year, so probably lots of changes to come. I have Excel shouldered WM3 X 18 and WM4 X 18 on order with stainless spokes. Thinking of running Avons, maybe 90/90 up front and 130/650 or 120/70 out back. Works Performance steel trackers on order. I need to sort out the sprockets, probably 530. Any recommendations from you other road racers on a good starting point?

Alan

JohnN · « **Reply #1 on:** January 20, 2015, 07:08:09 PM »

Welcome to WMRRA and I hope to see you at Pacific in April! Belly pan requirement is 2 quarts so you will easily pass inspection. After a couple of years of decline the grid sizes have started to increase but we are always happy to have more competition! Avon's are popular here with many 750's running them. On my 750 I use 17/46 at The Ridge but have tried 17/48 which also works. Pacific has been a struggle for gearing because turns 4 and 10 are just a little too slow for 2nd, but 1st is too low with the wide spacing so I'm still trying to find the best combination. The best scenario would be a taller 1st gear as all the other gears are well matched to the turns there. Maybe a Nova transmission is in the future but the remaining ratios on the Nova are completely different so hard to say which is best.
John

bwaller · « **Reply #2 on:** January 20, 2015, 07:34:14 PM »

I run the same rim widths with a 130/65 AM23 rear. It's a good choice on a narrow rear rim. I had been using the 90/90 AM26 front, but recently made some changes and decided on the 110/80/18 AM22. I like it better. The narrower tire might help with turn in some but the bigger footprint of the 110 might be better for your heavier 750. Avon claims the AM22 front & AM23 rear were designed to work well together and they seem to.

In case you haven't checked, the only other suggestion would be to use some load straps and "load the suspension" (front & rear) to 80% or so and physically lean the bike over to see where your belly pan touches. Better now than find out later it hurts ground clearance.

I use Works shocks too, they're a good simple shock. It's good practise to spend time on suspension. Measure full travel then 26-28% sag. I really like RaceTech springs. You can use their chart to get the right spring weight. The springs are shorter and allow you to use spacers to fine tune. I use 520 chain & sprockets on my 572, but if you have 530 stuff it'll same money for now.

Good luck with your preparations, I hope you have as much fun as the rest of us.

Haybus · « **Reply #3 on:** January 20, 2015, 08:01:23 PM »

Beautiful bike you have there John. Probably saw you racing at the Ridge last summer. Thanks for the ratio recommendation.

I'm hoping to make the first race, but still lots to do. When I'm there, you'll know it

. My bike will magically transform into something the size of a CB160 once I hop on it. At least it will appear that way. My friend Chris is building a Suzuki T20 to race too. His wife Katie was working track entrance last year, probably this year too.

Brent, thanks for the suggestion on tires and loading the suspension to check clearance. I think I'm ok for clearance, but won't really know until I get the rims/tires and shocks. As it sits now it's a little over 4" clearance. Could probably tuck it up a bit more with a little work. Your road racing thread has been fun to read. Lots of good info there.

Alan

turboguzzi · « **Reply #4 on:** January 22, 2015, 04:55:25 AM »

alan (and brent too

)

no need to risk hernia by leaning the bike..... or even load suspension.... take a big plywood plate, say 2 ft wide and a little longer than the wheelbase, place it at 45 degs just touching the tires and you should check for a 70-90 mm between plate and any "hard" parts....

bwaller · « **Reply #5 on:** January 22, 2015, 05:03:13 AM »

Ah there's always a trick up your sleeves TG!

Haybus · « **Reply #6 on:** January 22, 2015, 09:35:22 PM »

That's a good suggestion TG, and I've actually already done that

. The pan is nowhere close to the first thing that would hit in a lean. But I think I need to do it again with the suspension compressed like Brent suggested, but not until I get the new wheels and tires of course.

Question about brakes. I stuck with the stock set up, maybe not the best idea, but I only see the one option from EBC, and a few other no name brands from some of the supply houses. Any other options?

Alan

bwaller · « **Reply #7 on:** January 23, 2015, 04:51:44 AM »

Yes things change when the suspension is compressed. Belly pans are notorious for touching down at the wrong time! My original pan was mounted solidly at first but cracked at the mounts from vibration, watch for that. I ended making mounts allowing it to "hang" from the frame.

Brakes. Most will say you don't need them! I see you have two discs mounted. Just for #$%*s & giggles, you could consider shaving them to 5mm and drill and/or slot them. It certainly helps reduce unsprung weight and has no ill effect on braking power. In our race ass'n the class rules state "no floating rotors". My 276mm 550 discs are 2 lbs lighter each than stock. Try to find more aggressive pads. I followed turboguzzi and use small Brembo calipers which are still "period", but the stockers will work fine.

Haybus · « **Reply #8 on:** January 26, 2015, 10:14:25 PM »

More aggressive pads you say Brent? Hmmm, seems like my options are limited to good old fashioned organic that will probably give me a heart attack when I reach for the lever on lap 2, or a semi-sintered pad. Crap. I just bought new pistons and seals for the stockers.

Rotor drilling was in my plan. I've wanted a rotary table for awhile now, this is as good an excuse as any to get one.

Alan

Haybus · « **Reply #9 on:** February 22, 2015, 06:34:04 PM »

Not as much progress as I'd like, but the motor is back in, wheels are laced and trued, all new bearing in the hubs and steering stem. Shocks are in, but I've been out of town so haven't been able to pick them up. Hopefully next week end I'll have a roller again.

Alan

Jim F · « **Reply #10 on:** February 23, 2015, 03:24:36 AM »

Sweet

turboguzzi · « **Reply #11 on:** February 23, 2015, 04:48:59 AM »

we never do as much progress as we want, do we?

my 0.02: pads, not exactly racing compound but try ferodo platinum from vintagebrake.com... i did run the them early on on stock 500 calipers, where acceptable. invest time at home spacing the caliper brackets and sanding the pads in order to make full contact as they work at angle.... not fun work but amost with these..

JohnN · « **Reply #12 on:** February 24, 2015, 09:18:19 PM »

Looks good Alan, I think your progress will get you on the track when the season begins! I agree with Brent and believe your brakes will be sufficient. A bigger problem with these bikes is getting them to turn, and you have likely read the tips on raising the rear to reduce rake. This also increase clearance for lean angle. You can bring the trail back up with a late 70's 750K 45mm offset triple. Removing chain links to shorten wheelbase also helps.

John

Haybus · « **Reply #13 on:** March 01, 2015, 09:50:01 PM »

Little more progress today. Only had today to work. I've mounted lots of tires over the years, but these were the hardest ever. Then I had a heck of a time getting them up on the bead. Those Works shocks are super light, and very nice. If it happens to snow during the first race, my belly pan might come in handy as a plow.

I've got just shy of 5" of clearance to the belly pan. Sorry the picture is crappy, too lazy to go grab a real camera.

Thanks for the advice everyone, I really appreciate the tips.

Alan

Retro Rocket · « **Reply #14 on:** March 01, 2015, 10:02:01 PM »

How are you going to get pipes past that belly pan...?

Haybus · « **Reply #15 on:** March 01, 2015, 10:36:15 PM »

The pan has to come back off, I just threw it on to see how it all looks together and make sure everything still fit after finishing the welding. The pipes go between the frame and pan and they tuck up pretty snugly to the frame. I have to do some work on the rear pipe mounts. I'll post up pics when I'm done.

Alan

HondaMan · « **Reply #16 on:** March 01, 2015, 10:38:36 PM »

Quote from: JohnN on January 20, 2015, 07:08:09 PM

Welcome to WMRRA and I hope to see you at Pacific in April! Belly pan requirement is 2 quarts so you will easily pass inspection. After a couple of years of decline the grid sizes have started to increase but we are always happy to have more competition! Avon's are popular here with many 750's running them. On my 750 I use 17/46 at The Ridge but have tried 17/48 which also works. Pacific has been a struggle for gearing because turns 4 and 10 are just a little too slow for 2nd, but 1st is too low with the wide spacing so I'm still trying to find the best combination. The best scenario would be a taller 1st gear as all the other gears are well matched to the turns there. Maybe a Nova transmission is in the future but the remaining ratios on the Nova are completely different so hard to say which is best.
John

Which model gearbox are you using? Is it pre-1976 (i.e., K0-K6)?

Retro Rocket · « **Reply #17 on:** March 02, 2015, 02:58:36 AM »

Quote from: Haybus on March 01, 2015, 10:36:15 PM

The pan has to come back off, I just threw it on to see how it all looks together and make sure everything still fit after finishing the welding. The pipes go between the frame and pan and they tuck up pretty snugly to the frame. I have to do some work on the rear pipe mounts. I'll post up pics when I'm done.

Alan

Cool, love the Z in the back, we could possibly force ourselves to look at some pics of that bike too ....

bwaller · « **Reply #18 on:** March 02, 2015, 03:54:05 AM »

I know it's hard to make then swelt, but that thing is huge. Don't forget you probably only need to hold a couple litres for a dry sump engine.

JohnN · « **Reply #19 on:** March 02, 2015, 08:11:51 AM »

Alan:
You can reduce the size of the pan considerably and remain legal. Mine is much smaller but exceeds the 2 quart requirement and has passed inspection at WMRRA and AHRMA events. See photo- it matches the frame rail width and extends forward enough to leave room for the downpipes.

Mark:
I have a K6 transmission; any tips for 1st gear options? Something closer to 2.0 would be ideal.
John

Haybus · « **Reply #20 on:** March 02, 2015, 07:19:00 PM »

Now I'm thinking I'll keep the huge belly pan, just so people have something to talk about. Better they talk about the huge pan than how slow I was going, or "did you see that huge guy on that tiny bike." Who knows, maybe it's functional, like a NASCAR wing, but sort of reverse, and way different. Ok, maybe not that functional, but certainly fashionable. You know you'll all be wanting a huge wing under your bike now.

John, I like your minimal pan. I think it works well with your full fairing. I was going for something that would look like a chin fairing, but I kind of missed. Are you running baffles in your pipes? I made 2-1 pipes but don't have any baffles yet. I haven't fired it up, but I imagine it's going to be fairly loud.

Mark, transmission is K5. I haven't read all your book yet, but was there a certain year that's better than others? BTW, your book is great. I'm sure you've heard it a million times, but thanks for all you put into publishing that work of art.

My KZ was a frankenbike with a broken frame when I started. So I don't feel too bad doing what I did to it. 76 on the title, but someone turned it into an LTD and put a 77 1015 motor in with 29mm smoothbores and an RC pipe. That pipe is lovely above 4k rpm. It mostly looks like this, I don't have any more recent photos.

Alan

JohnN · « **Reply #21 on:** March 02, 2015, 08:16:42 PM »

Yes, I have baffles installed. WMRRA requires them, but has not conducted sound tests in the last few years. There are bikes on the track that don't have baffles and it does not seem to be a problem. This season there will be one round at Portland, and I've heard they are strict and will conduct sound checks.
John

Retro Rocket · « **Reply #22 on:** March 02, 2015, 09:15:07 PM »

Love the color of the KZ, whats it run like with the VM29's..?

turboguzzi · « **Reply #23 on:** March 02, 2015, 10:41:54 PM »

Quote from: bwaller on March 02, 2015, 03:54:05 AM

I know it's hard to make then swelt, but that thing is huge. Don't forget you probably only need to hold a couple litres for a dry sump engine.

+1,
way off in size.... you'll be dragging it big time, even going very slow...

looks like you'd need to cut off the parallel flanks to get it right

Haybus · « **Reply #24 on:** March 02, 2015, 11:02:15 PM »

RR - Took a bit of tuning, but I have them dialed in pretty good. I've never had any other carbs on it, so nothing I can compare with. It's super fun to ride. Seems like it has a lot more power than it probably does in reality.

TG - I've got so many other things to worry about before the first race. If that pan hits the ground I'll toss it and take up golfing or fishing or bumper boat racing.

News:

Author Topic: CB750 Racing (Read 32903 times)