Landracing Forum
Tech Information => Technical Discussion => Topic started by: Vintage_Honda_Garage on August 10, 2020, 11:15:54 AM
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Hello. I mentor a high school after-school bike building program and this year we are shooting for the salt with a M-BG Honda CB350. I have a few questions as this will be our first time doing something like this.
First question: Would a cafe style "bump seat" be considered partial streamlining? Something like this?
(https://i.ibb.co/gg8nCZ3/4149751.jpg)
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Nope, you are good ,,,,,, You should get a SCTA rulebook as it is pretty much the bible across all land speed venues..
Robert in California
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Nope, you are good ,,,,,, You should get a SCTA rulebook as it is pretty much the bible across all land speed venues..
Robert in California
I have the 2019 book. I just was uncertain if this bump would be considered streamlining or not.
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Next question for the bike guys, looking for tire recommendations. Stock size for this model is 18-100/90 in the front and 18-110/90 in the rear.
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These old 350's are raced in the UK on tracks and road circuits. Get a copy of "Classic Racer" magazine from the UK. There are ads in it for race tires in the narrower old widths and lots of other useful items.
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I think those sizes may be quite a bit larger than stock tires for the CB350. If I have the stock rim widths right (1.6" front a.k.a WM0, 1.85" rear a.k.a. WM1) they were set up at the factory with narrower tires like 3.00-18 or 80/90-18 front and 3.50-18 or 90/90-18 rear. Even those tire sizes seem a bit wide for the rim sizes - back in the 70s Honda seemed to fit wider tires on a rim than the tire manufacturers recommend these days. On a smaller displacement bike on the salt you might get a small advantage in rolling resistance with a skinnier tire. I think you could even go down to a 2.75-18 front and a 3.00-18 rear on the stock rims. If you're flush, Avon has race tires in the proper sizes. Don't know how much advantage they would have beyond a higher speed rating than some street tires in those sizes. Avon Streetrunners are S rated - 112mph and reasonably priced. I run S rated Heidenaus on my nitrous CB77. Record in the fastest class I run in is 108, so they are good for now. I run a very skinny front, 2.50-18 K40, on a kind of oddball 1.4" skinny wheel. Rear is a 3.00-18 K33 on an XL350 aluminum rim that is, IIRC, 2.15" a.k.a. WM2.
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I think those sizes may be quite a bit larger than stock tires for the CB350. If I have the stock rim widths right (1.6" front a.k.a WM0, 1.85" rear a.k.a. WM1) they were set up at the factory with narrower tires like 3.00-18 or 80/90-18 front and 3.50-18 or 90/90-18 rear. Even those tire sizes seem a bit wide for the rim sizes - back in the 70s Honda seemed to fit wider tires on a rim than the tire manufacturers recommend these days. On a smaller displacement bike on the salt you might get a small advantage in rolling resistance with a skinnier tire. I think you could even go down to a 2.75-18 front and a 3.00-18 rear on the stock rims. If you're flush, Avon has race tires in the proper sizes. Don't know how much advantage they would have beyond a higher speed rating than some street tires in those sizes. Avon Streetrunners are S rated - 112mph and reasonably priced. I run S rated Heidenaus on my nitrous CB77. Record in the fastest class I run in is 108, so they are good for now. I run a very skinny front, 2.50-18 K40, on a kind of oddball 1.4" skinny wheel. Rear is a 3.00-18 K33 on an XL350 aluminum rim that is, IIRC, 2.15" a.k.a. WM2.
All good info. Thank you. Our goal is 121 mph so we'll need a higher rated tire.
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Next question: Suspension.
What kind of setup do we want in the front and back? Stiff? Soft? Normal?
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Whatever suspension you choose, make sure that the bike and rider are as low as possible. Less wind resistance.
In your class, the stiffness of the suspension will not matter much. For high power, fast bikes that are prone to rear wheel slippage keeping the wheels on the ground is the most important thing so a fast rebound is best.
So in your case, the stock suspension will be fine, just make sure you set the height as low as physically possible and within the rules.
Good luck,
Don
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Whatever suspension you choose, make sure that the bike and rider are as low as possible. Less wind resistance.
In your class, the stiffness of the suspension will not matter much. For high power, fast bikes that are prone to rear wheel slippage keeping the wheels on the ground is the most important thing so a fast rebound is best.
So in your case, the stock suspension will be fine, just make sure you set the height as low as physically possible and within the rules.
Good luck,
Don
Awesome. Thanks for the insight.
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Great project bike with all kinds of speed parts available. I'm writing a book on "How to Make a Slow Bike Fast", so I'm very interested in your project.
Where are you located and where/when do you expect to have it finished?
Here is a photo of my M class 250 record holding bike.
(https://i.postimg.cc/0Qq5KPpc/TR25-at-Bonneville.jpg) (https://postimages.org/)
Tom
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Our goal is 121 mph so we'll need a higher rated tire.
Mickey Thompson makes a 2.50/2.75-18 drag front tire.
Avon Roadriders come in various street and race (AM-26) types, in sizes that might work.
If I ever get to the point of needing a higher speed rating I might look at that Thompson front and an Avon Roadrider 110/80 V 18 rear, which is rated for 2.15-3.0 rims.
My experience is limited to one year on very smooth hard salt and one year on rutted salt with soft spots where even my peewee nitrous 305 was spinning a little here and there. I was glad to have a fairly stock suspension with some travel on the rough stuff.
I'll suggest putting some energy into figuring out how to reduce your own frontal area as much as possible along with lowering the bike. Try to wrap yourself as tight as possible onto the bike. Get snug leathers that won't flap in the breeze. My bud Jaylin Billig says she "looks like a bug hanging on for dear life" on her CB175.
@Koncretekid - lovely bike!
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Great project bike with all kinds of speed parts available. I'm writing a book on "How to Make a Slow Bike Fast", so I'm very interested in your project.
Where are you located and where/when do you expect to have it finished?
Here is a photo of my M class 250 record holding bike.
(https://i.postimg.cc/0Qq5KPpc/TR25-at-Bonneville.jpg) (https://postimages.org/)
Tom
We are based out of Milwaukee, WI and hope to be at Speed Week 2021
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Our goal is 121 mph so we'll need a higher rated tire.
Mickey Thompson makes a 2.50/2.75-18 drag front tire.
Avon Roadriders come in various street and race (AM-26) types, in sizes that might work.
If I ever get to the point of needing a higher speed rating I might look at that Thompson front and an Avon Roadrider 110/80 V 18 rear, which is rated for 2.15-3.0 rims.
My experience is limited to one year on very smooth hard salt and one year on rutted salt with soft spots where even my peewee nitrous 305 was spinning a little here and there. I was glad to have a fairly stock suspension with some travel on the rough stuff.
I'll suggest putting some energy into figuring out how to reduce your own frontal area as much as possible along with lowering the bike. Try to wrap yourself as tight as possible onto the bike. Get snug leathers that won't flap in the breeze. My bud Jaylin Billig says she "looks like a bug hanging on for dear life" on her CB175.
@Koncretekid - lovely bike!
We ended up going with the AM-26 Avon Roadriders. 90/90 in the front and 110/90 in the back (smallest we could get). We also spent up a bit and got Nachi bearings along with aluminum rims.
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Another seat question.
Rule 7.F.4.b states "Rear fenders shall extend rearward to a point not less than a vertical line drawn through the rear axle. A seat that covers the rear wheel to the vertical line may substitute for the fender requirements."
This is the seat we are probably going to go with since we already have it (aka its free). This appears to fall within the aforementioned rule, correct? Also, this would not be considered streamlining correct?
(https://i.ibb.co/6YGw0WJ/IMG-9451.jpg)
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That seat should be fine for "M" class.
Just make sure that it is not over 36" high and doesn't extend rearward past the tire.
Here is the current holder of the record you are running on.
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In open bikes streamlining is anything in front of the rider that appears to direct or divert air.
Seat and tail follow partial streamlining rules.
I would not use the wrap around rear fender.... it will grab air and slow you down.... and if I remember the tail and rear fender rule say it can't extend past the back tire.
Use bars that force your elbows in to help the rider maintain a tight tuck. Smallest rider you have.... be sure your rider and owner is a BNI member and must have a motorcycle drivers license.
Good luck with your project... Become one with the motorcycle :cheers:
edit... Take a good look at the picture of the current record holder.... read the rules 50 times.... don't add anything to the rules... nothing is implied, they are what is written in the book.
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Seat and tail follow partial streamlining rules.
Are you saying that the blue seat in my previous post would fall under partial streamlining?
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Take a look at 7.F.12 Look at a few paragraphs into this section. It states that if a streamlined seat, tail section or fender is used, it can not extend further to the rear than a vertical line drawn at the rear edge of the tire (This is for non OEM components)...meaning that seat cannot extend beyond rear edge or tire. I used a level and stood it vertically against the rear tire and made sure my aftermarket rear fairing didnt extend past the rear tire. I had to fab up some mounts to move the rear fairing forward about 1.5 inches. Another option is to extend swing arm slightly to get rear tire further back. Rules state you are allowed to extend wheel base 10%. Good luck with your project!
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Seat and tail follow partial streamlining rules.
Are you saying that the blue seat in my previous post would fall under partial streamlining?
In a nutshell Stainless is saying the blue seat is ok to use in "M" class.
Don
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Ok. New question. This has to do with frame modifications.
7.F states:
"Perimeter-type frame engine cradle tubes must remain unmodified. Spar-style main frame spars must remain unmodified. "Main frame spars" are defined as the large formed tubes which connect the steering neck to the engine mounts and swing arm pivot."
My question basically is, can we remove/modify/relocate the circled section of our frame and still be within the rules?
(https://i.ibb.co/X3JWz3R/bodyframe-medium50100286020-B-01-32e0.jpg)
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That looks a lot like "large formed tubes which connect the steering neck to the engine mounts and swing arm pivot." to me... but luckily my opinion is just that... if you are planning something there... draw it up and submit it for a ruling... earlier the better...
:cheers:
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If my understanding of the rule is correct, that will land you in "A" class.
As Stainless said, ask for clarification and then let us know.
Don
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Are you wanting to get rid of that center tube to make room for a blower?
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Are you wanting to get rid of that center tube to make room for a blower?
Correct
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This is something a welder did for me decades ago. He tacked the new tubes onto the frame before he cut out the middle member. Otherwise, it is easy to get warpage in the frame when doing this job.
Be careful with some Honda frames like the SL350K2 and their dirt bikes. They are made from chrome-moly steel and are very hard to weld. Cracks form near the weld and the tube breaks after use. Special welding methods are needed to prevent this. Not all old Honda frames are mild steel.