B25 Sprint Bike....

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B25 Sprint Bike....

Postby sprinter » Fri Jun 24, 2016 8:58 pm

Hi,

I am new to this site, but have added a photo of my B25 based, 1/4 mile sprint bike....soon to be finished !

Thanks,
Jon

hangon (2).jpg

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Jack Gifford
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Re: B25 Sprint Bike....

Postby Jack Gifford » Sat Jun 25, 2016 7:12 am

Does "sprint" mean hillclimb?
After looking at the tires, I guess 'no'- you'd want more rear tire tread for hillclimb.
Is it a dragrace bike?
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Re: B25 Sprint Bike....

Postby hwan » Sat Jun 25, 2016 5:36 pm

SPRINT = DRAG BIKE

Dragging tends to be a knock out of two bikes, sprinting can be one at a time against the clock over 1/4 mile, straight, smooth tarmac (unless yuo run @ Melbourne !!).

Tidy looking bike consisting of a grass-track front end - welded to a vertical tube containing oil and rear end off something japanese - as i say - tidy and simple.
Now what you want is one of those AMR 300/500 superchargers (ebay ~£300) :lol:

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Re: B25 Sprint Bike....

Postby koncretekid » Sun Jun 26, 2016 1:38 am

Nice build! Bring that to Bonneville and set an altered class speed record- - the records are still quite low in the 250cc pushrod classes.
Tom
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Re: B25 Sprint Bike....

Postby hwan » Sun Jun 26, 2016 3:52 pm

The grass-track head angle and D1 Bantam front fork could be interesting ?

Still like it though - less is definitely more.

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Re: B25 Sprint Bike....

Postby minetymenace » Sun Jun 26, 2016 9:29 pm

I think you should blow it.
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Re: B25 Sprint Bike....

Postby hwan » Mon Jun 27, 2016 11:15 am

oh yes ...... as i say an nice compact AMR300/500 ...... :grin:

Sprint regs are a bit vague regarding which class you would then run in, but ...... classic 250 ?

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Re: B25 Sprint Bike....

Postby sprinter » Mon Jun 27, 2016 8:54 pm

Thanks guys,

I plan to run it at Weston Zoyland in Somerset in August.

You guys must be mind readers, as I have just bought an AISIN AMR300 Supercharger to fit over the winter ready for next year !

I have altered the head angle from 68 to 64 degrees and fitted a steering damper.

Front forks altered to 2 inches of travel.

Motor is set up for methanol with 32mm Amal MK2 Concentric carb.

Cant wait to test it very soon.

Jon

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Re: B25 Sprint Bike....

Postby sprinter » Mon Jun 27, 2016 9:11 pm

another angle....
sprinter.JPG

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Ian Hingley
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Re: B25 Sprint Bike....

Postby Ian Hingley » Mon Jun 27, 2016 10:24 pm

sprinter wrote:I plan to run it at Weston Zoyland in Somerset in August.


Let me know the date Jon and I'll try and get along to watch...

Ian
66 BSA Victor Enduro, 72 BSA Lightning, 62 BSA B40 trials, 89 H*nda Bros 650, 96 Aprilia Moto 6.5, 2000 Aprilia Moto 6.5

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Re: B25 Sprint Bike....

Postby sprinter » Mon Jun 27, 2016 10:31 pm

6th and 7th August...
Thanks Ian.

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Re: B25 Sprint Bike....

Postby Jack Gifford » Tue Jun 28, 2016 5:27 am

sprinter wrote: ... I have altered the head angle from 68 to 64 degrees...
I assume that's the included angle between the valves?
"Motley" bike history: Horex 400, 1940 HD 45 FH, HD Baja 100, '49 Indian Scout 440, Victor 441 Roadster, H*nda TL125, Guzzi V50, H*nda FT500, 400-4, '71 B50T
Aside from bikes: known as the "guru" of M/T hemi Pontiacs

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Re: B25 Sprint Bike....

Postby sprinter » Tue Jun 28, 2016 7:13 am

No Jack...thats the angle of the frames headstock (from horizontal).

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Re: B25 Sprint Bike....

Postby hwan » Thu Jun 30, 2016 9:02 am

It will be interesting how you get on, i was thinking the same B25/Supercharge/sprinting - as i've never 'blown' anything and i thought it would be fun.

But currently having a major downer on B25's - am now planning a 3hw (separate cams/gearbox/clutch that does need £600 to make i work!) but am slow getting the parts together - perhaps next year.

Interesting debate on the Austin 7 website - which blower to go for - amr300 or 500 - pro's and cons for each - also they appear to have decided that they are 'period' ???? (well acceptably so?) so should still be eligible for historic bike class's.

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Re: B25 Sprint Bike....

Postby sprinter » Thu Jun 30, 2016 4:51 pm

Thanks for your interest in my bike....
The AMR300 is ideal....but a much rarer beast than the AMR500...and is more expensive secondhand.
Both are just a type of Roots Blower...one with 0.3 litre per rev and one with 0.5 litre per rev.
The trickery comes in sizing the pulleys.....designing a large plenum chamber and mounting the back fire dump valve, etc.
Once you have got your head around all that, its an easy mod that makes the bike much more of a drag bike.

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Re: B25 Sprint Bike....

Postby hwan » Thu Jun 30, 2016 5:19 pm

according to Austin7 racers, its a bit more subtle, with the size 330/500 giving very different power curves - mind you they are talking 750-1172cc SV for circuit racing.
I suspect that the output/rpm curve will flatten off quickly - hence the different approach of 500 @ lower rpm than a 300 @ higher rpm.

As the B25 will rev safely i would have thought the 300, the better idea, with lots of compressor rpm at the crank rpm
What what i have read you need to be careful with cam overlap when blowing - the MX cam already has a lot of overlap for most applications - i would have thought something like a 4valve cam (blowing should give more air flow at low valve lifts, like a 4 valve engine) would be appropriate, though which one ??????

will be very interesting to see how it develops ...... would you be coming north - or at least to Santa Pod ?

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Re: B25 Sprint Bike....

Postby sprinter » Thu Jun 30, 2016 5:24 pm

I know what you are saying....
The main reason for building another engine is to build one with lower compression and a standard b25/44 cam .
I think its much simpler with a good blower set up.....and faster !
I would be hoping to Santa Pod next year at some point.

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Re: B25 Sprint Bike....

Postby sprinter » Fri Jul 01, 2016 2:44 pm

I now have all the bottom end together....it even has 4 gears and neutral !
Seriously, I cut away an old b25 case so that I could see the engagement depths, etc and dummy built the gearbox in that.
(Basically I copied the set up I saw on here)
Makes gearbox building much more straightforward for a newby like me !
b251.jpg


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