Lionel Brancquart's CBX

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Syscrush
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Lionel Brancquart's CBX

Post by Syscrush »

Hey all.

Lionel Brancquart and his (now departed) father built this racing CBX:

Image

My French is pretty bad, but it looks like this bike was once featured in RAD magazine, and I've seen claims of 200hp and a 20,000 RPM redline. I have a hard time with those #'s, but I'm wondering if anyone here has more knowledge about the bike or can offer up some pointers to more in-depth info on it.
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Re: Lionel Brancquart's CBX

Post by Jeff Bennetts »

There's some YouTube video of someone revving the piss out of it a track for an audience, maybe you can get some info from the videos?

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Re: Lionel Brancquart's CBX

Post by Syscrush »

Jeff Bennetts wrote:There's some YouTube video of someone revving the piss out of it a track for an audience, maybe you can get some info from the videos?
Yeah, I've seen a couple of videos of it, and I found this French writeup about the ignition system which also has pics of a bunch of engine internals, and references to engine speeds of 15,000 and 18,000 RPM. I'd really like to see something more in-depth and in a language I understand better. :)

This is probably the video you were mentioning:

https://www.youtube.com/watch?v=BYODFzLv3bY


(magic hand of admin)

It certainly sounds like it's getting to well over 10,000.

I'm also very interested in that $20,000 front end! :o
Phil in Toronto
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Re: Lionel Brancquart's CBX

Post by Mouse »

Syscrush wrote:It certainly sounds like it's getting to well over 10,000.
I'm also very interested in that $20,000 front end! :o
Got to wonder how much re-engineering went into it and how much of the original motor is left, plus how much of that 18,000 has power behind it? :neutral:
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Re: Lionel Brancquart's CBX

Post by Rick Pope »

Hmmm, a six turning 10k will sound a lot like a four turning 15k. Might there be a little exaggeration factor in there?

Not to say it wan't a great bike, but I've known some racing types to over state the facts...... :o
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Re: Lionel Brancquart's CBX

Post by Syscrush »

The ignition writeup shows some engine internal changes, but nothing exotic - forged pistons, bronze valve guides, titanium valves, and some kind of replacement connecting rods. To get to the 15,000-20,000 range, I would think you'd want to be rid of the stock cam chain setup, but that's pure conjecture on my part. If you're willing to inspect it after every race and replace as necessary, maybe it can work.
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Re: Lionel Brancquart's CBX

Post by Mouse »

When did Titanium valves stop being exotic? :-)
A stronger and lighter crank would be good. Knife edge? maybe.
Is the CBX an under or over square motor?
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Re: Lionel Brancquart's CBX

Post by EMS »

O.K. I do not want to be a spoilsport or smart aleck or whatever you want to call me. But getting to and over 15k in rpm with a CBX motor is going to be a challenge. Making the valves out of titanium will not by itself keep them from floating. The springs are the weak link here. I cannot see any spring design packaged in the given space that will get a CBX motor to 18,000 rpm
Overall, the CBX engine is a pretty heavy sister. Pointing towards the crank is something anybody with a little understanding of internal combustion mechanics would do for these kind of rpms.
Making the pistons lighter together with the rods was obviously addressed and looks good.

The stock CBX motor is 64.5 x 53.4 mm bore x stroke, which by definition is "oversquare". As the most common and easiest changes to the bore x stroke relationship is an increased bore, the engine becomes even more oversquare.
This is good for the ability to run at higher rpms, but it decreases the capability to incorporate higher compression ratios. Achieving optimum "breathing" is another challenge for oversquare motors. That limits the horsepower you will get out of it.

Finally, this is what it all boils down to:
If you're willing to inspect it after every race and replace as necessary, maybe it can work.

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Re: Lionel Brancquart's CBX

Post by Syscrush »

I found an article in Classic Motorcycle Mechanics here.

In addition to the valves, the cams are lightened, the tappets are somehow custom, the rods are forged and lighter than stock (they supposedly weigh 110g each), pistons are forged 12.5:1 units and crank is supposedly 7kg lighter than stock - no word on if it's a modified stocker, or forged or billet piece. I would think it would have to be forged to get the necessary strength-to-weight ratio, but I'm no expert.

Apparently that's a MotoGP front end plus 16.5" magnesium wheels and full carbon/carbon brakes, not the comparatively downmarket Superbike front end, so my $20k guess is likely way under. :o They got it via a connection with the Tech3 Yamaha MotoGP team and they were apparently on Colin Edwards' bike.
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Re: Lionel Brancquart's CBX

Post by Mouse »

I would never call you a "spoilsport" or a "smart aleck" EMS, because you have a far better understanding of these engines than I do. I need all the help I can get trying to understand the engineering that go's into them.
As to valve spring set up :?
Maybe :pray:
Moto GP front end eh? I agree :shock:
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Re: Lionel Brancquart's CBX

Post by Bird_Of_Prey »

I've heard this bike running - it makes you go weak at the knees and for sure it revs to 20k - it is fantastic.

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Re: Lionel Brancquart's CBX

Post by CBXTRA6 »

While I never saw personally the bike, I have 2 good friends in the french CBX club who are close to the guys @ Brancquard.
I also had my doubts about the 20 K RPM revving, but both swore that it was factual and due to the multiple internal changes done on the stock motor, most have been described in the Classic Motorcycle issue referenced by Skycrush.
In addition to the obvious lighter crank, conrods, pistons, cam profile, it also has a shim under bucket conversion like all modern high reving jap bikes. As for durability, it has already blown up at least once during an exhibition track event.
Not too surprising ! especially if it often gets the static paddock crowd show. Nevertheless, it is a remarkable CBX based monster and credit is due to the guys who put it together. When money is no object, "how fast do you want to go ? "
JP

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Re: Lionel Brancquart's CBX

Post by Syscrush »

Since posting this and doing a bit of follow-up research a year and a half ago, I've been daydreaming about a CBX converted to gear-driven cams. This is strictly in the daydream file, but I can't help wondering if it would be possible. Of course, everything is possible, it's just a matter of cost & reliability.

As strictly a thought experiment, what would be required? I may be wrong but I imagine that it could be possible to make a custom crank with a drive gear in place of the hy-vo sprocket (this would require a custom crank, right?), and a gear pack that's about the same dimensions as the cam chain plus tensioner, plus gear replacements for the second cam chain. Maybe it could be made to work with a single idler in between so that the direction of rotation could be preserved and the setup could work with stock cams. And the mounting points for the tensioners could serve as mounting points for the framework holding the gears.

If someone were to figure out and offer a kit, they could count on me to spend years saying how badly I'd like to install it and never actually doing it. :D
Phil in Toronto
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Re: Lionel Brancquart's CBX

Post by EMS »

Here is Roland Skate, who built two CBX racebikes "down under":
I still have some ideas that I'd like to bring to fruition, though I've shelved the plans for gear-driven cams, now that I can blame the engine case stud for the...blow-up, instead of a dodgy camchain
Don't know how easy it would be to install a gear drive in the center of the crank on a given motor. Most gear drives are on either end of the crank and cams.

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Re: Lionel Brancquart's CBX

Post by wyly »

after seeing a v-12 cbx I don't believe anything is impossible, it's just a matter of imagination, deep pockets and technical knowledge...

I discussed the mechanics of this bike with my mechanic and he says yeah it'll work...he estimates con rods of 110gms are likely titanium about $1k per, that front end about $30K...shaving 7kgs off crank, easy enough he says he can do that for me...the head of the Brancquarts CBX was rebuilt by companies that do the same for F1 Renault... my mechanic says he can certainly help my head breathe by cleaning up the head and putting in larger custom exhaust valves but it won't be cheap...but questions why I would bother when I could buy a newer used sport bike for less and will still perform better...

what astounds me most about the Brancquart cbx is the claimed dry weight, 174kg-382lbs :o where/what did they remove to lose 160lbs? http://survincity.com/2015/12/cest-bon-cbx/
CBX a work in progress, still improving...GS1150EFE completed and awaiting modifications.....RD350, remnants in boxes scattered throughout the garage

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