The MotoBi 250 Vintage Road Racer



Date: Sun, 25 May 1997

Subject: VintRR Oh no! Another vintage road race project

Hello,

My name is Michael, and I'm a projectaholic.

While at Craig's shop the last two days I did a deal with his friend Marc to buy his MotoBi (also known as Benelli) horizontal-cylinder 250cc four stroke stuff (at only $400 how could I go wrong?). This is a fairly complete chassis with presumably running engine and 2 additional engines. It appears that all the bikes are 4 speed transmission models. In the spares is a new race kit head (with "RR" lettering stamped below the exhaust port on the underside of the head) that looks to have much bigger ports than the standard head. There is also a new camshaft with "S" designation, but we don't know if that stands for Sport, Speziale, or Standard (maybe spatzle? - probably not since it is an Italian engine).

Craig and I spent about 40 minutes looking at the parts and scheming on what to change. The crankshaft is not a real impressive looking item. It has the same stroke as an XR200 Honda single (and is about the same width), and the mainshafts on the Honda crank are enough bigger that they could be turned down and modified to work in the MotoBi. This would also give a bigger crank pin.

The 4(!) ring piston isn't very impressive either, but it is the same bore as an XL250, and they are both flat top pistons so that is solved too. The Honda piston uses a 1mm bigger wrist pin, but since the deck height is a little lower that would probably be taken care of by ordering a slightly longer Carrillo rod to the appropriate length/ID dimensions. Welding in the combustion chamber will add compression without needing a dome on the piston.

Megacycle is supposed to have a master for the factory race kit cam, so I'll contact them for timing specs on that to see what cam I've got, or if the race kit cam will be at all suitable. Craig looked through some catalogs and found some 5.5mm stem titanium valve blanks in appropriate sizes, and I'm sure that R/D Spring will either have a suitable valve spring set or can come up with one once I send them a head. This time I won't worry about trying to come up with replacements for the OEM parts as I did for the Laverda - whatever retainers they've got on the shelf are what will be used.

Craig said the intake port didn't look too bad, but the exhaust is one of the wretched flat ports that the Italians seemed to love to put in their engines. I see a bit of TIG welding in the exhuast ports future.

Ivar very kindly took some time send me some MotoBi information. He told me that the factory bikes had trouble with the side thrust from the helical primary gears (much as do Ducatis - Nova sells a straight cut primary set for the Ducs) so this is going to have to be addressed.

The transmission is the other sticking point. All of the engines I got appear to be 4 speed boxes, which strikes me as less than optimum for a 250 race bike. 5 speed models were also sold in the States, but Ivar tells me that all the factory racers used 4 speed transmissions. The standard 5 speed has a wider overall spread than a Ducati 5 speed (and we won't even mention how it compares to an Aermacchi race gearbox!), and the top gears aren't very close together.

The immediate conclusion was to find another transmission to install (since I know of another Italian 250 racing bike that is running a CR125 6 speed). But wait - the MotoBi uses an English-style transmission with a mainshaft/layshaft setup (power comes in and goes out from the mainshaft), not a typical Italian/Japanese indirect crossover type box where the power comes in on the main shaft and leaves via the countershaft. The 4 speed cases are fairly narrow inside as well. Hmmmm, this is going to require more thought.

I've been emailing with a fellow in Australia who has some of the 5 speed engines, but even there new gears would have to be made, though this might be less trouble in the long run than finding a different transmission to swap into the engine. Then again, most of the tracks over here don't have (m)any first gear corners, so if I could build a dry clutch that would tolerate a lot of slipping on the start a close ratio 4 speed might be tolerable.

I don't know yet if Zanzani or others made straight-cut gear conversions or different transmission ratios, but I'll find out. I now have a (presumably) good address for Werner Maltry, who developed a lot of the factory racers and his own LinTo-type 500 twin based on the MotoBi single, so I'll drop him a line and see if he recalls any helpful information.

I'll build a new chassis for it. Ivar told me that Zanzani made a space frame for sale for the bikes, and I see no problem doing a replica of that or a LinTo 500. The lower rear motor mount appears to be usefully wide, but the top mounts are only 30mm wide and about 60-70mm apart. Hmmmm, maybe a bit more TIG welding will be called for here.

The engine has a pretty narrow chain line with about 10-15mm clearance to a 3.00x18 Avon tire, so it may need to be slightly offset in the frame to allow a 90 or 100 section tire.

I think that a set of late model external-damper Guzzi GP pattern leading link forks are likely, or maybe some Reynolds type LL forks, whichever seem to be more appropriate for the link geometry that I decide upon.

Before you know it (well, maybe late next year) I may have a perfect replica of the MotoBi racer that everyone wished they were riding in 1965.

As you might guess, any bits of MotoBi lore, legend or engine components are desired - you know where to find me. I guess I need to see if the Benelli club caters to MotoBi - or maybe I'll have to start a MotoBi club myself (in my copious spare time - right).

Cheers,

Michael


The MotoBi transmission ratios compared to some of the competition.



			% Drop				        % Drop				        % Drop	
Motobi 250		   		CRTT -5A		                CRTT -5B		
1st gear	20.56			1st gear	11.56			1st gear	2.26		
2nd gear	13.07	36.43%		2nd gear	9.02	21.97%		2nd gear	1.75	22.57%	
3rd gear	9.94	23.95%		3rd gear	7.09	21.40%		3rd gear	1.37	21.71%	
4th gear	7.55	24.04%		4th gear	6.04	14.81%		4th gear	1.12	18.25%	
5th gear	6.5	13.91%		5th gear	5.57	7.78%		5th gear	1	10.71%	
6th gear				6th gear				6th gear			
Overall:	3.163076923		Overall:	2.07540395		Overall:	2.26		
											
			% Drop					% Drop					% Drop	
Duc 250					CB175					B25 BSA		
1st gear	14.1			1st gear	2.769			1st gear	2.65		
2nd gear	9.62	31.77%		2nd gear	1.882	32.03%		2nd gear	1.64	38.11%	
3rd gear	7.5	22.04%		3rd gear	1.45	22.95%		3rd gear	1.24	24.39%	
4th gear	6.12	18.40%		4th gear	1.173	19.10%		4th gear	1	19.35%	
5th gear	5.4	11.76%		5th gear	1	14.75%		5th gear			
6th gear				6th gear				6th gear			
Overall:	2.611    		Overall:	2.769			Overall:	2.65		
											
			% Drop			                % Drop			             % Drop			
Greeves Silverstone			Motobi 200 (magazine reported ratios)	Honda CB160	
1st gear	2.39			1st gear	18	             	1st gear   2.769 
2nd gear	1.88	21.34%		2nd gear	10.9	39.44%          2nd gear   1.778       35.8%
3rd gear	1.4	25.53%		3rd gear	8.6	21.10%          3rd gear   1.318       25.87%
4th gear	1.1	21.43%		4th gear	7.2	16.28%	        4th gear   1.04        21.09%
5th gear	1	9.09%		5th gear					
6th gear				6th gear			
Overall:	2.39			Overall:	2.5	 	        Overall: 2.66

			% Drop			                % Drop			             % Drop			
Motobi 200-250 (factory manual)	        Motobi 125  (factory manual)
1st gear	2.615			1st gear        2.683	             
2nd gear	1.515	42%		2nd gear	1.665	37.94%
3rd gear	1.153	23.89%		3rd gear	1.267	23.9%         
4th gear	1.0	13.27%		4th gear        1.0	21.07%	       
5th gear				5th gear						
6th gear				6th gear			
Overall:	2.615			Overall:	2.683	 	




Check out the MotoBi pictures on the graphics page.



Update: 05/26/97

I just pulled the head off of the fairly completely assembled spare engine. It looks like I may have two 250 engines, as this one has turned out to be a 125!

First performance/reliability modification: The 125 had star-type lock washers under the head nuts. Lock washers under head nuts; will wonders never cease. Bad, MotoBi, bad. They'll get ditched and hardened and ground flat washers substituted.


And now for some MotoBi humor, courtesy of Ray Crenshaw on the Thumper List:

I wrote: Upon further investigation, one of my spare 250 engines that came with the bike has proven to be a 125. I'll have to think of something entertaining to do with the smaller engine to keep myself amused.

Ray replied:

Yes Ray, I think that is QUITE enough!


21 November 1997

I just weighed a complete 250 4 speed engine (engine, kick and gear levers, 22mm Dell'Orto carb with attached air filter), and the weight is 66 pounds (+/- .5 pounds).

The engine for my EX250 + 125GP =? project will be about 73 pounds, and I'm expecting that bike (with special frame, Dymag wheels, disc brakes, etc) to weigh in at about 185 pounds. The 250 Benelli road racer, presuming the 5 speed engine weighs about the same as the 4 speeder (and it should be lighter as it won't have the kick start stuff or flywheel/magneto), and granting that the wheels should be a bit heavier, should still be comfortably under 200 pounds dry. This is about 40 pounds less than a standard framed 250 Ducati narrow-case racer. If both bikes have 30 bhp at the rear wheel the MotoBi will have a 1.3 pound/hp advantage over the Ducati, or the equivalent of about 36 bhp. I like t!


Phil McCandless was good enough to send me some torque specifications for different fasteners from his owners manual on the 200 CC 4 stroke 4 stud engine

:

Phil adds: From personal experience: Take extreme caution when removing the pinion drive gear from the main crankshaft. Use a proper tool. It is on VERY TIGHT and must be taken off with the proper tool. The crank shaft WILL break if a conventional gear puller is used. Believe me!! It will happen!!


Here are some interesting measurements from various sports/racing MotoBi camshafts, supplied by Peter Oort:


Camshaft S M2 C2
Intake Open (BTC) 30 30 37
Intake Closed (ABC) 75 75 76
Exhaust Open (BBC) 75 75 76
Exhaust Closed (ATC) 30 30 32.5
Intake Lift in mm (.5 mm clearance 4.7 6.3 7.9
Exhaust Lift in mm (.5 mm clearance 4.7 6.3 7.3



Bore and stroke measurements:


Steve Crocker found himself with some time hanging on his hands and he scanned and converted to PDF some Motobi manuals for me to host. Surprisingly, the Cosmopolitan Motors phone number that appears on the documents will still connect you to the former Benelli/Motobi (and many others) importer. You can also reach Cosmo with a newfangled computer at the Vintage Cosmo website.



Building a new frame

After racing a borrowed Honda CB160 at the end of April 2009 I had some enthusiasm for finally getting to work on the frame for the Motobi roadracer project. I narrowed down my choices to one of the many different sketches I'd done.

I did things a bit differently than on earlier frames as I had some different tools available and the design of the frame was different. First, on prior frames I used a steering bearing cup for the steering head that had a narrow flange on it that was pressed into a tube that was the length of the overall assembly less the two flange widths. With the 30205 bearings I wanted to use that would require a 2.375" or larger tube to have enough wall thickness on the bearing cup. This time I machined the cups from 2.25" bar (with the bore left about .020" undersize) and put a small spigot on the lower end. This is pressed into a shorter tube and welded in place.

Second, since I was able to make the main lower tubes perpendicular to the steering head I decided to fixture the main frame on the base of the frame fixture instead of up on the steering head post. This simplifies the making of fixtures to hold some of the other tubes as I can work off the fixture base instead of 30" up in the air.

Third, I used software to generate templates for coping/mitering the tube ends. If you know the exact angles and offsets of your tubes the online tube coping calculator at Hal Eckhart's website is very useful. But rather than figure out all of that data I decided to use Rhinoceros3D, a NURBS modeling program. I did first do a side-view drawing in Alibre Design (Standard version) to establish the locations of the tubes and imported a point cloud of the nodes at the tube axis intersections into Rhino. Rhino makes it very easy to then snap a cylinder or tube between those points. The little black dots are the imported points:

Rhino view of the main frame

Rhino view of a tube intersection

Once those tubes are in place and the ends have been trimmed at the intersections the Rhino command "unroll developable surface" does just that, creating a template for the outside (for solid cylinders) or inside and outside if a tube. Here's a screen shot showing both the full templates (the printable versions had 13-15" removed from the middle so they'll fit on an standard piece of paper in my laser printer). The left templates are for the intersection of only two tubes, the right ones are versions that add in a cope where three tubes intersect.:

Rhino templates

After marking the tubes with the templates I use snips to cut as much metal away as was easily done. I tried then using a grinding wheel in my 4" angle grinder but what I found worked even better was my pneumatic 3" cut-off disk tool. That works better for plunging into the base of a narrow V cut and seems to work fine for trimming away other metal. You don't want to put a big side load on the cutoff disc, but in .049" tubing you don't need to do that as it is easily removed. You'll still need some files for final fitting, but the templates get you very close very quickly.

Here are photos from the first two days of activity in the garage:

Here's the third day after adding in the top tubes:

Here's today's photos of machining the bearing cups.

Oh well, time to pause and think. I've got another set of bearing cups already done. I may want to reconsider how the frame tubes are being run as a little more room around the carb might be nice to have. I might also do some practice with my TIG and maybe I'll give that a try, though I would have a choice between TIG braze or fusion weld.


Back to the home page
© 1996-2009 Michael Moore, last update for this page 21 May 2009

For more information contact us by using

http://www.eurospares.com/motobi.htm