Vintage Dirt Bike Q & A

I recently started a new project that will be strictly for racing. I am building
a 1978 CR125 sleeved down to 100cc and will be based on the FMF package
racers that were offered at the time.

I already have a lot of the parts, but have some questions about suspension
choices. The bike will be raced in the Marty Tripes 100cc Revenge Series so
I don't have to worry about AHRMA rules. I will be running an FMF "Godzilla"
swingarm which requires 13.5" shocks. I am looking at Ohlins or Works
Performance. Good, bad, any other suggestions? For the forks, I had planned
to use the 78 forks with an emmulator kit and beefed up springs. Again, any
body have a better suggestion?

I look forward to any and all input on this. Think of it as a blank canvas that
you can do pretty much anything with.

I'll post some pictures as the build progresses.

dogger

_________________
1973 CR250M
1979 CR250R
1993 CR250R
2008 CR500R
2009 CR250R

dogger,

It's great to have a new project heading into the winter, isn't it? Me, I've got the same old project ('MC175').

I'm sure you've got all this info already, but here's the 2010 Ohlins catalog:

http://www.bikehk.com/shop/power/2010.pdf

.and apparently Pro Pilot is the only place that carries the S36PR's ?

http://www.propilotracing.com/mainnewsuspension.htm

There's also Hagon - the 2810 and Nitro custom range don't look too bad:

http://www.hagon-shocks.co.uk/HagShocks.htm

...and the Works Performance Pro Racer dirt-track shocks look really nice:

http://www.worksperformance.com/pdf/ins ... oRacer.pdf

I installed the Race Tech emulators on my '74 CR forks. They didn't sit flat on the '74 split-pin (non-swaged) damper rods/pistons so I ordered the spacers, but then the preload was too great so it was off to the machine shop to modify them to center into the damper rod shafts and sit square on the piston tops (perhaps they'll drop right into the larger '78 pistons). The motor performance was so disappointing that I never took the bike off-road this summer, but I can tell you that the recommended 2 turns on the compression springs felt way too stiff on the road with 5W oil; I'd suggest 1/2 turn to start.

Are you planning on doing any porting?

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Thanks for the links Ray, some good info.

I will probably go with the Race Tech Emulators as well, there's not a lot of
options for the stock suspension. The only other choices I can think of is
going with later model forks or going with Simmons or other high dollar, ultra
rare setup - I'll call that plan B.

The engine is being built by Vintage Moto Works and the top end is being
sleeved down and ported by Eric Gorr. I just scored an FMF porcupine head
and will use a 32mm Mikuni carb and a hand fabbed cone pipe. Should be a
little rocket which is why I'm desperate for good suspension.

What's going on with your motor? That displacement size (175) should be
good for nice tractable powerband.

Don

_________________
1973 CR250M
1979 CR250R
1993 CR250R
2008 CR500R
2009 CR250R

Don,

One thing I looked into with my CR chassis was putting on forks from a new Yamaha TT-R230 (or similar). As far as I can tell, boring out the stock triple clamps maybe 3-4mm and either narrowing or adding axle spacers would net a 9.5"-travel disk-brake EVO-type front end.

Wow, a 32mm carb! I was going to suggest a 26mm or 28mm. The 2000 KX100 I worked on last year used a 24mm, with reeds, KIPS valve, and ports the size of matchbooks. I was unable to get engmod2t to show a power increase over 30mm with the MR 175cc motor below 10,000 rpm, for what that's worth.

The 175 is coming along slowly. It's my own fault I was disappointed with the motor. I had it ported to 70's-era "bigger is better" time/area values, put the motor together, and finished up the chassis. Soon afterward, I got engmod, completely redesigned the porting in the process of pipe development, then threw the new pipe on the 'old' motor when the weather got warm and was unhappy with the results (surprise!). Now I've got the motor apart to do it right, which is sure to be a challenge.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

That's the size the bikes came with back in the day and the size Eric
recommended to go with his port and head work. Besides, I doubt I
will be spending much time below 10,000 rpm. A motor this small can
only make decent power with lots of revs.

That will do it. Are you still experimenting with pipe design? How is
that engine software working out?

Speaking of pipe design, one of the benefits of going with a sleeved down
125 is since the stroke is stock, I can use a readily available 125 pipe instead
of needing a custom made piece.

After a little more research and feedback, I'm leaning toward Ohlins rear shocks
and the 78 forks with emulators and Progressive springs. I think that combination
will work good enough.

dogger

_________________
1973 CR250M
1979 CR250R
1993 CR250R
2008 CR500R
2009 CR250R

Don,

Right after I posted about the carb-size thing I thought, "hey, if anyone knows what he's doing it's this guy!", so I'm sure the 32mm carb will work great.


Quite well! There are tons of measurements that need to be taken for the program to work accurately, and it seems like I'm always finding ones that weren't perfectly accurate and need correcting, but the results change very little so I guess I'm OK. I don't know if the (crankshaft) horsepower and torque figures I'm getting will be achieved in real life, but the simulated powerbands correspond very closely with what I'm feeling riding the bike. The program's creator, Neels van Niekerk, has been extremely helpful and responsive to my questions, for which I'm very grateful.

I've found the MR175 motor is flow-limited pretty much equally by all three port systems (intake, exhaust, transfer), with max power & rpm being blowdown-limited by the top half of the (unbridged) exhaust port. I think your motor's max power may be more dependent on the intake flow/intake port timing and pipe design, though, as the stock ports are already generously sized and arranged for a mid-high powerband.


Yes - and that's got to be one of the most popular vintage porting arrangements ever, with practically everyone making a pipe for it at one time or another. I wonder if there's anyone out there making MT125R pipes?


I totally agree.

One other thing before I forget; I believe the CR125M ignition has a built-in rev limiter (I'd guess it's set to about 9,000 rpm, maybe higher). I remember a being a little shocked at how high the MR's OEM points ignition would rev in the few weeks I used it. Perhaps you're using a PVL and it's a moot point. The MT125R uses a different CDI box.

Ray

P.S. While I was developing the MR motor I used a max piston speed of 3,500 fpm (10,700 rpm for 50mm stroke) which was a figure I found on the Web; since then I see that 4-stroke guys are running speeds over double that, so now I don't know what to believe (a common occurrence for me after Web searches ). Here's an easy-to-use calculator:

http://www.csgnetwork.com/pistonspeedcalc.html

It's too bad piston mass isn't factored into the equation. The MR175 piston must weigh at least 25% more than the CR125M one.

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Have you considered using a sleeved down CR250M cylinder for your 175?
The port arrangement and timing should eliminate most of the limiting
factors you are facing now. Is that cylinder even compatible with the
MR175 case?

I am still deciding between the stock CDI or a PVL digital. The PVL allows
me to further tune the powerband and the digital map is much more
responsive than the stock analog. I'll need to do some more research on
that one.

When you are comparing the two pistons, are you talking OE castings or
aftermarket forged? I would think an OE cast 175 piston would be fairly
light even compared to a 125. The only motor I insist on a forged piston
is an open class, it's big and heavy but castings don't seem to last with
that level of power and vibration. I plan to use a OE Suzuki RM85 overbore
piston in this 100.

Don

_________________
1973 CR250M
1979 CR250R
1993 CR250R
2008 CR500R
2009 CR250R

Don,

It's funny you should mention the CR250M; I just finished modifying a '73 250 head to fit the 175. It's about 6mm bigger all around, and looks great on the MR cylinder; the huge Webco-style finning should be overkill on the smaller motor.

I don't think the 250 cylinder is compatible. The 250's transfer passages are wide/short whereas the 125/175's are long/narrow, and 3 bolts are about 10mm further from center with one about 15mm out. I think it would be a real challenge even if the rod/crank was similar. Getting just the head to fit was real interesting.

If I had it to do over again, I would have gotten a PVL. The Honda CDI's rev limiter isn't a problem in my application - it's actually good - but I believe there's also an auto-retard at low rpm to prevent starting kick-back that has made getting a decent idle difficult (the CR's full-circle crank works well with the tiny CR rotor, the lighter MR pork-chop crank doesn't). As close as I can tell, the Honda CDI adds about 12 degrees of advance below 5,000 rpm.

The piston comparison referred to OEM ART cast units. The 66mm 175 piston isn't much smaller than the 70mm 250 one, and the 56mm 125 piston seems like a dwarf in comparison (I haven't weighed them, so I don't know for sure). I've never had an ART piston crack even when the rings broke, but maybe I've just been lucky.

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

I had a feeling the 250 cylinder wouldn't work because you would already
have adapted it if it did.

I didn't realize the 175 piston was so big. It sound like Honda stuck with
the same rod length and just oversquared the engine for each displacement
increase. I totally agree about the durability of Art pistons.

Talked to Doni and Eric (they're building my engine), yesterday about the
PVL vs OE. Both said I should go with the digital version of the PVL. At
least that will save me from searching for an OE coil and CDI box.

I have to do some more welding (gussets) on the frame before I powder coat
it. I picked up some monster size footpegs from VMX so I decided I better
reinforce the peg mounts. The engine is due here in early December and I
will send the cylinder and head to Forward Motion for the sleeve down, port
and head work. Vintage Moto Works is also hooking me up with a nice set of
78 forks that I will restore and modify. Other than AMS, any idea where I can
get an OE looking set of red fork gators?

One good thing about using mostly aftermarket parts, the build goes a lot faster.

dogger

_________________
1973 CR250M
1979 CR250R
1993 CR250R
2008 CR500R
2009 CR250R

Yes, they just used the same 50mm CR125 stroke (with a slightly longer 109mm rod) and a giant piston to get 171cc.

Good call on the PVL. Apparently they have a really good rep.

I have a set of Daystar 58 (vintage/EVO) boots, in black. I never got around to using them, but they look like quality units. The red looks nice and bright:

http://www.denniskirk.com/jsp/tpl/produ ... 626&mmyId=

Ray

_________________
'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric