Board index Flathead Power-Technical Questions, Answers, and Suggestions Big Twin Flatties Gearing the 37U for the highway

Gearing the 37U for the highway

Post Fri Mar 03, 2017 11:41 am

Posts: 23
I am in the process of deciding what route to take with the gearing on my 37U. Heres what ive got so far. The engine has T&O wheels with a 4 3/8 stroke, polished, ported, relieved heads with big intake valves, WR manifold, CV carb. 4 vane upgraded oiling. 4 speed tranny that im going to rebuild.

I live in a remote area where the speed limit is 70-80mph just about everywhere. Takes an hour just to get to town. Id like to set the bike up to be used primarily for highway cruising. Im not expecting to do 75-80 all day but am wondering what gearing you guys would recommend. I have a 47-76 tooth primary belt drive im using. For the tranny sprocket ive heard to use a 24 tooth with a 48 on my rear mechanical drum. Also to use a close ratio first gear with this particular setup. Opinions? Suggestions? Cheers!-Matt

Post Fri Mar 03, 2017 7:06 pm

Posts: 1731
Location: Interlaken, NY USA

That gives you 3.234:1 overall ratio, not a bad place to start. If you can fit it, the bike should pull a 25 countershaft sprocket too. I go back and forth, between 24 and 25, but I have a hand shift lid. I'm not sure that the 25 will fit a ratchet top.
I'm curious about the rear, typically that sprocket is a 51 tooth. Thought you only found 48 t on the disk brake sprockets.
I've cruised the Thruway at 65-70 with the 25 and the bike's quite happy with it.
If you'd really like to improve your engine's performance, I highly recommend these heads from Finland.
They improve the combustion process, eliminate the hot combustion spot that tends to toast pistons.
I think they're around a grand now. I've had mine 3 years and am much impressed with them.

Post Fri Mar 03, 2017 7:42 pm

Posts: 569
Location: Wa, USA
I have a 38 U with stock stroke T&O wheels and 74 +.020 bore, relieved cylinders, 130cc heads, oversize valves, KNS 1 step up from stock intake cams, dual point auto advance ignition, CV carb and CHP oil pump and scavenger upgrade. I have a 5 in 4 with a 39 - 61 belt primary and a 23 - 48 rear. This setup with stock cams ran very nicely at all levels of riding. When I blew a head gasket sparking a major redo, I was doing 75 and accelerating on the freeway with 2 up. With the cams, the torque curve has moved up just enough that it now pings if I put it in 5th at anything less than 65 while riding solo. I no longer do much freeway riding with this bike (getting older and I have a newer dresser) so I am thinking about dropping the rear sprocket to a 50.

Post Sat Mar 04, 2017 12:25 pm

Posts: 23
PBI sprockets makes the 49T (my mistake not 48T) Part #2078 for $75. Im thinking of running that with a 24 tooth as I have a 53 ratchet top transmission. What about the close ratio 1st gear? Any real need for that? I only weight about 140lbs and dont plan on having a passenger much. Can I get away with a stock gear setup and the 24-49? Thanks rather only do this once :D

Post Sun Mar 05, 2017 9:35 am

Posts: 1731
Location: Interlaken, NY USA

The flatheads will easily pull the 2.6:1 first gear. I build all my trannys with the close ratio first AND third. I like the gear spacing, first-third are right where you need them when you succumb to "speed demonstrations". Third-fourth is a little wider gap than is ideal, but doesn't create a problem with day to day riding.
I've used the 2.44:1 first as well, but you really don't need that close a ratio. I ran that on a basically stock 80 for years and it works o.k, but the 2.60 is a better fit. I might add that I use these ratios on suicide clutches, have no control problems coordinating friction zone and starting out. You're using a hand clutch, so that's even easier.
I think the original 3.00:1 first was intended for heavy stock bikes and ease of starting out with a foot clutch.
Just my 2 cents

Post Mon Mar 27, 2017 12:37 pm

Posts: 307
Location: oklahoma usa
I'm a bit late to the discussion but for a long time I ran a 25 tooth transmission sprocket with no issues. I've even taken off from a dead stop in 3rd gear with no balking or lurching.

The bike will chug along at 70+ all day long at a fast idle.

Post Mon Mar 27, 2017 2:30 pm

Posts: 2770
Location: Los Angeles, CA
montuckymatt wrote:
PBI sprockets makes the 49T (my mistake not 48T) Part #2078 for $75.

Who and where is PBI?

Post Wed Apr 19, 2017 11:08 pm

Posts: 24
Location: Snohomish Co., WA
woody wrote:
With the cams, the torque curve has moved up just enough that it now pings if I put it in 5th at anything less than 65 while riding solo.

Hi, Woody, I met you last Saturday...
What grade of gas are you running? A bit more octane might help reduce the pinging.


Post Thu Apr 20, 2017 5:36 pm

Posts: 569
Location: Wa, USA
I am running regular. I have pretty low compression and have never had a problem with regular before. Experimentation has shown that the torque curve is noticeable higher. If I get it rolling fast enough, it runs fine. A change of gearing should get me back in the groove.

Post Thu Apr 20, 2017 8:20 pm

Posts: 1368
Location: Ojo Caliente,NM,USA
Think I would give a tank Premium a try. Did you degree the cams in when you installed them I've seen a offset key make or solve a little pinging. Or a change of riding style.

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