Posted July 29, 2005 - 03:52 AM
Im a newbie - so be gentle!
Ive built a XR650L engined road race bike and have a problem.
The gearing is too low. Im running 15/37 (higheest sprocket gearing I can fit!!) with a 17inch rear wheel (160/60 tyre) and the bike is struggling to pull past 105mph due to the engine revving past its peak power. Does anyone else have this problem?? Or know how to fix it cheaply? The engine is making 50HP at 7000rpm so that isnt the problem. I wish to be geared for 120mph or so.
Ive checked my primary drive ratios and theyre the lowest gearing so I cant improve that as far as I know. Out of interest, does anyone know what the honda 'power up' clutch gearing is??
Any help is much appreciated.
Posted July 30, 2005 - 06:45 AM
In order to get the stock XR650L to do 120 MPH at 7000 rpm, by changing the primary reduction only, it would have to be 1.825.
You may be better off doing some more open exhaust modifications. At 7500 rpm the rear tire speed is 115.62mph. I think the redline on the 650L is around 7550 RPM, but I don't know for sure. Back in 1996 I had a supertrapp on there with 8 disks. That engine revved out higher than with the stock muffler. Down a steep paved highway, I had the speedometer at an indicated 115, but I don't know how accurate that is.
You may be able to move your peak horsepower up in the rpm range a little bit, wich may allow you to reach a higher speed.
Posted July 31, 2005 - 09:46 PM
Originally posted by Lots of BikesBack in 1996 I had a supertrapp on there with 8 disks. That engine revved out higher than with the stock muffler. Down a steep paved highway, I had the speedometer at an indicated 115, but I don't know how accurate that is
Is that with stock gearing 15/45?
Posted August 02, 2005 - 07:22 PM
I heard that the XR600's that were used in the Baja races put the 15-tooth on the front, replacing the stock 14-tooth, commonly reached 115mph with 15/45 gearing.
Posted August 07, 2005 - 04:48 AM
Where did you get the 37-tooth sprocket?
My 650 is in a road race chassis - running in a RGV Suzuki frame etc.
I still havent worked out why my gearing is so low. Im beginning to wonder if I have the gbox out of a XBR500 in there!! But even this shouldnt make that much difference.
My bike runs an open megaphone pipe (80mm) with 60mm silencer, makes peak power (50HP) at 7000rpm but will rev to nearly 8K. The engine is a 675, with the stg1 Hotcam, VM40 race carb, so she's running quite well for a mildly tuned motor. More tuning is in progress but I need to sort the gearing issue out before I make any more power!!
Bike requires painting (and now a new cam cos of oil starvation - blown clutch cover seal over the crank!!!!) - heres a pic...
Posted August 07, 2005 - 12:02 PM
Compared to a streetbike, the XR650L is geared very very low!!! It is a dirtbike engine, meant for that purpose. A GSXR1000 will do 0 to about 70mph in first gear; in about 3 seconds. A 650L engine doesn't do 60 until it is in 3rd, and certainly not in 3 seconds.
If you measure the distance that your rear tire covers in one full rotation, I can tell you excatly how fast your bike should be going in every gear and any RPM. You could also just give me the radius, or diameter. The circumference equals the distance that your tire covers in one full rotation.
Posted August 07, 2005 - 04:43 PM
Anyways, I was looking to go the other way with my xr650l. I rode all weekend in the mountains on my new (to me) bike and it was so tall geared that most of the tight and technical stuff I never got out of first gear for miles of riding. I have had an XL350R for years and it was always able to get at least to 3rd in the same stuff. Though the XL is 6 gears and the XR is only 5 so with such wide gearing it just felt a little wrong. I wish first gear was split into 2 gears, 1 slightly under first for really slow travel and 1 halfway to the current 1st and 2nd gears, then there would be more ability with the stock gearing while still keeping a stable/workable high speed gearing.
Posted August 08, 2005 - 03:14 AM
the rear tyre Ive been using so far is a 150/60-17 and the final drive is 15/37. Seems to rev out at 105mph which doesnt seem right with my calcs?
Ive juts fitted a 160/60 rear to try n help the gearing a little.
Id dont expect it to have the legs of a GSXR1000 but it should at least do 120+.
My spreadsheet indicates it should be making peak power at about 125mph.
Im sure I'll find the problem eventually!!
Any help is much appreciated
Posted August 08, 2005 - 08:36 AM
Posted August 08, 2005 - 09:11 PM
According to my math, you should be doing around 38.95 miles per hour in first gear at 7000 rpm. If you can give me the exact distance that your rear tire covers in one full rotation, I can give you an exact number..
However, looking at the preliminary numbers that I have (I am guessing that your rear tire is 25 inches in diameter ) that rear tire should cover ground at 6.55 feet per rotation. At 7000 rpm, at the top of fifth gear, you should be doing 123.81 mph.
Of course these numbers do not take into consideration wheel and tire elongation due to centrafugal force. The tire diameter increases slightly at higher speeds. Then there is slippage and other factors..
But even with these numbers, you shouldn't be revving out at a measly 108mph unless your primary reduction has been altered. Stock primary reduction is 2.0294. If your primary reduction were 2.3265, then you'd be doing only 108mph at 7000 rpm in fifth.
And by the way... Your bike looks so totally cool!!!!! I've never seen someone take an L engine and mate it to a streetbike frame like that! AWESOME!!!
Posted August 09, 2005 - 11:27 AM
The primary drive is totally stock (Ive checked) and the only unknown now is my gearbox ratios. My speedo is a calibrated electronic one so shouldnt be far out at all. Im beginning to wonder if Ive only got four gears!!!
Regarding the bike build, its taken a long time to get it to this stage (on and off for 7 yrs as Ive been working abroad etc). The motor is making 50HP at the back wheel and Im currently having the stg 1 hotcam reground for more top end and revs. (the hotcam was scrap after 300 miles! - due to oil starvation). Im also getting the followers hard welded now just to be safe and cos they were worn obviously. Ive got the VM40 carb on the lathe at the moment and boring it to 42.5 to match my inlet port.
I hope to get a reliable 55HP for road and track use.
Oh by the way, you should hear her BOOOOOOM
Posted August 09, 2005 - 10:19 PM
Posted August 09, 2005 - 11:55 PM
You may want to cross-reference your electronic speedometer with a handheld GPS. You may be going faster than you think.. My ST1100 doesn't really feel like it is moving at 100mph.. Just a thought.
And you better make an MPG of your bike so we can hear how it sounds when moving.. I love hearing the big (real) thumpers rip!!!
Posted August 18, 2005 - 03:46 AM
where can I buy a 16T front sprocket?
(Theyre either like unobtainium or veryexpensivinum in the UK)
Posted August 18, 2005 - 05:31 PM
The XR600 from '88 - '90 has a 16-tooth available, but from there oward the largest is a 15-tooth.
I don't know what the difference between the older XR and the "newer" one in terms of countershaft splines, mounting, etc. A Honda dealership could tell you, but then again, so can sprocketspecialists.
Posted August 19, 2005 - 11:56 AM
Never mind Im tring to solve my low gearing problems with more engine tuning
Posted August 20, 2005 - 07:45 PM
Sprocket specialists has the broach tool needed for putting the proper splines in sprockets. If you contact them they can make a 16-tooth custom for you. It may be an arm and a leg in cost, compared to the usual sprockets, but still cheaper than paying for internal gearing changes.
Posted August 21, 2005 - 10:26 AM
Posted August 21, 2005 - 11:49 AM
FYI, Im currently having my hotcam stg1 rewelded and hard ground, same duration but a bit more lift (the original hotcam was worn away in 300 miles cos of an oil starvation problem). The same shop is hard welding my rockers, bead blasting the rocker cover and slotting my cam sprocket. I hope to have the motor rebuilt for next weekend as I have a track day arranged for early September. I hope to get 55HP (previously 50HP) out of her now as Ive also bored by VM40 Mikuni carb to 42,5mm. This now matches the intake ported Ive gas flowed. I must say the old stlye VM carbs still work really well on these motors. Easy to tune, no accel pump to mess with, and it still gives good power even in 40mm guise. I did have to machine the OD of the front of it to fit my OE honda inlet stub tho.
Fingers crossed - I hope the extra tuning will shift the torque up the rev range a little so that my peak power is nearer 7.5K - 8K. This should also help clear the gearing problem up as the engine will be more eager to rev...
Will let you know how it goes. If I get on the Dyno I'll post the grapgh on here again.
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