Author Topic: Lightened Crank, what's the impact? (Read 6434 times)

camelman · « **on:** March 14, 2012, 10:06:10 pm »

Does anyone have experience running a lightened crank on a street motor? If so, then what are your impressions for drivability and benefits/detriments? Any engine info would be helpful.

Camelman

geminimotors · « **Reply #1 on:** March 15, 2012, 12:17:04 am »

I can't speak for our SOHC bikes, but in air cooled VWs, a lightened crank and/or flywheel would let the motor achieve high revs sooner/faster, but at the expense of low end torque. Great for a drag car or sand buggy, but bad for mud bogs or trail crawling.

cbr954 · « **Reply #2 on:** March 15, 2012, 04:57:53 pm »

I run a lightened crank in all of my 750 motors. They do rev faster because there is less weight to get spinning. I don't really see a down side to them I am sure someone will disagree. Some say they loose low end torque, but I dont really notice it. I like them because the motors rev a bit faster.

MCRider · « **Reply #3 on:** March 15, 2012, 05:24:46 pm »

as CBR954 SAYS... throttle response is faster. I don't know if its torque or inertia, but smoothness off the line may suffer, and smoothness while riding at low rpms, depends on your riding style. If you get off the line fast anyways, you'l probably like it. If you keep the revs up all the time anyways, you'll like it.

I just naturally downshift at about 3500 rpm, I like to keep the motor spinning.

IF you slip the clutch and ride a lot below 4000 rpm, you'll like it less or not at all.

23tbucket · « **Reply #4 on:** March 15, 2012, 06:34:17 pm »

I think there could be a bit of a problem with trying to get a nice slow smooth idle? Also the engine could be much easier to want to stall out when taking off from a stop?

Really? · « **Reply #5 on:** March 15, 2012, 06:39:55 pm »

Interesting thread!

MCRider · « **Reply #6 on:** March 15, 2012, 07:11:14 pm »

Quote from: 23tbucket on March 15, 2012, 06:34:17 pm

I think there could be a bit of a problem with trying to get a nice slow smooth idle? Also the engine could be much easier to want to stall out when taking off from a stop?

Yes, that's right.

MCRider · « **Reply #7 on:** March 15, 2012, 07:20:46 pm »

DynoMan told me once that they get better 60 ft times, something that's very important to dragracers, with cranks that are heavier than as light as they can be. The very lightest cranks would hesitate when the load was applied, then catch and go.

Better to carry a little weight off the line.

The very lightest cranks may be more helpful in a roadracing environment where the rpms are high and the load isn't a light switch like in drag racing.

Now, as far as we are concerned, if you want the biggest impression at the Thursday night drive in blip the throttle contest, then you'll want the lightest crank possible.

tweakin · « **Reply #8 on:** March 15, 2012, 07:24:46 pm »

I am running a lightened APE racecrank in my bike and the idle is supper smooth and I did not notice any loss of torque. Granted I do have quite a bit of other modifications to my engine and am not using stock carbs.

cbr954 · « **Reply #9 on:** March 15, 2012, 07:58:55 pm »

I run an APE crank also. Mine idles good. I dont think the loss of bottom end inertia/torque is much of a factor on the 750 because it has a fairly long stroke as far as motorcycle engines go. I ride mine all day long with the lightened crank and lightened alt rotor. Different levels of crank balancing are available also..... if a person is worried about to light of a crank just get one that isn't lightened as much.

MCRider · « **Reply #10 on:** March 15, 2012, 09:39:11 pm »

Quote from: tweakin on March 15, 2012, 07:24:46 pm

I am running a lightened APE racecrank in my bike and the idle is supper smooth and I did not notice any loss of torque. Granted I do have quite a bit of other modifications to my engine and am not using stock carbs.

My project now has an APERaceCrank also. The crank was already lightened quite a bit, and it was an 888cc. With a seriously lightened rotor. I loved it, for about 50,000 miles. I never experienced anything I would consider a negative. Love that throttle response. Bigjay said he took a litle more off as he improved the balance.

I've got the Crower rods, which are a little heavier than the carrillos (didn't you get the carrillos?)

Don't have the new motor running yet, but i expect i'll love it too. Never had any trouble spinning the rear tire on the old motor. If I was a serious drag racer i would take DynoMan's advice and leave it a little heavier.

camelman · « **Reply #11 on:** March 15, 2012, 11:01:38 pm »

Interesting stuff so far. I'm looking at a street bike to take to the twisties and maybe some vintage racing. Everything so far is pointing towards a lightened crank for better response in the high RPMs. I like where this is going!!!

stueveone · « **Reply #12 on:** March 15, 2012, 11:05:26 pm »

Yezin my brotha! I've been building up a motor myself man. Got some new stuff to shoot the #$%* about. You gonna ride a week from this saturday? Hit me up dude!

stueve

Retro Rocket · « **Reply #13 on:** March 16, 2012, 12:00:09 am »

Quote from: camelman on March 15, 2012, 11:01:38 pm

Interesting stuff so far. I'm looking at a street bike to take to the twisties and maybe some vintage racing. Everything so far is pointing towards a lightened crank for better response in the high RPMs. I like where this is going!!!

It does have less flywheel effect at take off but is easily gotten used to, make sure you don't use the stock stator and rotor, with the ability of the engine to spin up faster the rotor puts more stress on the end of the crank, this can break cranks on the 750's.
I had a mid 80's 750 Ninja race bike i converted from the street, it had a lightened crank {among other things} and was a blast to ride, stupid fast, my mates GSXR1100 had a hard time catching it. Plenty of guys told Greg {the original owner} that there was too much weight removed but the bike was ultra fast both on road and track, the engine work was done by a HRC mechanic on an exchange program from Japan, he really got that engine working..... I have lightened cranks going in both my 750's....

camelman · « **Reply #14 on:** March 16, 2012, 09:40:40 am »

Retro, I hadn't thought of rotor issues causing issues with crank reliability. Have you, or anyone else, heard of CB engines other than 750s suffering broken cranks?

Stueve, let's ride! The Red Dragon is blowing a lot of smoke these days (bad valve guides) but I have 1050 ccs of Triumph at my disposal now.

I'll hopefully have my 466 kit back from the machinist this month, after which Rieck's services will hopefully help me create a new Red Dragon!

MCRider · « **Reply #15 on:** March 16, 2012, 09:46:50 am »

If I may, I've seen 2 and there have been 2 on the forum. It does/did happen. The first rotors were real heavy. Honda lightened the rotor themselves at about K2. I've never seen/heard of a OEM lighter rotor breaking a crank. They can be made lighter still, than the K2+ OEM rotor, but may not be necessary.

Power shifting and dead throttle down shifting is the culprit. Though who can blame someone with a 836 or otherwise kitted motor for a little light hearted power shifting once in a while?

Really? · « **Reply #16 on:** March 16, 2012, 09:49:09 am »

Y'all are spending my mental money!

Retro Rocket · « **Reply #17 on:** March 16, 2012, 03:29:10 pm »

Quote from: camelman on March 16, 2012, 09:40:40 am

Retro, I hadn't thought of rotor issues causing issues with crank reliability. Have you, or anyone else, heard of CB engines other than 750s suffering broken cranks?

Stueve, let's ride! The Red Dragon is blowing a lot of smoke these days (bad valve guides) but I have 1050 ccs of Triumph at my disposal now. I'll hopefully have my 466 kit back from the machinist this month, after which Rieck's services will hopefully help me create a new Red Dragon!

As MC said, it was a problem on the early 750's, depending on how much HP your planning on, i would still lighten later model rotors for the 750 on big HP motors, never really heard of it on other bikes but that doesn't mean it hasn't happened....

MRieck · « **Reply #18 on:** March 16, 2012, 05:00:21 pm »

Quote from: MCRider on March 16, 2012, 09:46:50 am

If I may, I've seen 2 and there have been 2 on the forum. It does/did happen. The first rotors were real heavy. Honda lightened the rotor themselves at about K2. I've never seen/heard of a OEM lighter rotor breaking a crank. They can be made lighter still, than the K2+ OEM rotor, but may not be necessary.

Power shifting and dead throttle down shifting is the culprit. Though who can blame someone with a 836 or otherwise kitted motor for a little light hearted power shifting once in a while?

Me neither. I have talked to guys about the heavier rotor helping drag launches but there was the problem of snapping the crank snout or bending the crank once they stared making big (for a CB) power.

cbr954 · « **Reply #19 on:** March 16, 2012, 05:54:46 pm »

My dad had a RC 1000 cobra motor back in 76. He went against RC suggestions and ran the oem rotor. It lasted a year or two but it did break the snout off the crank and made some air holes in the block along with it.

MRieck · « **Reply #20 on:** March 16, 2012, 08:53:30 pm »

Quote from: cbr954 on March 16, 2012, 05:54:46 pm

My dad had a RC 1000 cobra motor back in 76. He went against RC suggestions and ran the oem rotor. It lasted a year or two but it did break the snout off the crank and made some air holes in the block along with it.

My last engine made over 98HP....I have to honestly say I never had a problem with the late style rotor/balanced, crank slightly lightened. No disrespect intended BUT I never had a problem. The primary chains are another story.
Mike

754 · « **Reply #21 on:** March 17, 2012, 08:58:42 am »

just a few thoughts, correct/discuss with me if you disagree. I never ran a lighter crank..yet, but ran light alternator on a 836cc.
So I suspect;
Any deficiency caused down low by lightened crank can be overcome, by grabbing a hanful of R's and slipping(good) clutch, or downshifting.

Heavy rotor may feel it launches better, but more due to riding with dumping clutch to get out of the hole..(rather than proper clutch slipping holeshot)

dragracer · « **Reply #22 on:** September 16, 2014, 10:27:56 pm »

I'll keep my heavy crank with the rotor removed. Drag race only. Can't fathom any advantage to a lightened crank in a drag motor- balanced yes, light, no. In my opinion, we need that inertia off the line and for sustained rpm's down track. Had a good friend buy a ligthened GSXR 1000 crank from a road race friend of his to put in a drag motor. His ET went up over a 10th and a half in the 1/4 mile. Put another stock crank back in it and went right back to where it was to begin with- made a me a firm believer that lightened cranks are only good for street bikes, stop light to stop light and road racing where getting the rpm's up and down quickly while rolling in and out of the corners is all important.

PeWe · « **Reply #23 on:** September 17, 2014, 02:57:11 am »

Cheap mod when tuning the engine is a lightened alternator rotor, a must when I modified my engine many years ago. Impact? I do not remember how a stock CB750 feels. Quicker rev-up's I suppose.. When I hear stock CB750, my sounds like a racer

If I should need stronger rods I'll buy it complete with lightened crank, assembled, balanced ready to go.

Don R · « **Reply #24 on:** September 17, 2014, 09:52:02 am »

My cb750 mentor broke a crank with a non lightened rotor. it was on an 836 kitted, cammed ported motor though.

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Author Topic: Lightened Crank, what's the impact? (Read 6434 times)