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  • Press Fit Question

    This is a bit of a long-winded explanation to get to what is ultimately a pretty simple question... bear with me...

    So, I did some work on this motorcycle countershaft last weekend. It's for a supermoto race bike (4-stroke dirt bike with 17" wheels and slick tires). The wider wheels and tires generally cause clearance issues with the drive chain. A common solution is to add spacers to the countershaft and chainring in order to move the chain a little bit further away from the tire.

    Many countershafts have a threaded hole in the center with a corresponding bolt to keep the countershaft sprocket in place. This Suzuki used a circlip, though, as seen by the groove in the pic. Adding a spacer put the sprocket directly over that groove leaving the owner with no way to secure it.

    The shaft did have a nice smooth hole in the center, but no threads and I didn't feel like I'd have any chance of threading it. I decided to turn a bolt to down for a tight interference fit, press it into the hole, add a couple of really quick tacks with the tig and chase the threads with a die. Seems to fit the bill, but here's my question...

    While I doubt that it would pose any issues in this particular case given that the fit between the splined countershaft and the countershaft sprocket are generally quite loose. What effect on the OD of that shaft would my press-fitting this bolt into the hole have? Any? I didn't mic it before and after... probably should have. Obviously there are things to consider like the material, thickness, amount of interference, etc... Generally speaking, though, to what extent do you guys consider a press-fit to alter the OD of the part that has been pressed into?



  • #2
    Did you check the hardness of the shaft to see if tapping the hole for a bolt would have been possible?

    Comment


    • #3
      How much press fit did you use? I have never measured for expansion of the part after pressing something in but I doubt if it would be very much in your case and as you said, the wear of the splines and sproket would still have a loose fit.
      It's only ink and paper

      Comment


      • #4
        Originally posted by Glenn Wegman
        Did you check the hardness of the shaft to see if tapping the hole for a bolt would have been possible?
        A carbide auto center punch left no mark on it. That was as far as I pursued the threading idea.

        Comment


        • #5
          Is there a possibility of making offset sprockets?

          Comment


          • #6
            Originally posted by Glenn Wegman
            Is there a possibility of making offset sprockets?
            Not really. Most of these guys run non O-ring chains and replace the drivetrain on the regular, run different gearing for different tracks, etc. The cost of that approach wouldn't fly.

            Carl, the amount of press fit I shot for was a little bit over .001.
            .0015 maybe? Admittedly, I just shot for that number and it seemed to feel right at the press.

            Comment


            • #7
              the only problem id have with this setup is a nut is basicly a natural press fit extractor, your talking a lot of force once some mechanic torques down on on that..


              Maybe its overkill, but id tig the hell outta it, well, at least a full bead around where the two parts join, standard fit threads don't distort too bad from welding heat in my experiance, and the nut should'nt even get near the end of the threads by the sound of it (or add washers to insure it won't)

              If the welding heat screws up your spline fit, a diamond coated file should be able to restore it quickly enough reguardless how hard it is, but I kinda doubt the larger part would get hot enough to seriously distort.. But then, im not experianced in welding.
              Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

              Comment


              • #8
                Im sorry but by an engineering viewpoint --- You have an engine that produces more horsepower - you hook it up to tires that are wider and bite more --- and now you want to toss the final drive loads off the shaft end even further?

                First off it wouldn't surprise me if you started munching countershaft bearings,

                Secondly, I would be extremely careful when messing with original design in this area, there is no room for error, I ran a motorcycle service department for many years and can attest to just how nasty it can be when one loses their chain up front but its still connected to the rear wheel...

                Even Glens suggestion (that really sounds good at first) of perhaps making an offset sprocket could raise holy hell and Ill tell you why, these systems are designed to float and while engaged in the splines they really don't have any side forces being thrown at them -- they seek an equilibrium in the middle and find their "happy place"
                Now lets say you took a box stock sprocket turned its teeth down and just used its internal splines yet welded another sprocket right along side of it and turned the inner splines off of it, You now pretty much have a factory hook up and nothing can go wrong right? wrong --------- due to the teeth being offset from the splines the free-floating internal splines will "walk" under pressure over to one side and stay there till they either pop a circlip or wear at a interference plate.

                All I can say is be careful...

                Comment


                • #9
                  Here is my solution to a similar problem. The sprocket is spaced further away from the engine by using an adapter to fit the original sprocket, that mounts the new sprocket further out. This saves making a female spline and getting it properly heat treated. Here are some pics.

                  This is a stock sprocket that fits the tranny spline. I used trig and the DRO to lay out 15 bolt holes in the adapter. I used 5/16" SHCS to fit into the teeth of the #50 (525,530) sprocket. I could have adapted the final drive sprocket to the shaft, thinking a taper-loc mount would be strong. But I found one of these double drive sprockets and adapted it.

                  You can see the front view here.

                  It is hard to see, but there are 15 SHCS coming out each side of the adapter, which also carries the bearing shaft. LocTite the bolts. The bolts on the one side engage the stock sprocket with the spline, and the bolts on the other side engage the double drive sprockrt. The double drive sprocket then drives the chain to the rear wheel.

                  Here is the overall side view.

                  The bearing is a self-aligning flange mount ball. The support plate mounts on specers from the swingarm bolt and two engine mounting bolts.
                  --Doozer
                  DZER

                  Comment


                  • #10
                    It might be a good idea to drill and ream a dowel pin hole and press in a dowel
                    to keep the stud from coming out. That's the main problem I see.

                    Comment


                    • #11
                      Tig

                      I agree with Black Moons' post. I'd carefully TIG the stud into the countershaft the full circumference and you'll likely have a fine solution. If you're worried about too much heat, well TIG is great for control or you could make an aluminum "heat sink" that slid onto the splines for your welding operation.

                      We're not talking about a high torque fastener here, you just have to/ must keep the sprocket on. Use Loctite for sure!

                      Super moto is a lot of fun!

                      Comment


                      • #12
                        Originally posted by A.K. Boomer
                        Im sorry but by an engineering viewpoint --- You have an engine that produces more horsepower
                        Huh? More horsepower? Who said anything about that? Should I tell this guy the modification got him an extra 5hp?

                        Jokes aside I respectfully hear you on the caution. Duly noted. I'm a racer myself and literally have the scars to go along with it. This guy is a personal friend.

                        This isn't exactly uncharted waters. Best guess would be 90% of the bikes on the grid have spacers. This particular bike has 3 seasons of racing on the motor WITH the spacer in place (don't ask what they were using to secure the sprocket before ). No countershaft bearing or seal issues. I rebuilt three motors for three different guys this winter, all with spacers that I didn't install, none with any problems. My personal racebike has a spacer. Par for the course. We're talking about a .125" or so spacer... No one is moving the chain line 1.5"
                        http://www.motostrano.com/hocrf45cosps.html

                        Also, not to be argumentative, I'm not a service manager, but from what I have seen your statement about countershaft sprockets floating on the splines and needing to find their happy place is not true in this case. Overwhelmingly the norm is to have a bolt or nut that secures the sprocket tightly against a spacer/shoulder on the shaft (Honda, Yamaha, KTM, Kawasaki... all the current 450CC MX bikes except Suzuki, actually). No movement. Even so, with a stock setup the amount of play between the sprocket, shaft and circlip is very slight. I generally only see road and supermoto race bikes, though... maybe, cruisers, etc are different? I dunno.

                        Comment


                        • #13
                          I believe an option (in the future) would be to simply have an internal thread EDM'd into the end of the shaft. Then a bolt could be used to pull the sprocket down against the spacer.

                          Comment


                          • #14
                            doozer: Very good solution! I like the extra bearing you added for support (Takes care of extending the chain so far), And the heads of the bolts will act like a spider coupleing (allowing minor misalignment without binding)
                            Being its made outta standard bolts and a standard(ish?) sprocket for the only wearing elements, it can be easily replaced if ever wear becomes a problem. (or padded with some rubber if it ever managed to wear a gap)
                            Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

                            Comment


                            • #15
                              Originally posted by Magee
                              Huh? More horsepower? Who said anything about that? Should I tell this guy the modification got him an extra 5hp?
                              My bad -- when you stated "race bike" I thought you meant "race bike"


                              Also, not to be argumentative, I'm not a service manager, but from what I have seen your statement about countershaft sprockets floating on the splines and needing to find their happy place is not true in this case. Overwhelmingly the norm is to have a bolt or nut that secures the sprocket tightly against a spacer/shoulder on the shaft (Honda, Yamaha, KTM, Kawasaki... all the current 450CC MX bikes except Suzuki, actually). No movement. Even so, with a stock setup the amount of play between the sprocket, shaft and circlip is very slight. I generally only see road and supermoto race bikes, though... maybe, cruisers, etc are different? I dunno.

                              It depends on the bike model not the make --- Yammerhammers, Kamakazies, Hound dawgs , and Susies (along with about a hundred other makes from around the globe) ALL have models with free floating sprockets that are either circlipped or have a splined washer that is installed and then rotates in a slot and is held by two or three little bolts that connect it to the sprocket --- When I went to Hound Dawg school they stated that this was a method utilized to stop chain variances from reeking havoc with either loosening a fixed sprocket assembly and/or causing these variances to be directly transmitted to the countershaft bearings and therefore causing pre-mature failure of said bearings --------- for what its worth...

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