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| Viewing Page 1 of 1 (Total Posts: 8) |
| Author | Comment |
Ron Liddle
Feb 8, 07 - 8:38 AM |
Hodaka trials motor information needed
In the process of freshining up a trials motor for a friend. The base motor is a Combat wombat which appears to have an o3 cylinder. The gear set and primaries all appear to all be from the combat wombat also. This motor has had a steel sleeve pressed over the clutch housing apparently to add dynamic weight to the crank shaft assy. What I want to know from someone experienced with hodaka trials motors is how much weight is enough? The owner is concidering purchasing one of the weighted flywheels with the brass sleeve pressed over it. Is this a worthy adition or not. We are going to use the Combat Wombat ignition which utalizes a flywheel already. This engine also uses a wombat carb and the cylinder head has had it's area increased I suspect to lower compression. It also used a modafied wombat two ring piston which I plan to use another the same again. I have no experience in hodaka trials engine build up other than where the parts go. Any help would be appreciated. Question 2, would the adition of a lower primary ratio be of any advantage? At present and as it probably worked previously it uses the 59 tooth primary of the Combat wombat. I have come up with an even lower ratio primary combination which with some minor machine work will work. I just want to know from someone knowledgable with trials motors if this would be any advantage. 
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Ed
Feb 8th, 2007 - 10:23 PM |
Re: Hodaka trials motor information needed
Ron: My own experience suggests something like - all the flywheel weight you can put in there will be almost enough. OK, that is probably an over-statement but . . . I have approx. 2+ extra pounds on the clutch and approx. 3.5 extra pounds on the magneto flywheel on my reeded Combat Wombat trials motor. It seems a bit abrupt to me when the 3.5 pound weight is removed. I'd have the 03 cylinder on my engine, except it wasn't being built yet when the engine went together in 71/72. Been gonna get around to installing an 03 cylinder for some time now . . . but keep getting side tracked with thoughts of installing a Wombat cylinder (piston port) for Classic class. So I end up just leaving it alone. I'm curious, what compression does the engine you are working on have? I need to re-check mine just for kicks. I can't remember what it was the last time I checked. I've GOT to start a note book!!!! Gearing? I'd recommend aiming for about 50:1 overall ratio in first gear for a Hodie 125 for trials. I don't think I know any rider sensitive enough to tell the difference between getting there by adjusting primary gear ratios and getting there through sprocket selection . . . so I wouldn't recommend spending machine shop time to modify primary gear sets. Ed Keep the rubber side down! |
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Ron Liddle
Feb 12th, 2007 - 6:36 AM |
Re: Hodaka trials motor information needed
Thanks Ed. I received this engine in a box. Waitting on the top end parts so after it is assembled I will try and get a compression pressure for you. Now this may be a dumb question but why exactly do you want added dynamic weight to the rotating assembly. Is it for a smoother idle? because as all my years of engineering I can only see this added weight slowing down throttle response but giving a smoother engine as the engine comes up to speed. Is it to tone down the throttle response.Also I am at present working on a lower primary ratio for the trials motors. Real busy at the moment with several projects but I will get to it. I have been informed that this would be quite an advantage. But you know I don't ride trials myself so I am trying to understand what you fellas need. |
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Ed
Feb 13th, 2007 - 7:13 PM |
Re: Hodaka trials motor information needed
First - the added flywheel weight for trials. There are multiple factors which are deemed valuable for trials. Among them are smoothing of the power pulses of the engine and energy storage (these two things are very closely related). Increased rotating mass @ the crankshaft absorbs a portion of the "punch" when the engine fires and stores that energy for release during the "coasting" portion of the crank rotation. The end result is smoother power delivery and reduced chance of breaking traction . . . and traction is of paramount importance in trials. If you imagine riding on smooth, mossy and muddy rocks . . . smoother power delivery is worth its weight in gold . . . and it helps relieve a little bit of the requirement for absolutely perfect throttle control on the part of the rider in order to retain traction. Energy storage in the flywheel also allows a rider to rev 'er up and blast a steep climb where there is good traction at the bottom and then "feather" the throttle and "coast" on "fly-wheel smooth" power up gnarly, pebbly or muddy steep climbs. Most trials riding involves engine operation from idle up to and including the blazing speed of 3,000 rpm. The previously described "blast" might involve revving to 8,000 or higher for a very few seconds. Get that extra flywheel weight spinning that fast (8,000) and it has some significant energy stored to help you on your way. At the lower rpm's, the energy storage keeps 'er ticking over for the tight turns. You are quite correct that all that mass does slow down throttle response, but the benefits described above are worth it. (Keep in mind that the closer you approach "explosive" throttle response, the more you will break traction as you "snap" in a bit of throttle.) While a racer wants ALL the HP NOW for maximum acceleration out of corners - and wheel spin is one of the ways to get that - A trials rider wants his tire "hooked up" at ALL times. Breaking traction is one of the first sins in trials. Flywheel weight helps keep that tire "hooked up". Finally, the ideal would be "tuneable" flywheel weights. In bare, clean, rough granite bed-rock sections with copious traction everywhere, less flywheel weight would be fine. In round river rocks with slimy mud - or slimy, muddy grassy stream banks - more flywheel weight will tame the power and help maintain traction. I've got to research some ratio numbers before commenting on gearing and primary ratios. Ed |
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Ed
Feb 16th, 2007 - 2:57 PM |
Re: Hodaka trials motor information needed
OK! I've found the time to be able to offer some comments on gearing. As mentioned earlier, I would recommend targeting about a 50:1 overall reduction in 1st gear for a Hodaka 125 with an 18" modern (radial) rear trials tire for trials competition. Since the engine you are working on has the Combat Wombat straight cut primary gear set (this is good because the spur gears are more efficient - if a bit more noisy - than the helical cut Wombat primary gear set) with a primary drive ratio of 2.75:1 and you have the Wombat/CW 1st gear of 9TX31T with a first gear ratio of 3.44:1, then you'll want about a 5.28:1 final drive ratio (sprockets) to end up with 50:1 over all gearing. Assuming a 13 tooth counter shaft sprocket, you'd need about a 68 tooth rear sprocket. You mentioned having been advised that a lower primary ratio would be "quite an advantage" for trials motors - but no information was offered regarding what advantage would be gained. I'm not quite sure why I'm not on your trusted advisor list, but I would disagree with that (as mentioned previously in this thread). Please note that I do understand there are situations where a lower primary drive ratio might indeed offer noticeable performance increases . . . but I'm pretty darn sure trials isn't one of them. I suppose it would be good to confirm that the words "lower primary drive ratio" were meant to mean a higher NUMERICAL ratio (ie "granny" gearing). IF you have some way of modifying Super Rat (93) primary gears (16T X 62T)with a primary ratio of 3.87:1 for use on the 125 . . . the rear sprocket could drop to 48T or 50T . . . which could cause problems (see below). 1) The engine you are working on MAY return to a chassis which has longer shocks installed in order to reduce rake and trail. This also causes a "steeper" swing arm . . . and MAY call for a large rear sprocket to keep from "sawing" the swing arm pivot area with the chain. It could be that a 68 tooth rear sprocket is much better than a 50 tooth sprocket in such a situation. 2) Note that we have already discussed the addition of fly-wheel weight to smooth or tame throttle response for improved traction . . . so why would it be advantageous to do a bunch of extra machine work to create special primary gears which are intended to make throttle response more "snappy"? Ed Keep the rubber side down! |
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Ron Liddle
Mar 26th, 2007 - 5:59 AM |
Re: Hodaka trials motor information learned new info to ponder
Thanks Ed for all the great feed back on this subject. I have done some research and further R&D of my own and with another trials hodaka expert. Some of which we have already acomplished and some which will be incorporated later. Lower primary gears are available with the use of the 93 100 rat gears. This was acomplished by mating a 100 right side crankshaft half with the wombat 125 left side.This allowed for the use of the primaries of the 93 100 rat which are of a lower numerical ratio. Like you said "granny" OK sinse the drive ratio of the rear wheel is much lower and much less stress is placed on the clutch load we used the 100 rat clutch with only one set of springs no inner springs. This makes the clutch much easier for the rider's operation. A 1" steel plate was machined and secured to the wombat flywheel. Now on a previously proven trials motor rebuild I recently completed I noticed a weight added to the outside of the wombat clutch. This really crowded the space under the clutch cover and required some work with a die grinder for it to clear.It also was a source of a possible leak. After thinking about adding even further weight to the rotating mass and in need of more kinetic energy. And after realizing that the clutch was under so little stress due to the decreased load which the lower gearing provided. The next change will be to use the ACE 100 clutch which is of a much smaller O.D. and will allow for weight to be added without causing a clearance problem.I will let you know how this all works out.We are also concidering adding a better quality main bearing which is designed for a greater load. Now ED I know you will think this over and respond with your thoughts and expertise.I do appreciate it all.And if this all seems way out to you, remember some things just have to be tried. If it doesn't work out then so be it the knowledge is worth the effort.
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Ed
Mar 29th, 2007 - 4:55 PM |
Re: Hodaka trials motor information needed
Ron: I was wondering if you were working on a way to put the 100cc straight cut primaries on a 125 . . . sure enough, you were. That will allow setting up gearing for 2nd gear to be the main gear for most sections (with 1st available for really, really tight stuff) -- without having to run a rear sprocket approaching the size of the rear wheel. That's kind of neat. Adding flywheel weight on both sides of the crankshaft certainly seems better than adding weight on only one side, doesn't it? Higher capacity main bearings seems like a passably good idea, though I'm not sure they are needed. My own trials engine ran from '72 thru '78 - got parked till the winter of 2003 and then was cleaned up and started (did not open the top end or bottom end). It still has all the original bearings (crank and tranny) - with no outward signs of failure. I wish I could remember what I did to prep that engine for storage!!!!! BUT . . . two weekends ago the original clutch side main seal finally "gave up the ghost". Guess you just can't depend on those clutch seals for much more than 35 years! ;-) So I'm going to have to "go in" and replace the clutch side seal. I'm running the internal debate right now . . . new bearings? or leave 'em alone? Bye for now, Ed . . . and keep the rubber side down! |
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Ron Liddle
Mar 30th, 2007 - 9:06 AM |
Re: Hodaka trials motor information needed
Really Glad to read from you ED.Sounds like you are great on maintenance. On Main bearings, I found that the best way for me is a couple differen't ways. 1 I set a dial gauge on the crank and lift up and down. Sometimes just lifting up and down on the end of the crank is good enough. The favorite one for me is to hook up my timing buzz box to the ignition just as I was going to time the engine. Rotate across the timing marks until the point of change is noticed.Then just before the point you have noticed I lift up and down on the crank with my hand. If the buzzer changes tone then I decide how lose is too lose for the main bearings.As you no loose main berings will scatter the ignition timing and cause inconsistancies. To a point and for those not so criticle it can be let go quite a bit.Anther method is to run the engine while running a timing light across the timing marks.Worn main bearings will show as an inconsistant firing point. And you know lose mains and old seals don't work out too long? |
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