tipsy geometry problems

Discussion in 'Rack Mounted Engines' started by mechcd, Dec 4, 2008.

  1. mechcd

    mechcd Member

    I have a weird problem at hand here.

    When I was riding my bike as a regular bike, i had the spring and suspension stuff set as it was out of the box. The rear triangle sat parallel to the ground. I had the seat up high so the pedals were in the right spot for my leg length. All was great :biggrin:

    With the engine on there, I have the urge to keep the seat low, as I feel like i'm going to fall over with it up high. I would also like to be able to put a foot or two down at stops.

    If i leave everything the way it was, but with the seat as low as it will go, the exhaust hits the seat and pedaling is horrible as expected. This is the first pic. The seat is in my trunk so its not in the shot.

    In the second pic, i start to jack up the suspension. This gets the seat away from the engine and makes things feel a bit better. The problem here is that it makes the entire bike higher and i can't get on and off easily. Pedaling is still bad because the seat is low. The front forks are sort of straight up and down and i don't know if thats a problem or not.

    My motorcycle instincts might be kicking in here, but it seems like i wanna make the suspension as low as it will go. This allows me to put the seat up more for pedaling, and give the front forks more of a rake. I was thinking of putting a 24" wheel on the back to help a little. The seat still hits the exhaust :icon_cry:

    Now that I've said all of this, I have no idea what I want to do. I guess I want to lessen the tipsy feeling I get by having the engine mounted up high.

    Cruiser frame? My wallet says no. I thought about rotating the GEBE mount so that the engine rides way to the back and low to the ground.

    Time for an EH025 or ECO4? :surrender:

    Attached Files:

    Last edited: Dec 4, 2008

  2. mechcd

    mechcd Member

    I started whacking things with a big dead blow mallet and got my engine mounted back farther so the exhaust doesn't hit the seat.

    Now I need to learn how to ride a bicycle all over again :D
  3. SimpleSimon

    SimpleSimon Active Member

    The "tipsy" issue is a CoG one, and common on rack mounts. Moving the engine back and down will help some, but be careful you don't end with a "tail wagging the dog" issue in exchange. As for the exhaust issue, why not pull your muffler, make a template of the exhaust pipe and flange, then fab up a 180 degree U bent pipe to point the exhaust behind you, or at least a 90 degree to point it out to the side?

    Really, it just a small flange which could be easily rought cut from mild steel, with bolt and pipe holes drilled. A bit of work with a file will smooth up rough edges and round corners pretty quickly. Make two flanges, get a pipe of appropriate size bent in the curve needed, pay a welding shop to weld the flanges in place. Then just bolt it together again.

    As shown in your pics, really all you need do is cut off the flange on the exhaust, and swing it over to the side.
    Last edited: Dec 5, 2008
  4. SirJakesus

    SirJakesus Guest

    Nice bike! ;)
    Yeah you have the idea. If the mallet thing didn't move it back enough get some tough flat metal stock and make a new center mount for it. The bike is always going to feel a bit squirrelly if you're anywhere over 5ft 10in or so. It's a rather stubby bike. Make the suspension stiff to reduce bounce from the un-suspended weight. I wouldn't change the position of the suspension mount... it just seems like a bad idea to me... no science to base that on.
    It looks like you have the original supersonic (or whatever) black rims on there. I would highly suggest zip tieing the cross points or being prepared with heavy duty spares. I warped the **** out of mine before the bike was even motorized.

    I'm hoping I was part of your inspiration :) but if not whatever, the frame itself makes for easy motorizing. If your spring blows out and gets play in it they can be had for cheap at, just so ya know.

    After inspecting the pictures some more it looks like you already have an upgraded spring. Thats probably whats throwing your rides height off. I think the replacement one I got was 4 or 5 inches, the shortest available to replace the original. Your current mounting is probably bringing the bike back to its normal height whilst giving a bit more travel. On another thought you may want to wrap that exhaust with some black insulation tape. Looks like you have a good chance of burning yourself on that bare pipe.
    Anyways, cool to have another xr75 rider around. If I can suggest, get new grips, and be careful of the stock stand.
    Last edited by a moderator: Dec 5, 2008
  5. mechcd

    mechcd Member

    Yeah, your bike definitely got me started :D

    I had it up and running on Saturday with the seat lowered and the exhaust out of the way. It was a lot easier to mount/dismount and stop with a foot on the ground. As for the spring, its a stock one from another bike, just a tad longer.

    Then I tore everything apart again tonight :goofy:

    The front rim is the stock aluminum one with a new tire and tube

    The rear rim is one i acquired from the local bike grave yard (bike rack). I kept the aluminum rim. I'm not really familiar with this stuff but I figured I could use it later with new spokes and hub. I'm going to use one of those bolt wire pliers to tie the spokes down.

    It seems like my back end is all messed up. The rear triangle got bonked a few times as a regular bike. To get the GEBE ring to fit, I have to pull the wheel to the right. Now the front and rear tire aren't aligned. I have a feeling this is the cause of my wobble. They are both parallel, but not in line with each other.

    Hrm.... I have access to the junky NEXT bike (frame?) that all the other assorted parts came from..... i wonder if the rear would swap out, prolly too much work.

    Eventually I want to flip the engine upside down so the carb is facing forward. At that point I'll chop the exhaust and route it decently. I do have access to a MIG and stick welder. I can booger two pieces of pipe together.

    Whew, this whole thing ended up being a bit harder than i expected. The GEBE is a bit fiddly itself as I need to unbolt about 6 things anytime i want to move something a smidgen.

    SirJakesus: If i recall you have the Staton CVT hub. In general, how much do you pedal? I would like to simplify things and leave the derailer set in one position.
  6. mechcd

    mechcd Member

    Like this :D

    I haven't welded the pipe, so I stuck a piece of tubing in there for the mockup. The muffler bolts to the same fender mount as the GEBE frame.

    Its a heck of a lot easier to get on and off of now.

    The gas tank rears its ugly head again and has no obvious place to reside

    the guitar hero game in the left corner isn't mine :(

    Attached Files:

    Last edited: Dec 8, 2008
  7. SirJakesus

    SirJakesus Guest

    Yeah if the wheels are out of line the bike is always going to fight itself and the handling is never going to be quite right. In fact I think mine is a bit out of whack from my peddaling days but theres enough adjustment space with the horizontal dropouts to realign things for the most part. I still feel a little bit of wobble at low speed, I think its just the nature of the beast to some extent. The sacrifice of a $100 full suspension bike. Get your jollies with it and upgrade later is my take on the xr75. The frame hasn't failed yet despite my constant abuse and heavy motor setup. I've read of other people having to bend their frames to get GEBE rings to fit correctly... Something about a frame spreader/scissor jack, wood and a mallet on both sides. Search around I'm sure the process is on these forums somewhere since I'm not a golden eagle expert.

    As far as pedaling is concerned... The GEBE and Staton Nuvinci are two totally different animals. I have gearing going all the way from low where I can peak out the engine revs and only be moving 9mph to peaking and going 32mph. Technically I can shift with the nuvinci and not have to pedal at all but it definitely aids in much faster take offs and for climbing hills. If you're looking to avoid shifting as much as possible while motoring I would suggest probably 2-5, 2-6, 3-4 or 3-3 (front-rear) gearing to have low enough gearing to push off and just high enough to give your engine a boost up hills. If it helps try moving controls around so you can operate the throttle and your preferred shifter at the same time for zippy acceleration. Hope you kept the gripshifter and cable for the rear if you did want to shift on the fly.

    Whatever you do don't give up, MB's are a lot of work to get set up initially but afterward its mostly maintenance work and the occasional screw up or accident repairs. But you should definitely focus on getting that rear wheel on there straight. It's never going to be as fast, safe or fun with the rear wheel pulling your arse into the curb. PM me or I'll be watching this thread. Let us know your progress MB brotha!
    Last edited by a moderator: Dec 9, 2008
  8. mechcd

    mechcd Member

    I ended up using the same gas tank mount and drilled a few more holes to bolt it to the former bottom of the engine. It still starts hard. I'm still blaming the weather as the plug has that nice tan color to it. Putting a drop or two of gas on the air filter helps things along. Plus my kill switch wire melted onto the block. I wondered why the engine would die when i hit bumps :idea:. The booger runs for like 2 minutes with the fuel petcock at the engine turned off. Last time i threw a sandwich bag over the air filter to kill it :grin5:

    I do still have the rear grip shifter and derailer system on and potentially working. The cable is currently zip tied to the frame. It wouldn't take much to fix, but it seems like I keep taking the back wheel off a lot and i'd rather adjust it once everything else works.

    I need a cone wrench. badly. I need to tighten the bearings and give it another ride. That might cure my problems. I did see the several threads about the car jack wood etc. I gathered the stuff to do it, looked at the rear triangle and decided some vice grips and love :shout: would make it fit better.

    The laundry list goes like this then
    1. Tighten bearings
    2. Attach exhaust with something other than melted dishwasher hose :rolleyes7:
    3. kill switch, shifter, back brakes -in whatever order

    I was looking at getting another bike, but my silly side got me and I bought the guitar hero game that was already residing in my apartment. Evidently my friend didn't want it or have space or something. I always joke my mom that if I don't eat, I can watch cable tv.

    I saw a new version of the XR-75 a Wal-mart. I hazily remember seeing disc brakes, but don't quote me on that. Either way, if i do get a new one, a longer frame would make things feel a little better. I like MTBs for the suspension and stuff. Cruisers look more solid, but the Cranbrook things I find here are drum brakes with no shifters or suspension.

    If it's not horribly cold or wet tomorrow, I might go for a short ride. Of course I'm up at 2:30 am again. upstairs neighbor came back after a 1 week gap.... diabetes doesn't like me occasionally.... BUT there is snow!!!! This calls for hot chocolate, bagels, oj, and biking!
  9. beast775

    beast775 Guest

    hello mechcd

    hope your getting the bike straightened out.its the worst part of motorizing a bike.once thats done then things start working out.i have had my staton kit on 6 different bikes,till i found one i you have diabetes?ive had juvenille diabetes,,type 1 for 34 years this year:ack2:..good luck.
  10. SirJakesus

    SirJakesus Guest

    I saw the new XR, it has dual linear pull brakes like the older version. Though they look beefier. My bro has the XR200 which has front disc brakes and rear liners and it seems like a good bike. Or they have the NEXT mountain ridge with front disc brake. All of these bikes have a similar solid rear triangle that makes mounting a motor simple... however that GEBE ring is going to complicate things on any of them since they're narrow and don't give the wheel much clearance. I would say just work on the 75 for awhile and get all the kinks out that you can and ride it until you find the perfect donor bike.
  11. mechcd

    mechcd Member

    I haven't posted in a while so I thought its time for an update.

    The rear brakes are back in action. It barely clears the GEBE "strap", but it never rubs according to the chalk i put on it.

    I got a cone wrench and tightened things up in the back. Alas, I can't really try it out.

    I've tried to start it several times since my last run but no luck. Trying in 10F weather was just for exercise :dunce:. The gas actually froze in the line. It was 40F a few days ago and i got it to sputter and cough, but no starts. More choke, more sputter.... more tugging.. and my arm gives out. Somehow I used half a tank of gas doing this (4oz?). To me this means its lean for the cold weather, but i really don't want to take that silly throttle cable assembly and needle thing apart anymore. I'm still somewhat regretting using a pocket bike engine. The sweet high pitched roar of how every many horsee-powers keeps me tugging away.

    I dumped my now month or two old gas mix in the car. Now my car exhaust smells like 2 stroke. I must have the low smoke, high smell oil :thinking:

    The brazing on the exhaust joint broke loose somehow. I guess I need to sneak it into a lab or shop and hit it with a MIG real quick. That U shaped bend right out of the engine gets pretty darn hot.

    Does anybody have some ideas for a gas tank? I might have pictures of my current setup in another thread. My 8 oz chainsaw tank is pretty nice, but its not enough gas for this thing. Something like a 2 liter pop bottle turned sideways would be great.

    I've been type 1 diabetic since I was 2. I haven't really known anything else. I've had a pump since 2004 and it has made things like traveling or a random off schedule day much easier to handle. :cool:
  12. mechcd

    mechcd Member

    Another update:

    The XR-75 is pretty much back to being a frame and a pile of parts. I was riding it around in the snow and somehow got something stuck in my rear spokes. I was going down hill pretty fast, and on the way down, the rim got torn up, the rear brakes bent, and the derailer is iffy. :whistling: It was a fun slide downhill though! I didn't have any of the engine stuff mounted.

    I'm not sure what I want to do with it now. i need yet another rear rim. I might go with a 24 inch to make some room in the back and to ease up on the gear ratio. The GEBE is getting to be a major pain to work on. In order to get the wheel lined, up the wheel nuts are loose. those also hold the motor bracket on. So basically it takes a person to hold the bike up, a person to hold the engine, and a third person to get the wheel aligned.

    I have an XR-250, but the GEBE ring doesn't fit at all on it. The spoke pattern is wrong and it wouldn't clear even if it did clip on.
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