cfd511 Posted December 29, 2023 Share Posted December 29, 2023 So today was the first day towing a low boy on my 92 superliner with 350. I noticed a front end bounce while cruising 45-50 mph. it was bad enough that it made me uncomfortable to say the least.it got new king pins. I did notice the motor is moving a lot so I think I have a broken engine mount. its going to mechanic in the am. any idea so I make sure he looks at everything. Quote Link to comment https://www.bigmacktrucks.com/topic/73698-superliner-front-end-bounce/ Share on other sites More sharing options...
mrsmackpaul Posted December 29, 2023 Share Posted December 29, 2023 King pin on the grease plate to far back Shock absorbers on the front axle crook Paul Quote Link to comment https://www.bigmacktrucks.com/topic/73698-superliner-front-end-bounce/#findComment-544246 Share on other sites More sharing options...
Adkat Posted December 29, 2023 Share Posted December 29, 2023 Steer tyres out of balance, for some reason always worse at 80kmh/ 50mph. Engine mount shouldn't cause bounce at cruise, but on/off power would be noticeable. As Paul said, pin too far back, not enough weight on steer axle, crook shocks 1 Quote Link to comment https://www.bigmacktrucks.com/topic/73698-superliner-front-end-bounce/#findComment-544247 Share on other sites More sharing options...
JoeH Posted December 29, 2023 Share Posted December 29, 2023 Rubber bushings probably bad on the engine mount, no big deal to replace. Could have loose bolts threading the rear engine mounts into the bell housing. Bellhousing is probably aluminum, so hopefully they aren't stripped out. How's your 5th wheel positioned? Is it dead center over the tandem or is it forward enough to put some weight on the steer axle? What weight rating springs are on your steer axle? Too light could be too weak to steady out any uneasiness in the chassis. Does it do this around 45-50mph on all roads or just on one particular road at a certain spot? I've got roads around me that certain bends make you feel like the tie rods are about to fall off. Only does it at one pair of bends, everywhere else it does fine. Had another road started making my triaxle buck for a couple hundred yards, always the same exact strip, then they came through and paved the road and the truck goes smooth as glass over it now. Quote Link to comment https://www.bigmacktrucks.com/topic/73698-superliner-front-end-bounce/#findComment-544256 Share on other sites More sharing options...
JoeH Posted December 29, 2023 Share Posted December 29, 2023 But my 1979 r686st does probably the same hop you're talking about. Endt676 motor with a 2 stick 6 speed. 5th gear, ~45mph. Solution? Just don't drive at 45 mph. Even though it might be a very comfortable speed otherwise. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/73698-superliner-front-end-bounce/#findComment-544257 Share on other sites More sharing options...
mechohaulic Posted December 30, 2023 Share Posted December 30, 2023 bad or broken engine mount should show a gear shift moving erratic . cruising 45-50 would be a "no strain "condition for drive train. engine mount/ rubbers wouldn't be under load to twist UNLESS the cruising condition involved torque. change the front tires with another set if possible or have those balanced to confirm they aren't the problem. had a brand new truck with vibration at 40-50 mph = turns out the balance weight fell of the rim == think I'm showing my age again. tires were bias not tubeless . also how is the word "bounce " used in reference to problem . truck bouncing up/ down or is it shimming at 45-50/ wandering ?? I agree with all above comments terms of 5th wheel set wrong etc. was the low bed empty or hauling a D-10!! (i know it can't just point of issue). 1 Quote Link to comment https://www.bigmacktrucks.com/topic/73698-superliner-front-end-bounce/#findComment-544279 Share on other sites More sharing options...
cfd511 Posted December 30, 2023 Author Share Posted December 30, 2023 So the truck doesnt shack at all. It a up and down . Feels like there no weight on front end . Drives fine when no trailer on the back. Only gets real scary with volvo 160 on a lowboy. King pin is dead center of tandem i believe . Will confirm in am . I def feel the motor mount . Feels like engine gunna twist out of truck. Quote Link to comment https://www.bigmacktrucks.com/topic/73698-superliner-front-end-bounce/#findComment-544290 Share on other sites More sharing options...
JoeH Posted December 30, 2023 Share Posted December 30, 2023 I think you need the king pin forward 6 inches or so. Wiser heads will steer you straight. If your 5th wheel is too far back you can do this... https://youtu.be/udE01fKbWCk?si=PhfTDYy6-Snp3YSc 1 Quote Link to comment https://www.bigmacktrucks.com/topic/73698-superliner-front-end-bounce/#findComment-544293 Share on other sites More sharing options...
mrsmackpaul Posted December 30, 2023 Share Posted December 30, 2023 It doesn't have to be very far forward of the center, maybe only a inch or so It depends on the local laws a weight limits on axles Should be a full load on the steer when you have full weight on the drives This will vary on wheel base and laws etc You should be able to get it pretty much spot on first go Weigh just the steer axle no load Weigh just the drives no load Get some old style kitchen scales and a piece of say 2 x 1 wood long enough to measure your wheel base at scale you choose, maybe 1 inch equals 1 foot Then two kitchen scales, something like this Maybe use a 1 - 200 scale as in 1 pound on the scales equals 200 pounds in the real world One scale is drive axle center the other is steer axle Put the 2 x 1 on top with scaled wheel base to match the measurements on the stick and center these marks in the middle of each scale Load these up to match the tare weight From the gross weight your allowed to carry you subtract the tare Scale this weight to the same ratio as tare weight Then place this on the stick and move it along until the weight is in the ratio from steer to drive axles This is your king pin location This was how things were done before computers and trucks run at maximum weight By doing this you can't over load the steer but always have plenty of weight to make the truck handle properly Paul Quote Link to comment https://www.bigmacktrucks.com/topic/73698-superliner-front-end-bounce/#findComment-544295 Share on other sites More sharing options...
70mackMB Posted December 30, 2023 Share Posted December 30, 2023 8 hours ago, JoeH said: I think you need the king pin forward 6 inches or so. Wiser heads will steer you straight. If your 5th wheel is too far back you can do this... https://youtu.be/udE01fKbWCk?si=PhfTDYy6-Snp3YSc l just saw that video the first of the week! LMAO! That poor puppy dog. l missed where the 5th wheel was. l thought it was because of all of that RAW power under the hood! drip drip drip lol .....Hippy Quote Link to comment https://www.bigmacktrucks.com/topic/73698-superliner-front-end-bounce/#findComment-544305 Share on other sites More sharing options...
mechohaulic Posted December 30, 2023 Share Posted December 30, 2023 would think it has a slider 5th wheel, easy check would be move wheel one way or other. post was put up on Thursday with unit going to mechanic Friday (yesterday); hope this has a follow up. noticed on many post placed on BMT forums NOT MANY update on results . 1 Quote Link to comment https://www.bigmacktrucks.com/topic/73698-superliner-front-end-bounce/#findComment-544317 Share on other sites More sharing options...
mechohaulic Posted December 30, 2023 Share Posted December 30, 2023 = what was used to tow lowbed before Superliner showed up?? if there was another tractor. how do "two" units compare for specs- wheel base/ 5th wheel settings . first trip with super/lowbed- does that mean new to truck or new truck trailer combo ?? Quote Link to comment https://www.bigmacktrucks.com/topic/73698-superliner-front-end-bounce/#findComment-544319 Share on other sites More sharing options...
mrsmackpaul Posted December 30, 2023 Share Posted December 30, 2023 7 hours ago, mechohaulic said: would think it has a slider 5th wheel, easy check would be move wheel one way or other. post was put up on Thursday with unit going to mechanic Friday (yesterday); hope this has a follow up. noticed on many post placed on BMT forums NOT MANY update on results . I have never seen a 5th wheel that slides in Australia I have me a feeling they are not allowed out here for what ever reason Swishman will know more than me about this stuff But yes if the 5th wheel can be slid then forget my long winded drawn out procedure and put the scales back in the kitchen for the cook Paul Quote Link to comment https://www.bigmacktrucks.com/topic/73698-superliner-front-end-bounce/#findComment-544344 Share on other sites More sharing options...
mechohaulic Posted December 30, 2023 Share Posted December 30, 2023 1 hour ago, mrsmackpaul said: I have never seen a 5th wheel that slides in Australia I have me a feeling they are not allowed out here for what ever reason Swishman will know more than me about this stuff But yes if the 5th wheel can be slid then forget my long winded drawn out procedure and put the scales back in the kitchen for the cook Paul NOT a long winded procedure but free knowledgeable advice from reputative source. have ALWAYS had respect for the Australian culture and it's people.so close on the globe yet so different in operations. Quote Link to comment https://www.bigmacktrucks.com/topic/73698-superliner-front-end-bounce/#findComment-544346 Share on other sites More sharing options...
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