macattack Posted November 25, 2011 Share Posted November 25, 2011 hey.ok ive been battling this shimmy problem i have on my mr688s for close to 2 years. starting to drive me nutso. i dont operate it very much once twice per month. here is a brief description of my problem.at about 35-40 mph i get a shimmy/wobble in my truck. above and below that its fine.i replaced the following items.rear bushingsinsulator pads all.tie rod endswheel bearings.king pinsflex bar on front between spring hangers.replaced and retorqued hanger bolts.retorqued frame bolts.balanced my front rears.all u joints.few other misc items i cant recall now.still have the shimmy!!!.my question is should i have my fronts balanced 425 65r 22.5? i was told if my steering wheel does not shake its not in the front. but the mr688s cabover truck front tires are basically behind the seat. so is it possible the tire shop could be wrong. the fronts are on aluminum rims, i dont see any weights on the front rims at all.i had my front rears balanced with beads. tire shop told me it may take up to 100 miles before the bead bag breaks and balancing begins. is this correct?also i noticed that the front timbren type rubbers on the front springs are in contact with the trucks frame without weight. i dont think this is correct. maybe my problem is the previous owner installed oversized rubbers causing the wobble through out the truck.any ideas what else i should check? sorry for rambling on and on. Quote Link to comment Share on other sites More sharing options...
macattack Posted November 26, 2011 Author Share Posted November 26, 2011 my steering wheel is nice and steady. drove nearly 70 miles today, the rear balance didnt seem to help. feels like someone is pushing on the cab from left to right. one of these days i will find him. lol Quote Link to comment Share on other sites More sharing options...
MACKS Posted November 26, 2011 Share Posted November 26, 2011 I would suspect the front tires.. Quote Link to comment Share on other sites More sharing options...
macattack Posted November 26, 2011 Author Share Posted November 26, 2011 thanks got to get them balanced. my fronts are brand new 425 65r 22.5 on alum rims. i know the prev owner had to install longer studs and spacers for the front rims. truck previously had some narrow front tires.thanks Quote Link to comment Share on other sites More sharing options...
Mack Doctor Jr. Posted November 27, 2011 Share Posted November 27, 2011 power steering pump may be bad. I had the same problem on an R model. It would only get bad if i crossed a set of RR tracks kinda fast.It ended up being the pump.Some of the old style power steering pumps had internal filters on them, u may need to check that out. Im not sure how new your truck is or if it is the same style im thinking of. Quote Link to comment Share on other sites More sharing options...
macattack Posted November 27, 2011 Author Share Posted November 27, 2011 hi.thanks never thought of the pump. its a 96 mr 688s. my avatar is a pic of it. Quote Link to comment Share on other sites More sharing options...
Rob Posted November 27, 2011 Share Posted November 27, 2011 power steering pump may be bad. I had the same problem on an R model. It would only get bad if i crossed a set of RR tracks kinda fast.It ended up being the pump.Some of the old style power steering pumps had internal filters on them, u may need to check that out. Im not sure how new your truck is or if it is the same style im thinking of.This kinda interests me. A power steering pump is a positive displacement pump meaning as the rotor turns moving the internal vanes, or gears, a certain amount of fluid is displaced through the balance of the hydraulic system as flow. Unless your pump was worn internally allowing internal bypass of the fluid, (cavitation, and increased heating as a byproduct) it is difficult to understand how the pump would be at fault. This would be easy to see as increased effort needed to turn the steering mechanisim, (due to low system pressure) or the system fighting the driver attributed to worn parts.I don't know if the steering gear has been checked for proper preload, or over center operation in this application, but I've seen a lot of steering parts replaced as servicing technicians either did not troubleshoot correctly, or owners didn't want to pay for the service. On most all accounts, someone is not happy with the turnout. Many people, (including technicians) do not either understand the need for proper mesh of the steering gear, (recirculating ball type) or the wear that occurs internal to the steering gear, and it's effects. I only bring this up because I have seen several steering gears just plain worn out yet it is overlooked and bad front end parts are blamed. Again someone is usually dissapointed and feels taken advantage of, or not treated fairly.If the steering gear has not been checked/adjusted and over center torque measured, I would start there. Yearly changes of power steering fluid, (no matter what you prefer) along with the filter will provide many more years of trouble free service than a unit that never sees anything but a top off when low.Hope I didn't bust anybody's balls cause didn't mean to.Rob Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment Share on other sites More sharing options...
shortstack Posted November 27, 2011 Share Posted November 27, 2011 I agree with Rob but my first thought is the mention of the previous owner changing the front wheels/tires. Too many times I've seen problems overlooked because it was assumed everything was done right. It could also be that the different parts aren't working together or another problem (possibly a worn steering gear like Rob said) is aggrevated by the load the bigger tires put on the entire setup. Just a thought. Trey Quote Link to comment Share on other sites More sharing options...
macattack Posted November 27, 2011 Author Share Posted November 27, 2011 what should i look at as far as the tires being changed to 425 65r 22.5. i know the previous owner stated he had to adj so the wheels the tires would not rub when turned.thanks for the info.the truck seems to steer great from one extreme to the other. Quote Link to comment Share on other sites More sharing options...
Rob Posted November 27, 2011 Share Posted November 27, 2011 what should i look at as far as the tires being changed to 425 65r 22.5. i know the previous owner stated he had to adj so the wheels the tires would not rub when turned.thanks for the info.the truck seems to steer great from one extreme to the other.Sounds like he adjusted the steering stops which would have nothing to do with the problem you are experiencing. These are just bolts that thread into the spindle forging limiting how far the wheels will actually turn, or cut.Rob Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment Share on other sites More sharing options...
macattack Posted November 27, 2011 Author Share Posted November 27, 2011 is there anything he could of screwed up on when changing to the front floats? Quote Link to comment Share on other sites More sharing options...
Rob Posted November 27, 2011 Share Posted November 27, 2011 I would really hesitate to say anybody "screwed up" at all. I think something is probably not working together quite correctly. Could be something as simple as wheel bearings in the hubs slightly worn, rim offset improper for the truck given the tire size, loose parts as mentioned earlier, etc. It also could be runout of a part slightly bent, or a belt slightly out of whack in a tire. If you could get a different set of tire and temporarily install on the steer axle to evaluate any change in the problem(s).Rob Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment Share on other sites More sharing options...
macattack Posted November 27, 2011 Author Share Posted November 27, 2011 how can i tell if its the proper offset? Quote Link to comment Share on other sites More sharing options...
Rob Posted November 27, 2011 Share Posted November 27, 2011 how can i tell if its the proper offset?Tire dealer should be able to tell you offset from the rim number stamped into it. Research through a Mack dealer if your hub/axle combination will support the offset installed. The more offset the rim has, the more loading of the wheel bearing in the direction of the offset. The installation of float type tire changes the geometry of the front end including the steering angles at times. You may have something fighting design parameters with the installation of non standard tires for the wheel hub combination.Rob 1 Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment Share on other sites More sharing options...
macattack Posted November 27, 2011 Author Share Posted November 27, 2011 thanks rob. i will check into this. Quote Link to comment Share on other sites More sharing options...
Mack Doctor Jr. Posted November 27, 2011 Share Posted November 27, 2011 Im not saying what I did makes sense for sure. I just know what I did and it fixed it. The truck had the pump with the res. mounted to it. ( u know the style clear back by the fire wall where u couldnt even add oil to). I put the same style (used) back on it and it did the same thing. So we broke down and put a Vickers pump on with a frame mounted res. and it fixed the problem. I dont know why but it did the trick. Maybe u can put a flow meter in line to check things out? Quote Link to comment Share on other sites More sharing options...
macattack Posted November 27, 2011 Author Share Posted November 27, 2011 thanks i will check that out also. Quote Link to comment Share on other sites More sharing options...
Rob Posted November 27, 2011 Share Posted November 27, 2011 Im not saying what I did makes sense for sure. I just know what I did and it fixed it. The truck had the pump with the res. mounted to it. ( u know the style clear back by the fire wall where u couldnt even add oil to). I put the same style (used) back on it and it did the same thing. So we broke down and put a Vickers pump on with a frame mounted res. and it fixed the problem. I dont know why but it did the trick.Maybe u can put a flow meter in line to check things out?Keep in mind how a hydraulic power steering system works and what it takes to make it work.Pressure equates to how easy it is to turn the wheels.Flow equates to how fast you can turn the wheels.Pressure gauges and flow meters are part of a thorough power steering system checkout. You use these to check operating parameters of the system, ensure relief valve settings are correct and that the system is not destroying itself internally from binding, (as in the case of altering factory settings). Over center torque settings is a critical setting due to the steering gear wear greater in this area. If the over center torque is not, or unable to be set appropriately, the steering mechanism will never be able to work correctly. This is set by the depth of mesh of the worm and sector shafts internal to the steering gear and measured at the steering wheel hub with the pitman arm removed from the steering gear. It is measured in inch pounds and should be up to three times more resistive in the center position than off center. As mentioned, if over center torque cannot be acheived, the system cannot work correctly and a plausible by product is erratic operation.This whole subject seems vaguely familiar somehow.......Rob Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment Share on other sites More sharing options...
macattack Posted November 27, 2011 Author Share Posted November 27, 2011 thanks for all the responses. i am going to tackle this problem this week. i will post what i find. Quote Link to comment Share on other sites More sharing options...
mackcl96 Posted November 27, 2011 Share Posted November 27, 2011 thanks got to get them balanced. my fronts are brand new 425 65r 22.5 on alum rims. i know the prev owner had to install longer studs and spacers for the front rims. truck previously had some narrow front tires.thanksWhat the previous owner did with spacers and studs is not correct and MUST be removed. even if it is not causing the shimmy..Your wheel is not siting on the hub bosses.Stud and hub mount front float rims... use the bosses for stress relief Quote Thanks for hearing me out.You can have the soap box now---------JIM Link to comment Share on other sites More sharing options...
Rob Posted November 27, 2011 Share Posted November 27, 2011 What the previous owner did with spacers and studs is not correct and MUST be removed. even if it is not causing the shimmy..Your wheel is not siting on the hub bosses.Stud and hub mount front float rims... use the bosses for stress reliefI never read that part of the prior post, or forgot. Definately needs to be corrected before further t/s can take place.Rob Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment Share on other sites More sharing options...
macattack Posted November 28, 2011 Author Share Posted November 28, 2011 sorry for misleading you about the spacers and longer studs. that post never came out sounding like it should. but actually it has the proper offsets installed. i was not sure what they were called so i just stated spacers. i did have the fronts off for the brake work and kingpins. mechanic said that its all fine. gonna throw some beads in my front floats and see what happens.sorry for the bad post. Quote Link to comment Share on other sites More sharing options...
redington Posted November 28, 2011 Share Posted November 28, 2011 I am not an expert on this subject but I have found that if the tire technician did not use adequate lube the tire may not set properly on the rim. You can look at the ring on the bead and see if it is even all the way around. When the tire is new it should have a dot to place next to the valve stem so that minimum to no weight are needed, Often when placed properly the wheel and tire are balanced with no weights. If the truck had steel wheels as new and later aluminum wheels are used the studs are often too short. Aluminum wheels are thicker than steel and have to be replaced. If the tire is correctly mounted, the bearings properly torqued to spec and torqued properly, front end alighned to spec, and shocks in good condition, you can try a set of Centramatic wheel balancers. They will keep the assembly properly balanced throught all speeds over 25 MPH. I had a problem on my R Model and the wheel balancers took out all of the mid speed vibration. Good luck! Quote Link to comment Share on other sites More sharing options...
macattack Posted December 3, 2011 Author Share Posted December 3, 2011 I would suspect the front tires..did the front balance on my floats and fixed my problem. had beads added and was told it will take a few miles for the bag to seperate and spill the beads. but for now its definately helped my problem. thanks everybody. Quote Link to comment Share on other sites More sharing options...
mackcl96 Posted December 4, 2011 Share Posted December 4, 2011 sorry for misleading you about the spacers and longer studs. that post never came out sounding like it should. but actually it has the proper offsets installed. i was not sure what they were called so i just stated spacers. i did have the fronts off for the brake work and kingpins. mechanic said that its all fine. gonna throw some beads in my front floats and see what happens.sorry for the bad post.I'm curious what these (proper offsets) you have that are between the rim and drum ,,,,,,,,little concerned on your safety and the people aroundyou ........Could we see pictures Quote Thanks for hearing me out.You can have the soap box now---------JIM Link to comment Share on other sites More sharing options...
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