Can you Run FAT tire on an Old Mopar with Success???

[graybo]

I have a nut and bolt restored Superbird Clone that I put 8" wide 60 Series Bf Goodrich tire on all four corners. She was very hard to drive with factory Mopar gear box and cheap Monroe shocks.    I just installed Bilstein shocks from PST and they made a huge difference.  I put an extra firm power steering box from STEER &* GEAR which really did make the steering stiffer / more firm feel.  But it still pulls the car left or right on poor roads (crowned roads).  On the concrete highways it drives like a dream.  Anyone else trying to run really fat tires on an old Mopar and having this road feel?  Should I just put a 70 series tire on it?  Please share your thoughts..  ONLY IF YOU ARE RUNNING FATTIES ON THE FRONT WITH SUCCESS.   

[skip68]

Beautiful car graybo.   
I'll let someone else answer as I've only ran 60 series on mine.  Sounds like an alignment issue to me.   

[myk]

What you're describing is bumpsteer; no amount of tire width will eliminate your symptoms.  What you need to do is increase your positive caster settings in your alignment.  With stock suspension you will be limited in the amount of positive caster you can dial in, so you will almost always have the problem you describe; old Mopars are just built this way.  If you want to go further in reducing bumpsteer let us know...

[rt green]

make SURE your center link is level. from pitman to idler arm.

[c00nhunterjoe]

I ran 245/60s on the front of mine on an 8.5" wheel. It drove great. No issues. Thats 26.5 tall and 9.6 wide. Stocl steering and suspension parts. Only change was lowering power steering line pressure

[Kern Dog]

I run a 275-40-18 on the front. The car handles fantastic. Bump steer? Huh?? Not here!

[cdr]

post some pictures of your steering linkage under the car.

[graybo]

What you're describing is bumpsteer; no amount of tire width will eliminate your symptoms.  What you need to do is increase your positive caster settings in your alignment.  With stock suspension you will be limited in the amount of positive caster you can dial in, so you will almost always have the problem you describe; old Mopars are just built this way.  If you want to go further in reducing bumpsteer let us know...

Okay guys.  UPDATE.  The new bilsteins really firmed it up.  And a Steer and gear box made it A LOT more firm steering.  Drives great on most roads...but it still wants to pull the car left or right on grooved roads (caused by a lot of truck traffic)  The dip or crown in the road pulls the front tire.  I don't know how to explain it better.   And yes I am interested in hearing more about the caster setting BUMP STEER please.  Thanks folks.  My front tires are 255 60 15 on 8 inch wheel with 4.5 inch Backspace.  They look great on the car so I don't want to change.  HELP

[graybo]

post some pictures of your steering linkage under the car.

CDR... I will post pics later today.  What are you looking for in the steering?   I got all stock a arms, lower control arms, torsion bars, etc...  All stock with New PST front end kit(rubber not poly).  Thanks  I hope you can help me.  This is driving me nuts. 

[XH29N0G]

Running 245/60/15 on 7" similar behavior to a small degree, recently rebuilt stock set up.  Will also be interested in hearing more about how to address this and like this thread.

[HPP]

What are your alignment specs and tire pressures and how old is your restoration? Since you have a metric size called out, I assume you have radials too. Is that correct?

What you are describing is called tramlining. Some of this is inherent with wider tires having more contact with the road surface so they generate more directional force. They will tend to find the path of least resistance which leads to them following ruts in the road. It can be highlighted or minimized by changes alignment, tire pressure, age of components, and tire construction.

Bump steer is what occurs as the tires alter their desired path based in suspension compression or extension. Driving over a railroad crossing or through an intersection and feeling the car pull one direction or the other as you go over irregularities is an example of bump steer.

[graybo]

HPP  YOU HIT THE NAIL ON THE HEAD!!!  TRAMLINING?  To answer your question this restoration is BRAND new.  I just finished the car this July 2015 and I'm out running the HELL out of it to find my weak spots.  Every Bushing, Nut , bolt, spring, shock, is brand new.  I had my car alligned at a very reputable place for old mopars.  These people really know allignment for old cars and have never had an issue with them.  Since you seem to know the exact problem...Can you help with the exact fix?  Thanks.  Graybo

[graybo]

HPP.   Yes I do have radials.  Bf Goodrich Radial T/A.  I do not know the allignment specs.  That is a little above my pay grade. 

[RallyeMike]

As HPP said, what you are describing is not bump steer. Still, one part of the solution as suggested can be to have the alignment shop maximize your caster. I'd have a second shop look at it and set it. They can confirm the first shop's alignment. It's best to find an old-school alignment shop - don't take it to local chain tire store.

Another part of this might be expectations? A 45 year old Mopar even with 100% new everything and radials will still not drive like a modern car. It will still have a tendency to follow the grooves, fall off the road crown, and wander around compared anything even semi-new.

I'm running 245/60/15s on front of 2 of my Chargers, and 255/50/16 on another.

[HPP]

If all your components are brand new, then that is a good thing. It eliminates worn parts as a variable. Radials also are more tolerant of alignment variables than the original bias play tires are. The cord construction and rounded shoulders of a radial will normally resist tramming better than bias ply will. However, to utilize the effectiveness of that construction, you have to alter the alignment specifications.

If you had it aligned at a reputable shop, that is typically a good thing. If you can, try to find the spec sheet they used or call and ask them if they have any records of what they did. Many shops use the stock specs for restorations. This is good if you stick with bias plys, but  if you have radials, you really should make sure they did their best to maximize the settings to the radials capability. This is why knowing what you have is important. A radial will tolerate a much wider range of adjustment than a bias ply, so you can alter the stock settings to improve the steering response and feel of the car. In stock form, most classic mopars have negative caster, positive camber, and some toe in. With a radial car you should almost have the opposite - positive caster, zero to negative camber and some slight toe in. You want positive caster because of is naturally self centering, improves the return to center action of the steering system, and increases stability at highway speeds. All issues you are describing.This is the big gotcha with a classic mopar, its hard to get positive caster  in them.  The bushing cams used to set the alignment in the upper control arms have a very limited range of adjustment and you may only get 1-3* positive. You can aid this by using an offset bushing and you may get an additional couple of degrees out of it, but really the easiest way to get adjustment is with new upper control arms so you can get up towards 5* positive.

Continuing with the alignment specs, you want slight negative camber in your static settings. Slight being -.5 to -1* This keeps the tire vertical when you turn and the body rolls. This is especially important if you have the stock, softer torsion bar or smaller anti-roll bar. If you are aggressively hitting corners, you may want to be higher up on the negative scale such as -1 or more.

For toe, you want about no more than 0.125" total toe in with both sides. Again if you are aggressive, you may want to reduce this to .0625" total. Toe allows the friction of the road to push the tires in to a neutral position while moving forward. The less toe you have, the more "darty" the suspension will feel, but the more stable it will feel upon corner turn in. .125" total is a reasonable compromise to friction and turn in for most drivers. Some shops may report this in degrees. Just ask them to translate it for you.

For tire pressures, you may consider dropping it a couple of pounds to give the sidewall some additional give to absorb the tramming. You might consider using a chalk or crayon line on the tread face to determine pressure for optimum tread contact. This isn't always the highest recommended pressure on the sidewall. Make a mark across the tread, drive around the block and see if it is all scrubbed off. Adjust up/down until you scrub it all off.

Ultimately, as other have said, there will always be some slop in the system that you can't totally eliminate, and  a worm gear steering box will always have some amount of play, especially in the center of its travel, but you can significantly improve things.

[graybo]

If all your components are brand new, then that is a good thing. It eliminates worn parts as a variable. Radials also are more tolerant of alignment variables than the original bias play tires are. The cord construction and rounded shoulders of a radial will normally resist tramming better than bias ply will. However, to utilize the effectiveness of that construction, you have to alter the alignment specifications.

If you had it aligned at a reputable shop, that is typically a good thing. If you can, try to find the spec sheet they used or call and ask them if they have any records of what they did. Many shops use the stock specs for restorations. This is good if you stick with bias plys, but  if you have radials, you really should make sure they did their best to maximize the settings to the radials capability. This is why knowing what you have is important. A radial will tolerate a much wider range of adjustment than a bias ply, so you can alter the stock settings to improve the steering response and feel of the car. In stock form, most classic mopars have negative caster, positive camber, and some toe in. With a radial car you should almost have the opposite - positive caster, zero to negative camber and some slight toe in. You want positive caster because of is naturally self centering, improves the return to center action of the steering system, and increases stability at highway speeds. All issues you are describing.This is the big gotcha with a classic mopar, its hard to get positive caster  in them.  The bushing cams used to set the alignment in the upper control arms have a very limited range of adjustment and you may only get 1-3* positive. You can aid this by using an offset bushing and you may get an additional couple of degrees out of it, but really the easiest way to get adjustment is with new upper control arms so you can get up towards 5* positive.

HPP very informative.  Thanks A LOT. 

Continuing with the alignment specs, you want slight negative camber in your static settings. Slight being -.5 to -1* This keeps the tire vertical when you turn and the body rolls. This is especially important if you have the stock, softer torsion bar or smaller anti-roll bar. If you are aggressively hitting corners, you may want to be higher up on the negative scale such as -1 or more.

For toe, you want about no more than 0.125" total toe in with both sides. Again if you are aggressive, you may want to reduce this to .0625" total. Toe allows the friction of the road to push the tires in to a neutral position while moving forward. The less toe you have, the more "darty" the suspension will feel, but the more stable it will feel upon corner turn in. .125" total is a reasonable compromise to friction and turn in for most drivers. Some shops may report this in degrees. Just ask them to translate it for you.

For tire pressures, you may consider dropping it a couple of pounds to give the sidewall some additional give to absorb the tramming. You might consider using a chalk or crayon line on the tread face to determine pressure for optimum tread contact. This isn't always the highest recommended pressure on the sidewall. Make a mark across the tread, drive around the block and see if it is all scrubbed off. Adjust up/down until you scrub it all off.

Ultimately, as other have said, there will always be some slop in the system that you can't totally eliminate, and  a worm gear steering box will always have some amount of play, especially in the center of its travel, but you can significantly improve things.

[Kern Dog]

Yes.

[XH29N0G]

nice car

[Paul G]

I am running 255/60-15 tires on the front of my 72. On a deeply grooved road it will do as you describe. Sort of dart back in the groove if it gets out of it. But that only happens on the same section of a really bad road.

I have the caster maxed out with some noticeable negative camber. No alignment shop for me. I can screw it up several times before it gets right, and it costs me less that way.

Tire pressure is lower than what you think. Tire sidewall says 35 psi max, thats not a normal cold tire pressure. I found that about 28 psi cold all around works well. 255/60-15 on 7" wheel fronts, and 275/60-15 on 8" wheel rears.      

[A383Wing]

I'm running 60 series tires on my cars also, no issues...I did have a problem with the Daytona one time...it followed every rut in the road...turns out the toe was off....fixed that and it's fine now