1972 MC 350/Auto - Suspension Upgrade

[B-Man]

Starting this thread to document my upcoming suspension upgrade on my 1972 Monte Carlo.  I will post some pics this weekend, but verbally / textually here is what I'm starting with and looking for:

HANDLING: Looking for tighter handling for everyday - sort of aggressive - driving.  Although I live in New England, the roads are actually pretty nice where I live.  At some point I would like to put my MC on a track, but I dont think I need to go to adjustable shocks / struts, etc.  Unless there is a 'might as well' sort of thing, and they are easy to adjust and set up.

STANCE: My MC has what I would consider a high, stock stance.  And I like it!!!  While I normally seek to lower my cars a bit for that aggressive stance, I'm really liking the way my MC sits.  And I dont want to mess with it too much if I dont have to in order to achieve my goals

RIDE: I dont want the think to shatter when I go over a speed bump!  99% of my driving is nice, flat, paved roads with an occasional dirt or stone road.

 

REAR: Right now there is an old pair of working air shocks in the rear.  Everything else looks original and stock

> What sort of high-end rear shocks has everyone converged on?  Hotchkis, QA1, Bilstein?

> How about some 'solid' trailing arms instead of the stock c-channel sort of trailing arms?

> Rubber or polyurethane bushings?  Any preferred brand?

 

FRONT: Right now there are some rusty old, standard-looking shocks.  The springs have new-ish black paint on them, so I'm assuming they were replaced during the previous restoration.  The shocks may actually be a good brand / model, just old and tired

> What sort of high-end front shocks has everyone converged on?  Hotchkis, QA1, Bilstein?

> Are adjustable fronts really worth the upgrade?  Any hands-on use and experience out there?

> 'Solid' A-arms and control arms ... worth it for regular driving, or are those upgrades strictly for definite track usage?

> Rubber or polyurethane bushings?  Any preferred brand?

 

TIRES: 235/60-15 all around.  And I like the stance and fit.  I could see going a little bigger in back, but it would just  be for aesthetic.

> Any opinions on the right size for everyday, aggressive driving and maybe some track time?

> What brand is generally preferred here on the forum?

 

STEERING: I've read some threads on changing the gear ratio on the steering box.  Is it worth it?

[Blackhawk]

Bilstein is a good choice for non-adjustable monotube shocks, and I would think that they would be a good middle ground between soft stock replacement shocks and adjustable monotubes. Adjustable shocks would be great if you were looking to really fine-tune the handling.

For the rear lower control arms you can buy a boxing kit to prepare the arms for mounting sway bars, or you can buy the upgraded arms. There's no need to buy adjustable lower arms.

The rear suspension is made to have some flex, so I would recommend rubber bushings to avoid binding. You could install roto-joints too, I don't think any brand is better than the others for that.

Aftermarket upper front control arms are great for fixing the suspension. The stock suspension has negative caster, which you can make positive with a stack of shims or by changing the control arm. This would give more of a modern car handling. If you go down this route make sure that the arms actually move the ball joint for added caster, the cheap white-box arms rarely do. UMI, some BMR, QA1, Ridetech, or the big names will typically have this added and advertised. A few of the lower control arms adjust the caster as well.

Going to a taller upper ball joint can improve handling as well by fixing the camber curve caused by the short stock spindle.

235/60-15 is a little short compared to the stock sizes of 215/70-15 and 215/75-15. A common size is 255/60-15. I don't think there's much of a difference between the BF Goodrich Radial TA, Cooper Cobra, or the Mastercraft Avenger GT. I'm not aware of any higher-performing tires in a 15" wheel, and the sidewall is going to add quite a bit of compliance to handling.

Going to a Jeep Grand Cherokee or other aftermarket quick-ratio steering box gives a more aggressive feel compared to the 'one-finger' stock steering. In the past people would also use 80's GM steering boxes, but those won't allow the wheels to turn as far.

[cbolt]

I can only speak to the shocks and rear control arms/sway bar because those are the only components I have installed that deviate from stock. I was lucky enough to drive my car on the Bristol Motor Speedway about 10 years ago and immediately decided to switch to Bilstein shocks on all 4 corners and boxed rear control arms with a sway bar. Just those upgrades made a world of difference just in body roll alone. I think I went with PMI for the rear set up, which comes in a kit with poly bushings installed.

 

I also swear by the BFG T/A radials (225/70-15) but folks will complain about them as the lettering tends to turn brown over time. Hey, from the drivers seat they look great, that's all I know. If you are driving next to me and can read the lettering then your head is spinning so fast that you likely have other problems beside my brown-ish lettering on my tires, right?

 

Jokes aside, most folks here have switched to the Cooper Cobra's with no handling loss vs. the BFG's. 

[Dtret]

WOW. That’s a lot of information you’re looking for. I’m a stock guy so I can’t help. 

[B-Man]

@Blackhawk and @cbolt , WOW, thanks so much for the detailed and informative responses!!  I will definitely use that info to make some purchases over the next week or so, and I will report back what i get.

Bilstein all around is where I was "leaning" (haha).  And while I love the idea of adjustable I'm not convinced I would ever really take advantage of it, and would probably end up leaving them in some ill-suited adjustment. 

For the rear, I will check out the PMI setup and some other options.  I lean towards rubber instead of poly, so would go that route first.  But boxed control arms with a sway bar - noted.

For the front, the camber and caster commentary from @Blackhawk will need some more thought and research by me.  I think I understand and can visualize the geometry of your statement, but I will have to sketch it out to understand what's going on.  But I believe you, no doubt.

For tires, ok thanks for the data points.  As I stated, most of my every day driving is a little aggressive admittedly.  So keeping the sidewall height down, without driving on 'rubber bands' is going to be the key there.   I will probably end up with the BF Goodrich Radial TA or Cooper Cobra.  The final size will be a little dependent on what I end up doing with the suspension I would think.

For the steering box ... aftermarket Jeep Cherokee seemed to be the way to go from my reading.  I will go that route.  I will have to do something with the power steering pump and lines I think - the steering 'sticks' every now and then which is scary (not related to steering box I dont believe).

@Dtret , Yeah I normally keep everything as close to stock as possible on my restorations.  My 72 MC has had a previous restoration and a lot of hands in the cookie jar.  So I'm working with what I have, and keeping as much stock as I can.

Thanks again guys.  Awesome info

[jft69z]

Glad you were able to get a lot of detailed information to help with your project!

Have you considered becoming a dues-paying member of the club, which would be a good way of helping to keep this place running? The added benefit, noticing that you're from CT, you could maybe hook up with another member we have from there, and possibly caravan down to the Carlisle Mini-meet with him in a few months and hang out with us in person!

It'll be the best $25 you'll ever spend, and the hotel room discount from being a member covers that small cost many times over.

Thank you,

Joe T

[B-Man]

@jft69z , yeah I sent Rob a check and have been emailing with him.  I should be all set up soon.  I had a heck of a time logging in all year and finally just emailed him - should have done that right away.  From now on you will probably see WAY MORE participation from me than you'll have ever hoped for, haha.

As far as Carlisle goes, yeah I will make it out there at some point.  A good, local friend - and car guy - is from nearby in PA and is always driving back and forth.  We've already planned to make it a 2-bird-1-stone situation

[jft69z]

That's great Brendan, thank you!

I know Rob's out and about right now, but I'm sure he'll see this and double check the status of your check. After that you will be good to go.

Keep in mind, you DON'T need to bring a car to come to the Carlisle meet, just come anyways and hang out. A bunch of us have done that, and I can guarantee that nobody made us feel anything less than welcome. Our other member I mentioned earlier also attends the Syracuse mini-meet, so keep that on your radar as well, that's another great time (and closer than Carlisle too)..

Thanks again

Joe T

[B-Man]

REAR SUSPENSION - I just found the YouTube video below showing how to 'box in' lower control arms and set them up for sway bars. Cool video! But no way am I set up to make time for that right now. I will be buying my way out of this eventually, haha!

And as I go down the path of new shocks, boxed in lower control arms and a sway bar in the rear it's quickly obvious that some sort of packaged kit for the entire rear, and probably front too, is in fact the better / easy way to go. Just pricey $$$$. And I don't want my Monte to be up on jack stands all summer either. The new 3-bay garage is functional but taking way too long to finish up. I can't wait for that either.

So ... for now I'm just going to get a set of Bilstein monotube, non-adjustable shocks all around. Since all of my bushings look to be in good shape, thanks to the previous restoration, there is no doubt that will make enough of a difference for what I'm currently going for.

Specifically, here is the plan

REAR & FRONT

NEW BILSTEIN FRONT & REAR SHOCKS FOR 68-72 GM A-BODY, GAS SHOCK ABSORBERS, 1968 1969 1970 1971 1972 CHEVY II CHEVELLE CONCOURS EL CAMINO MALIBU NOVA OLDSMOBILE OLDS CUTLASS F85 PONTIAC GTO LEMANS https://a.co/d/eqUmpU5

It should be interesting to see what improvement comes out of just replacing the shocks

[Rob Peters]

On 3/24/2023 at 12:50 PM, B-Man said:

@jft69z , yeah I sent Rob a check and have been emailing with him.  I should be all set up soon.  I had a heck of a time logging in all year and finally just emailed him - should have done that right away. 

 

Brendan,

I checked the club mailbox this morning and your check has not yet arrived.  I think it was around the 19th or 20th that you told me you had arranged an electronic check to be sent for your club dues.  You may want to check to make sure whoever was supposed to send that to me has sent it because I am not sure it would take that long to arrive in the club PO Box.

rob

[Montemedic]

Good gas shocks such as Bilstein, front and rear sway bars make a big difference. Do you have a rear sway bar on your Monte?  It’s an easy bolt on part.

[Montemedic]

You said your car was restored prior to purchasing it. How old are your body bushings, or have they ever been replaced? 

[B-Man]

@Rob Peters - all good buddy.  I think I told you the bank said the check should arrive "by the 27th".  It will get there when it gets there.  Again, all good.  Just happy you were able to get me back and logged in here so that I can participate!

@Montemedic thanks for the encouragement on the Bilsteins.  They seem like the common and correct choice for now, no doubt.  My MC has a front sway bar (covered in oil and muck but the bushings look passable), but no rear.  I didnt look closely at my rear lower control arms and I'm not sure if they have the mounting holes for the sway bar.  They are definitely not "boxed".  You're right I should probably just buy a sway bar for now if the lower control arms have the mounting holes.  I can always upgrade to boxed control arms later and keep the sway bar.   Hmmm, yeah ... definitely ordering a sway bar too!

@Montemedic I mentioned its an older restoration, and it went through Maple Motors at some point (not sure if thats good or badl; they seem legit).  In general all of the bushings look to be in really great shape.  They must have been all gone through at some point and replaced with good rubber - not polyurethane I dont believe.  So I will leave them as much as I can for now.  Again, a complete suspension kit is probably in my future at some point.  Unless one is actually failed or looks old I will hold off on replacing any.

[B-Man]

Follow up, I just ordered this sway bar from Summit, along with a new starter (different thread)

https://www.summitracing.com/parts/opg-rp01702

 

Edited March 25, 2023 by B-Man
misspelled word

[B-Man]

And ordered this set of Bilsteins off Amazon

https://www.amazon.com/BILSTEIN-ABSORBERS-CHEVELLE-CONCOURS-OLDSMOBILE/dp/B01BMXJB58/ref=sr_1_1?crid=9MJPJQMSL74&keywords=bilstein+shocks+68-72+gm+a-body&qid=1679782630&sprefix=bilstein+shocks+68-72+gm+a-body%2Caps%2C115&sr=8-1

[B-Man]

@Blackhawk, I stand corrected ... my current tires are 235/70-15 ... not/60. The size certainly looks reasonable and they don't rub.

Looking at the sticky'd thread on wheel and tire database, the Google Doc link doesn't work anymore. So I'm trying to gather data to make my next tire purchase.

The current rally wheels are in great shape and I like them (can't attach pics yet, ugh). They are 15x8 and I measure a 4.25 in backspace.

Question: Do you think I'm good to go ahead and order up some 255/60-15 BF Goodrich Radial TAs?

 

[MCfan]

Brenden,

Like you, I'm a Mechanical Engineer so I dig this mechanical fit stuff.  🙂

I used the Wheel Offset Calculator at tiresize.com to compare your proposed set up of 255/60-15 tires on 15x8" wheels with 4.25" backspace to your existing setup of 235/70-15 tires on the same wheels. Here's what I found:

First, you can use the table several pages down in the Wheel Offset Calculator to determine the offset for your wheels which is -6mm.  This is confirmed by subtracting the backspace of any 8" wide rim at zero offset which is (8+1)/2 = 4.5" from your wheel's measured backspace of 4.25".  So, 4.25 " - 4.5" = -.25' which converts to -6mm offset.

On the front: Since the 255/60-15 tire is 10" wide and 27" tall  (versus 9.3" wide and 28" tall for the 235/70-15 tires) your inside and outside sidewall clearances will be .35" less. I think you can probably get away with that if nothing is rubbing now.

On the rear: The reduction of your existing inside and outside clearances will be the same (.35"). HOWEVER, the sidewall clearance with your wheel well lip will only be around .44".  I have confirmed this by comparing your proposed setup (using the Wheel Offset Calculator) to my existing rear setup where I have measured my sidewall clearance at .75". 

I suggest you carefully measure the sidewall clearance to your rear wheel well lip of your current setup at the narrowest point and decide if you can afford to lose another .35" of clearance there.  Don't forget to allow for some body shift during cornering or hard acceleration/braking.

Wider tires on the rear of any first gen Monte really need to be mounted on wheels with about +12mm offset (5" BS on an 8" wide wheel) to move the combo inboard (away from the quarter panel) where there is more room.  Wheels with -6mm offset will actually move your combo outboard about .75" more than is ideal. I think your proposed combo will probably fit on the rear, also, but you'll need to decide if the small outside clearance is livable.   Good luck.

[Michael Boyte]

Very informative piece of information there .

Thanks for posting this !

[B-Man]

@MCfan, holy cow ... wow, that is some really great fact finding!!!  Thank you so much for taking the time to do that, and to type it all out here.  Many others will benefit from that exercise down the line.  I will check out the tire size calculator - I've used that one before, very cool - and will check your numbers.  I will also get the current wheels / tires back on and drop the jack stands so that I can check the current clearance against what you've calculated.

Note that I'm pretty sure the rally wheels are 8in wide.  Hard to measure directly with the current tires on there of course.  I will eyeball it again or come up with a better indirect measurement method, but I highly doubt they are 7in wide.  That will make a difference in the calculations of course.

Thanks again.  Super cool of you to do all that!

[Blackhawk]

Whether stock of aftermarket, the wheels should have either a code or the offset stamped on the barrel that would allow identification. I believe 15x8" stock rally wheels (for a Corvette) has a backspace of 4". Remember that an 8" wheel will measure about 9" if you go from lip to lip.

 

I'm running 255/50-17 on 17x8 wheels with a 4.5 backspace in the back, and it's tight but not terrible. You will want to make sure that your body is centered on the frame by locating the alignment holes on the driver's side frame, they're a hole with a slot through it.

[MCfan]

3 hours ago, B-Man said:

Note that I'm pretty sure the rally wheels are 8in wide.  Hard to measure directly with the current tires on there of course.  I will eyeball it again or come up with a better indirect measurement method, but I highly doubt they are 7in wide.  That will make a difference in the calculations of course.

Brenden, you are absolutely correct. I wasn't sure which rally wheels you currently have/use.  If they are stock first gen Monte rallys (RPO ZJ7), they are 15x7 with +0.30" (+7.65 mm) offset.  However, if not stock Monte rallys, they might well be stock Corvette rallys which are usually 15x8 with 4.25" backspace (-.25" or -6 mm offset).  Of course, they could also be any of the custom made rallys (I have both 15x7 and 15x8 rallys from WheelVintiques, both with custom offsets of +12mm).

Note that when discussing and comparing wheel/tire combos it is imperative to only use OFFSET measurements, NOT backspace!  All wheel manufacturers specify offset measurements/dimensions because offset is completely independent of wheel/rim width!  Offset is measured from the centerline of the wheel, regardless of width, to the mounting surface of the wheel on the hub. If that surface is outboard of the centerline, the offset is positive (+); if inboard of the centerline, it is negative (-). Offset measurements are always expressed in millimeters (mm) to avoid confusion with backspace measurements which are always expressed in inches (in).

Note also that most car enthusiasts discuss and compare wheels by referring to their BACKSPACE measurement.  However, that can be dangerous as backspace for any given wheel is totally dependent on wheel/rim width.  Backspace is measured from the mounting surface of the wheel to the inside rim lip/edge.  Therefore, you can only compare wheels by their backspace measurement if they have the same rim width.  The backspace measurement that corresponds to a zero (0) offset for any wheel always = (rim width +1)/2.

For example: A 7" wide wheel with a 4" backspace has zero (0) offset (mounting surface aligns with the centerline of the wheel) while an 8" wide wheel with a 4" backspace has a -12mm offset (mounting surface is .5" inboard of the centerline) which pushes the wheel tire combo .5" further outboard. Study the table given a couple screens down in the Wheel Offset Calculator tool to see the relationship between Offset and Backspace for any Rim Width. The relationship is Offset = Backspace - (Rim Width+1)/2 

You can easily measure the backspace of any rim by laying it face down, then laying a straightedge across the inside rim lip and measuring down to the mounting surface, however, you MUST also know the rim width to know its actual offset measurement (see equation above).  Rim width is easy to measure on a naked rim but more difficult to determine if you have a tire mounted.

[jft69z]

21 minutes ago, MCfan said:

 stock Corvette rallys which are usually 15x8 with 4.25" backspace

You sure that's not 4", Dennis?

The Corvette 'AZ' 15x8 wheels on the back of my Camaro measure 4" backspace.

The 'FW' 15x7 wheels in front have a 4.25 BS.

[MCfan]

Joe,

Thanks for your input.  When I was considering going to 60 series tires (255 front, 275 rear) several years ago, I considered a set of four used Corvette rallys that were 15x8 with 4.25 bs but rejected them due to their -6mm offset.  I don't doubt that there are other Corvette (or Corvette-style) rallys (maybe from different years) with different backspaces.  The rear rims on your Camaro are even more severe with -12 mm offset so the Camaro and Monte rear wheel wells must be somewhat different dimensionally due to their different body styles. First Gen Montes like around +12 mm offset in the rear, especially with wider tires.

Your front 15x7 rims with 4.25 bs (+ 6 mm offset) are almost ideal for a first gen Monte as the factory rallys are very close at +8 mm offset).

I think the Wheel Offset Calculator at tiresize.com is a fantastic tool to do both visual and quantitative comparisons of wheel/tire combinations but you have to input accurate offset measurements to use it.  That's why the included conversion table available on that tool is so useful.  But to get the correct offset (in millimeters) you have to first select the correct rim width and backspace measurements in inches.  Having actual fitment/clearance measurements of an existing wheel/tire combo with known specifications as a base to compare any other proposed combination is the ultimate decision aid and I think that is where Brenden is headed.

[jft69z]

10 hours ago, MCfan said:

Joe,

Thanks for your input.  When I was considering going to 60 series tires (255 front, 275 rear) several years ago, I considered a set of four used Corvette rallys that were 15x8 with 4.25 bs but rejected them due to their -6mm offset.  I don't doubt that there are other Corvette (or Corvette-style) rallys (maybe from different years) with different backspaces.  The rear rims on your Camaro are even more severe with -12 mm offset so the Camaro and Monte rear wheel wells must be somewhat different dimensionally due to their different body styles. First Gen Montes like around +12 mm offset in the rear, especially with wider tires.

Your front 15x7 rims with 4.25 bs (+ 6 mm offset) are almost ideal for a first gen Monte as the factory rallys are very close at +8 mm offset).

I think the Wheel Offset Calculator at tiresize.com is a fantastic tool to do both visual and quantitative comparisons of wheel/tire combinations but you have to input accurate offset measurements to use it.  That's why the included conversion table available on that tool is so useful.  But to get the correct offset (in millimeters) you have to first select the correct rim width and backspace measurements in inches.  Having actual fitment/clearance measurements of an existing wheel/tire combo with known specifications as a base to compare any other proposed combination is the ultimate decision aid and I think that is where Brenden is headed.

 I did measure my wheels at the time, that's when I found that chart I posted a while back. That chart only lists one BS option for 15x8 rims (not that the list is a comprehensive, end of all measurements bible  .) I searched/googled again today for the Corvette wheel sizes, and it seems most said 4" BS, but then a couple others mention 4.25 (some were aftermarket wheels). Most sources said there was only one Corvette 15x8 wheel, and it had 4" BS though.

If anyone has anything different than a 15x8 'AZ' stamped Corvette rim, please post it here & I'll update the list.

https://www.firstgenmc.com/forums/index.php?/topic/26766-gm-rally-wheel-id/#comment-247088

 

[B-Man]

@Blackhawk @MCfan and @jft69z, you guys are a HUGE help!  Thanks so much for all this, and no doubt its a repeat of stuff already on this forum.  But I appreciate you regurgitating it  so that I can figure out just the right tires and not waste $800!

@Blackhawk, you've absolutely pointed out an error in my above stated rim width measurement of 15 x 8.  I was eyeballing lip to lip, sort of through the tire since the current tires are all still mounted.  I eyeballed 8in lip to lip which means I probably have 15 x 7.  Once I get the tires dismounted I will take some actual measurements for sure.  And what do you mean by "the barrel" so that I can look for the code and compare it against the list that @jft69z posted and linked above?  Do you have an example photo of where that code is found?

@MCfan stated to Joe that "Your front 15x7 rims with 4.25 bs (+ 6 mm offset) are almost ideal for a first gen Monte as the factory rallys are very close at +8 mm offset)."  I 'm nearly 100% convinced that I have these "almost ideal" wheels on my MC :).  I'm sure I measured the backspacing correctly at 4.25in.

And in other news, my new rear sway bar came in!!   As well as my new high-torque starter (other thread).  I'm still waiting for the lower control arm boxing inserts and my new set of Bilstein shocks to come in.  Note that my plan at the moment is to 'loosely' install the boxing inserts into my lower control arms, and use the tube / standoffs that came with the sway bar.  In the next couple of weeks I will be over at my buddy's house who has a welder and a shop, so we should be able to weld the inserts with the lower control arms installed (fingers crossed) - it's just a mild hack until I upgrade my suspension completely with billet or properly boxed lower control arms

Thanks again everyone.  Super helpful community here.  Hopefully @Rob Peters cashes my check soon and updates my account so I can be even more active!!!