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Re: Best steering/suspension/brakes for 71 B body? [Re: AndyF] #2565142
10/16/18 11:31 PM
10/16/18 11:31 PM
Joined: Sep 2006
Posts: 1,161
Los Angeles, CA
JF_Moparts Offline OP
super stock
JF_Moparts  Offline OP
super stock

Joined: Sep 2006
Posts: 1,161
Los Angeles, CA
Originally Posted By AndyF
255/45-17 is a decent size tire. It is way bigger than anyone had back in the 70's but way smaller than what BMW or Porsche engineers would put on a car your size these days. So you just have to decide how "modern" you want to be.

A 17 inch rim allows you to run a 13 inch brake rotor which should be big enough to stop that car. Do you know how much the car weighs and what the weight distribution is? If you haven't had the car scaled then that should be a high priority. You need to know what you're working with before you start working.

I assume the car is at least 4000 lbs and probably has 52 or 53% on the front tires. So there is a lot of weight up there which means the tires are getting tortured. A 255/45-17 tire can handle around 1500 lbs of load but that doesn't mean it is going to handle great when it is fully loaded.

Since you have a running combination you should be able to crawl underneath it with a tape measure and figure out if you can put a bigger tire on it. If you can great, if not then you know what your limitation is. If you are limited to a 255 section width then consider a bigger wheel. Perhaps a 255/40-18 or a 255/35-19 would work better for you. A 19 inch rim would allow a 15 inch brake rotor which is what the Porsche guys would use these days on a 4000 lb vehicle with 500+ horsepower.

Go spend some time on the BMW website and look at the tire sizes and brake rotor sizes they are putting on their 4000 lb cars. Or visit your local Dodge dealer and look at what the Dodge engineers put on the Hellcat Charger. That is a big car and they put big wheels, big tires and big brakes on it. Once you have the correct size wheels, tires and brakes then you can fine tune the spring rate and shock valving.

You'll also need to stiffen up the chassis a bunch. Ma Mopar made these cars as limp as a noodle back in the day. XV is out of business but you might be able to dig up some of the stuff they published on chassis stiffness. They were the only business to ever take the subject seriously but the market must not have been big enough to keep them in business. Basically you'll need to add some crossmembers and a monte carlo bar.


Awesome response, thank you!

I do need to get that car weighed. I've always assumed it weighed around 3500 lbs, based on nothing more than old Car & Driver articles and the fact that I have aluminum heads, intake, and lighter front seats!

When I first moved to SoCal I assumed there'd be a huge drag / SCCA scene here, but as far as I can tell there's not too much due to the high cost of real estate gobbling up places where one might try to race. I think I can weigh the car at Irwindale.

I do wish XV was back in business. Before I had any budget, all I was able to afford from them was their seat belts, which I still use and which were well designed.

I also looked at the Corvette C5/C6 suspension site posted above, but you need to replace your entire rear frame to make that work. I want to keep the car somewhat original, so that's a no-go.

Re: Best steering/suspension/brakes for 71 B body? [Re: JF_Moparts] #2565181
10/17/18 01:19 AM
10/17/18 01:19 AM
Joined: Nov 2016
Posts: 200
Colorado
B
Bob J Offline
enthusiast
Bob J  Offline
enthusiast
B

Joined: Nov 2016
Posts: 200
Colorado
Look at US Car Tool Or XV Racing and Firm Feel

This is a must read K Frame weldup

My dash is out so I welded up the cowl pinch welds and the steering column Brace. The cowl use to flex pushing on the steering column brace before not so much now. The spot welds holds the car together but lets it flex a lot. Weld up as much you can. Tie the sub frame connectors to the set mounts. A lot of this is low cost with big returns and most will not show. It just takes time to prep and weld.
Good luck as I like the 71 Plymouths as I had a 72 RR/GTX and a 71 RR.

Last edited by Bob J; 10/17/18 01:21 AM.

07 Porsche C2 997.1 6spd DD and SCCA Solo II SS
68 Plymouth GTX Convert. 493cu Hotkiss Wilwood Classic air, Building for Solo II Cam
65 Chrysler 300 Convert. project
57 Desoto 2dr. Firesweep 392 Hemi,Wilwood Project with son
2003 Dodge Dakota R/T "DEAD"
Re: Best steering/suspension/brakes for 71 B body? [Re: JF_Moparts] #2565370
10/17/18 02:30 PM
10/17/18 02:30 PM
Joined: Jan 2003
Posts: 30,945
Oregon
A
AndyF Offline
I Win
AndyF  Offline
I Win
A

Joined: Jan 2003
Posts: 30,945
Oregon
Did you read my book? The book might be the first place to start. I think it answers a lot of your questions.

https://www.amazon.com/Mopar-B-Body-Performance-Upgrades-1962-1979/dp/1613252501

Re: Best steering/suspension/brakes for 71 B body? [Re: AndyF] #2565439
10/17/18 04:38 PM
10/17/18 04:38 PM
Joined: Sep 2006
Posts: 1,161
Los Angeles, CA
JF_Moparts Offline OP
super stock
JF_Moparts  Offline OP
super stock

Joined: Sep 2006
Posts: 1,161
Los Angeles, CA
Originally Posted By AndyF
Did you read my book? The book might be the first place to start. I think it answers a lot of your questions.

https://www.amazon.com/Mopar-B-Body-Performance-Upgrades-1962-1979/dp/1613252501


Nope, I didn't know about the book. It's now in my Amazon cart.

To date, most of the info I've used for these susp mods has been Ehrenberg's tech articles in Mopar Action and 'Super Street Mopar' by Mike Martin.

Re: Best steering/suspension/brakes for 71 B body? [Re: JF_Moparts] #2566564
10/19/18 04:55 PM
10/19/18 04:55 PM
Joined: Jan 2010
Posts: 1,440
NW Chicago suburban area
Mopar Mitch Offline
pro stock
Mopar Mitch  Offline
pro stock

Joined: Jan 2010
Posts: 1,440
NW Chicago suburban area
JF -- You should be able to safely run 275-40-17 tires front and rear on 9"rims, such as the YearOne 17x9 Mopar rally rims.. as they have the correct offset to fit our cars... without any major concerns. And seek some 200 TW tires for that size.. choices are minimal from the different brands... 18s are where its at today for best selections.

Keep in mind that these shorter sidewall and diameter tires do change the ride comfort.. and watch out for potholes as the rims can more easily get damaged.

If you're not up to getting larger TBs, such as FF's 1.18 (and you wouldn't regret it... only wondering why you didn't get them sooner!), then get double-adjust shocks.. QA1 or Vikings.. or similar... double-adjustables.

Skip the exotic rear suspension that your thinking about... Hotchkis or FF rear leafs would suite your needs just fine.

Consider a larger rear sway bar beyond the typical .75"... look for or have made at least .875 or 1.0"... because your car is front-end heavy.

Have you re-positioned the battery into the trunk yet?... = about 1% weight transfer.


Mopar Mitch "Road racers and autocrossers go in deeper and come out harder!"... and rain never stops us from having fun with our cars... in fact, it makes us better drivers! Check out MOPAR ACTION MAGAZINE, August 2006 issue for feature article and specs on my autocross T/A!
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