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Old 04-23-2009, 07:35 AM
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Front and rear suspensions working together

I see lots of threads on putting a front subframe under an older vehicle. Also replacing a straight axle with IRS from a Jag, T-bird, Vette, etc.

Made me wonder - does any consideration need to be made how the front and rear suspensions work together when you swap in one or the other from another vehicle?

Or do you treat the front and rear as essentially independent from each other?

Or is there just an assumption that the factory engineers took care of all that and we don't need to worry about it?

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Old 04-23-2009, 07:50 AM's Avatar Moderator
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Good observation. You are 100% correct in your intuition, those swaps do usually mess up the Ackerman steering a little. Turning centers of the front wheels are supposed to meet in the center of the rear end. Changing the wheel base or track width of the components ruins that relationship. Saving grace is it rarely causes any harm other than possibly slightly faster tire wear but doesn't affect driving feel or safety.
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Old 04-23-2009, 08:04 AM
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Thanks Willys.

Doesn't help me all that much yet, but it gives me a direction to Google.
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Old 04-23-2009, 08:27 AM
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Essentially there is no real correlation between the front and rear suspension so they can be handled as two separate entities. Like Willies said, not all front suspensions are interchangeable between different cars. The front suspension geometry should be set up for a specific wheel base to maintain correct Ackerman. The reason that manufacturers have gone to a fully independent suspension front and rear is because of the light weight cars ride much smoother with less unsprung weight. The weight of a heavy front or rear axle beating on the road feeds back into a light car with a noticeably rougher ride where as most of the weight of an independent suspension is on springs with the rest of the car, Called sprung weight. The main thing to consider when choosing suspension components is track width and the correct attachment of the components. There is a lot of geometry involved in all suspension components that should not be taken for granted. The manufacturers spend millions of dollars designing this stuff and even they have compromised with some of their stuff. Be very careful of some of the aftermarket stuff. Some of those units are dangerous and they use us as a testing ground.
I hope this clears things up for you a little bit.
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Old 04-23-2009, 10:43 AM
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I would recommend copies of Tune To Win and Engineer to win by Carroll Smith and read everything our own Billy Shope has written to get more information on chassis setup. there is even analytical software available for reasonable that will help you with chassis setup. there are lots of factors to consider when setting up a chassis as ackerman is just one factor to consider..relative roll stiffness front to rear affects oversteer/understeer of the car..spring rates and shock valving are things to consider if you want a good ride and decent handling..then there is turning characteristics to look at..Does the car turn in like we want in a corner. Does it launch and run straight when you get on it..

Waay more than can be handled in one short post but perhaps if one studies all of this one can become more knowledgeable in chassis..I mean no offense to anyone but I have found that the typical hotrodder is happy if the springs hold the car up as he works on engine power..but then one can have all the HP he can get and it is not all that much help if he cannot get it one the ground and handle it..

I have tried most all of it and now do what is known to work..
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Old 04-23-2009, 10:46 AM
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I agree with everything so far. Basically, they can be treated as completely separate entities, or tuned together. You can always slap a mustang front end and a Jag rear end on a car and drive it without really tuning it. This is what most hotrodders do, and as long as they aren't really terrible they drive it like that and put up with the quirks as long as the car is cool. Like this Model A that I helped a buddy build uses the front axle from a 31 Lincoln, the rear axle from a 48 plymouth, the steering box from a 64 Olds, a steering column from a 50s buick, and a completely custom fabbed steering linkage. You shift the auto tranny by grabbing the shifter on the side of the tranny through a hole in the floor. If it doesn't handle well or ride nice, who cares? It looks really cool

Most guys slap stuff together the looks cool, or that the magazines say is cool, but how they work together is up to the end user. Putting on the right spring rate and shock damping as well as how they set up the geometry is different for every car if you want it to be ideal.

So in answer to your question, I would say that attention SHOULD be given to how they work together and what components will do the best for your application. But, once they're in they will all provide a certain camber/caster curve that can be fine-tuned with adjustments. Its more the tuning that determines safe/optimum suspension geometry than the actual part selection.
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Old 04-23-2009, 11:05 AM
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Thanks for all the replies.

I do have some suspension books. Unfortunately, they're "up in the attic somewhere". I don't recall too much info about the relationship between front and back, but I'll have to give it some more reading once I dig them up.

I guess I'm curious about the more narrow case of mixing two OEM independent suspensions, under the assumption that they are kept stock. Those who are putting a front IRS subframe from a vehicle that already had a live rear axle under another vehicle that has a rear axle are probably in safe territory, but I wonder about putting, for example, a Jag or T-bird or corvette IRS onto a car with an existing IFS that was designed to work with a live axle rear.

Seems like there should be some interactions to consider, especially since it would be done (presumably) to improve handling - it's a lot of work just for looks. But it's a bit difficult to picture all the interactions in my head compared to how just the front, or just the rear work. It never seems to be discussed when these swaps are talked about, so I was curious if it was just not an issue, or if it's just overlooked.
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Old 04-23-2009, 07:33 PM
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Check out Bob Bolles in circle track.The front and rear suspensions need to work together he go's way in depth about moment center, roll centers and getting both ends of the car to work together to get the best handling.The 2 must work together as they are joined by a rigid frame and what affects one end also affects the other.Just for example if you car is handling well now and you put on a rear sway bar or increase the spring rate in the rear it would then tend to oversteer .But putting a stiffer bar or springs in the front will tend to understeer.All this is very dependent on roll centers as different roll centers also change these tendencies.The lower the roll center the more the car wants to roll and must be countered by stiffer springs and bars.Higher takes less spring rate to control body roll so a softer spring can be used for a better ride but handling will suffer.
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Old 04-23-2009, 09:11 PM
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I agree with barnym17 insofar as their most definitely is a relationship between the front and rear suspensions. With that being said, that doesn't mean that you can't install a Jag IFS and a 9" Ford with Hotchkiss suspension and make it work together very well. The key is knowing (or to think that you know) what the goals are for the car, and what the characteristics are for the suspension you are thinking of installing. For example, if you want a canyon carver suspension and install a C-6 Corvette front end and then install ladder bars with a track locater on the solid rear axle you will likely be sorely disappointed when you dive into that first corner.

There are certain basic design considerations that I take into account when laying out a new suspension project for a front engine rwd car.

Roll center height-Have the rear roll center higher than the front
Roll stiffness-Front roll stiffness higher than the rear (to promote under-steer at the limit)
Spring selection-Having the rear wheel rate different (typically lower) than the front. Notice I said wheel rate, not spring rate.

Etc. etc...

This is stated very basically and there are exceptions for every one of those rules. The point is, be realistic about the use of the car, do your homework regarding suspension and handling, or just find someone who has been there to get advice from (and drive for yourself one of their finished projects to help solidify your trust in them), and get to cuttin and weldin!

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