clubber

I'm hoping that I can get some help in trying to fix an FB rear suspension from this section. I have a Maxton Rollerskate that has stock FB rear suspension but has some capability for a "fix". I have about 6 inches in front of the front mounting points for the trailing arms that I can use to make longer one's as necessary. I can also move the arms up or down by a few inches if necessary.
I've done some research and what I've read is the upper links are supposed to be about 70% the length of the lowers. However, the stock links are about 13 and 9 inches long so, that should be about right from what I have read. Obviously the website I read this on is wrong or there is some other factor that I'm not figureing in. Has anyone got info on how to set this up?

mustanghammer

iTrader: (3)

The answer is yes....and no. It depends on your application, the springs that you select and how you drive. Given the option to choose I went with a three link design on my racecar. There are 4 links that work but they are a radical departure from the Mazda design.

Here is a link to an extensive thread on the subject.

https://www.rx7club.com/1st-generation-specific-1979-1985-18/fb-rear-suspension-geometry-problems-options-solutions-876479/

nofords

iTrader: (9)

3 link all the way.

A good link with rod ends, not poly bushings goes a long way to being more stable and smooth operation.

clubber

Yea, I read that thread and that's where I got the idea to fix the rear sus. I tried to buy a tri link from G force but he's retiring and doesn't have anymore of the kits. In that thread, it says that a 4 link can be made better than a three link for putting down power but there isn't a how to on 4 links.
I've been searching for a while but I haven't found any kind of a tutorial on how to set it up. I found one page that said that the upper links were supposed to be 70% the length of the lowers but since the length's are 13 inches for the uppers and 9 for the lowers, that means Mazda did it right and there shouldn't be the binding problem. My car has a panhard bar so the watts link problems are already gone. Since I have the room for changing the mounts on my car, I was hoping to get some idea of what was needed. My fall back solution if I can't find how to setup a 4 link is to go to a 3 link and I've got more room to do that than a normal FB.

mustanghammer

iTrader: (3)

Couple of things - the Tri-Link that G Force sells was designed to fit an RX7 chassis and conform to a rule set that did not allow for the arm to enter the car's interior. As a result it was a compromise solution and the geometry was not that good. Certainly better than the OE 4 link and great on a stock HP car but not on a racecar with 200+ HP.

If you still want one - call Charlie Clark at www.kcraceware.com. He can fab one for you.

A better solution - if you have the realestate to work with - is a straight third link setup with the upper arm located in the correct plane.

mustanghammer

iTrader: (3)

I would recommend picking up the book "How To Make Your Car Handle" by Fred Puhn. It is an old book but the information is timeless.

Regarding 4 link rear suspensions, I have seen them installed on allot of fast SCCA club racing cars. Typically the upper and lower links are pretty close to the same length and they are as long as possible. The E Production RX7's in this area use a "4 Link" rear suspension with an upper link that is pretty close to the same length as the lower arm. Charlie Clark at KCRaceware designed it.

Back in the 90's Charlie attempted to make the OE 4 link work on his ITA RX7 by changing the bushings over to spherical bearings in all of the arms. The result were a series of lower control arm failures. Ask him about it.

j9fd3s

iTrader: (3)

in the factory suspension its not the length that is wrong, per say, its that the upper arms point IN, so when the car rolls, the arms actually try to pull out of the body (and sometimes do, if its rusty).

the factory solution, outlined in the competition book (www.foxed.ca) used the stock arm locations in the body, and different pickup points on the axle, so they could run longer upper links. the competition lower links are actually slightly shorter (5mm) rod ended and adjustable. the uppers are rubber bushed like stock.

the factory also lowered the roll center by moving the watts link pivot way down, but a panhard does the same.

the KC raceware solution is much simpler.

so its pretty clear on a 4 link, longer and straighter upper arms are the way to go.

clubber

I've got the book "How to make your car handle" that I bought new when it first came out (yea, I'm old). It doesn't cover stuff like link design but more explains how one works. You guys have provided some good info and I'm thinking about calling KC racewear to see if he has any ideas or hardware to help me out. Thanks for the help so far. One last question, Any ideas on how the arms should be tilted up or down ? I think my make your car handle book has that info but any experience by you guys will only help the process. It is kinda fun to actually try to get info on engineering this stuff. Thanks again, I already feel I have much more info than a couple of days ago.

peejay

Q. Can the stock 4 link be fixed?

A. Yes:



(Ignore the gear-oil spewage)

Barring that, if you have to keep it 4 link, you could try using a set of lowers as the upper links. Make the upper links parallel, and the attachment points *slightly* closer together at the front than the back (vertically wise)

j9fd3s

iTrader: (3)

this is what the hatchi guys do. they also have a bracket to lower the pickup point for the rear of the lower arm.

mattp

iTrader: (3)

Have you looked into getting a copy of Race Car Vehicle Dynamics by Milliken & Milliken? It's a good source.