home made k frame
I've designed a few K-members, here's one that I used to produce that allowed putting SBC/LS engines where a rotary used to be in the '93-up Mazda RX-7. Made mostly from 14ga sheet, lighter than the factory unit but plenty durable for autocross and road racing. Pretty easy to make a jig for as the stock subframe had great suspension geometry, just needed a redesign to make room for a rear sump pan and lower V8 crankshaft location...
There's a lot to be considered when fab'ing up a K-member from scratch. If you want to install a rack where one didn't exist, you need to find a rack with an effective width that works with your existing control arm geometry.
Back in the mid '80's when I designed my first Dirt Latemodel chassis, I basically designed the front frame around the steering rack that I planned to use. I chose a Ford Fairmont power rack, as it was cheap and easy to find in junkyards at the time. I started by drawing a front view mock-up of the components on poster board, which allowed me to work out control arm locations/lengths that worked with the Fairmont rack width and spindles I planned to use. The goal was to minimize bump steer and give me a front roll center that worked. The downside to using a Fairmont rack was 3 turns lock/lock, but I fixed that by making a home-made quick steer box out of a T-350 planetary. Effectively converted that 3 turn rack into a 1 turn rack. I had more time than money, using the junkyard rack just made sense.
With all the front suspension mounting locations worked out, I made a simple jig that located the steering rack in relation to the upper/lower control arm locations. Then built the rest of the car around that jig.
There's also spindles to consider. If you are converting from rear steer to front steer, simply swapping the spindles side/side will have a huge effect on your Ackerman geometry. Ackerman geometry is designed mostly into the spindle arm and controls how much the toe changes as the steering angle changes. Swapping the spindles side/side will have the effect of converting a rear steer car's stock Ackerman geometry to anti-Ackerman geometry....
...Textbook Ackerman generally keeps all 4 tires aligned 90* to a common radius center as the front tires are steered. Basically the inside front tire needs to turn a tighter arc than the outside front tire. For a daily driver on the street, textbook Ackerman will minimize tire wear. It will also make the car easiest to push by hand regardless of steering angle.
...Zero Ackerman is where both front tires steer in the same direction without regard to how tight the steering arc might be. I've seen lots of tube frame drag cars with no Ackerman.
...Anti-Ackerman is something we use with the dirt cars as it helps keep the car from spinning out under power. With the tail hung out thru the turns, anti-Ackerman keeps both front tires pointed in the direction that the car is traveling. Generally makes a car handle better when it gets out of shape under power, but hard to push by hand around corners in the pits.
With driving Outlaw Dirt Latemodels in my background, I don't mind some anti-Ackerman in my street/strip rides :)
Grant
I've designed a few K-members, here's one that I used to produce that allowed putting SBC/LS engines where a rotary used to be in the '93-up Mazda RX-7. Made mostly from 14ga sheet, lighter than the factory unit but plenty durable for autocross and road racing. Pretty easy to make a jig for as the stock subframe had great suspension geometry, just needed a redesign to make room for a rear sump pan and lower V8 crankshaft location...
There's a lot to be considered when fab'ing up a K-member from scratch. If you want to install a rack where one didn't exist, you need to find a rack with an effective width that works with your existing control arm geometry.
Back in the mid '80's when I designed my first Dirt Latemodel chassis, I basically designed the front frame around the steering rack that I planned to use. I chose a Ford Fairmont power rack, as it was cheap and easy to find in junkyards at the time. I started by drawing a front view mock-up of the components on poster board, which allowed me to work out control arm locations/lengths that worked with the Fairmont rack width and spindles I planned to use. The goal was to minimize bump steer and give me a front roll center that worked. The downside to using a Fairmont rack was 3 turns lock/lock, but I fixed that by making a home-made quick steer box out of a T-350 planetary. Effectively converted that 3 turn rack into a 1 turn rack. I had more time than money, using the junkyard rack just made sense.
With all the front suspension mounting locations worked out, I made a simple jig that located the steering rack in relation to the upper/lower control arm locations. Then built the rest of the car around that jig.
There's also spindles to consider. If you are converting from rear steer to front steer, simply swapping the spindles side/side will have a huge effect on your Ackerman geometry. Ackerman geometry is designed mostly into the spindle arm and controls how much the toe changes as the steering angle changes. Swapping the spindles side/side will have the effect of converting a rear steer car's stock Ackerman geometry to anti-Ackerman geometry....
...Textbook Ackerman generally keeps all 4 tires aligned 90* to a common radius center as the front tires are steered. Basically the inside front tire needs to turn a tighter arc than the outside front tire. For a daily driver on the street, textbook Ackerman will minimize tire wear. It will also make the car easiest to push by hand regardless of steering angle.
...Zero Ackerman is where both front tires steer in the same direction without regard to how tight the steering arc might be. I've seen lots of tube frame drag cars with no Ackerman.
...Anti-Ackerman is something we use with the dirt cars as it helps keep the car from spinning out under power. With the tail hung out thru the turns, anti-Ackerman keeps both front tires pointed in the direction that the car is traveling. Generally makes a car handle better when it gets out of shape under power, but hard to push by hand around corners in the pits.
With driving Outlaw Dirt Latemodels in my background, I don't mind some anti-Ackerman in my street/strip rides :)
Grant