Red-Rx7

Howdy all.

I wanted to open up a conversation about the bumpsteer received in the FD3S cars with the Pettit/Alamo subframes used.

As many of you know, these subframes allow the 20b to be installed without modifying the firewall. Yet, you have to adjust the steering system to make it all fit.

Some people (usually people who own them) say it takes a tad to get use too, but its "over concerned", and there is no real bumpsteer.

Others (usually people who never has driven one) notes that the bump steer is horrid and completly makes the car not even feel like an RX-7.

Now, those with experience, please give me your thoughts. Also, Why cant the steering system be moved around to eliminate these woes?

Thanks,

Mike

peejay

The problem is that when you move the rack down, you also need to move the steering arm on the spindle down as well, so that you don't get a situation where suspension compression gives you toe-in (and droop gives you toe-out) because the tie rod is trying to describe a different arc than the A-arms.

Even then, you're still mucking things up because if you move the rack higher or lower, the inner mounting points need to be adjusted inboard or outboard so the tie rod is the proper length for the rack's height. You *can* compensate the length by bending the steering arm on the knuckle, but then you are also altering the Ackerman, which may or may not be desired.

But let's go back to the original issue: toe in on compression, toe out on droop. Imagine you're turning in to a corner. The car leans and the suspension motion makes you turn in even more. Okay, you can learn to deal with the non-linearness of that. What happens if you hit a bump... OH **** happens - car turns in harder as you hit the bump. This can result in understeer, oversteer, or maybe just an uneasy sense of skittishness.

Red-Rx7

Peejay..

Ok, excellent response on the bumpsteer... But what is the most logical way to fix this... assuming you have no choice (Ie, don't say don't touch it).

Mike

j9fd3s

iTrader: (3)

well you said it yourself. move the motor back, no more bumpsteer problem. or you maybe could split the difference, move the rack a smaller amount. think about it though the reason the fd is so hard is because they didn't want it in there.
is there a factory 20b (13g) fc? yes
is the 20b in prodution before the fd? yes

mike

peejay

ok, in a nutshell, without playing with one, my guess is lower the rack, and have the spindles bent (VERY precise work) down and outward (might not clear the brakes) which helps bumpsteer but hurts Ackerman, so move the rack forward to help Ackerman.

Or, attack the trans tunnel with a sledgehammer and a 6-pack of your favorite beer and mount the engine back instead of mucking about trying to fix bumpsteer

94RHDFD

iTrader: (3)

I want to lay rest the bumpsteer problem........there really isn't a noticeable difference under 140mph, over that the car does get alittle wigglely, but what street car doesn't at that speed. You can either kill your power steering or deal with it, but how often are you going to be above 140mph anyway.

94RHDFD

iTrader: (3)

BTW, I use the Pettit subframe

Red-Rx7

Interesting! Thanks for the response.

So, you moved the knuckle up? And there is no big mystery on the "BUMP STEER" as everyone makes it out to be?

I would love to talk to you more about your setup.

Thanks,

Mike

twint78

u can take it to a shop that deal with steering and suspension and they will be able to work out the "bump steer" for u.

bermuda20b

I also use the pettit subframe and spindles. I race the car in the 180-190 mph range in Nevada and do not have problems. The car was a bit wiggley (as described above) so I removed the power steering. The slow down in response was all that was needed.

t-von

iTrader: (8)

Bringing this one way back.

With all the talk about lowering the rack and creating bumpsteer, what about all the Fd's that have aftermarket suspension upgrades? Think about this for a sec. Tein S tech springs will lowering the body 1.5" in the front. With the body lowered 1.5", the rack will move down 1.5" however, the stock spindle arm will remain at the same height. So my question is, would a lowered Fd experience bumpsteer as well?

GT1-20b

Simple solution,
Cut 3" of the LIM.
Make a custom sec. Fuel rail.
Use a Turbo II Water pump set up.
Make a custom oil pan.
Engine will fit nice without touching the rack, subframe, or fire wall
Will also be able to use the OEM Strut bar accross the top of the engine.
And if I may, removing the PS. belt is just a "bandaide" to an existing problem.
When at speed, if the feeling is comfidant, then why remove the belt ?

RotorMotor

iTrader: (5)

im not liking what im hearing..... i thought that pettit "solved" the bumpsteer problem with the 2nd version of the subframe/spindles???? i hope ******* so, because thats teh one in a box in my garage. i think we need to hear from someone like david hayes who just recently had pettit do his car... -heath

Howard Coleman

iTrader: (6)

Mike,

Thanks for revisiting an important subject re the 20b fd.

The amount of bumpsteer can vary from almost nothing to huge as in Dick Lambert’s case. That may be why we are seeing varying opinions as to whether it is an important item..

I posted the following re Dick’s situation in the “ just scaled a 20b” thread….

“Dick’s car runs either a Pettit subframe (or something similar) w extended steering arms off the upright. it was built, i understand, by a mazda engineer in florida. dick has owned it for a year and a quarter. i did drive it to the Rotary Revolution (you are all going this april 21st, right? dick's 3 rotor will be there as well as my twin To4 2 rotor)

so i have about 700 miles in the driver’s seat.. (T66/ fresh Pettit motor/haltec e11)

as is, the car drives poorly. as soon as you turn the wheel. it has toe in bumpsteer. i did a bumpsteer on it and it specs out like it drives...

at one inch of bump the toe in is .249!
2 inches .288
3 inches .531!!!!!!!!!!!!!!!!!!!!!

here's what happening:

with toe in on bump, the outside wheel turns in response to the steering wheel. then the car rolls and the front outside wheels goes into bump starts turning more, which creates more roll and more turn in. yikes!

in order to keep the car from continuing this spiral a counter turn with the steering wheel is needed to correct every turn.

having raced and designed and built SCCA tube frame (mazda) racecars for 22 years i can not tell you how bad this dynamic is for the car and driver. if it was my car i would not have driven it around the block before fixing it.

bumpsteer should be toe out and be at most .01 to .02 per inch of suspension travel. this car has half an inch toe in!!!!!

i do not know if it is Pettit engineered or a knock off, or just someone's project so i am not laying blame anywhere. i have the Highest Regard for Cam Worth.

i will be fixing Dick's car when i get the time... it will involve relocating the outer steering pivot point downward.”

As Peejay points out, not only is bumpsteer really bad in the turns but every bump elicits toe steer. If, for example, you hit a bump with the right front tire and it compresses the RF an inch on Dick’s car the RF tire steers a quarter of an inch to the left!!!!!!!! So the LF is tracking straight and the RF is steering left!

This just wrecks tires which are made to roll not drag across the pavement and screws up all the tire-stick that was engineered into the fd, and the car hunts right and left as you drive straight on a bumpy (aren’t they all) road .

So you are driving the most advanced sportscar suspension on the planet and it works as intended all the time except when either front tire is hitting a bump or turning???????????????????

Every turn requires a counter turn.

Best quantify and fix the problem.

There are various posts on this thread ranging from no problem to big problem. That’s because, if properly modded, the fd20b can have virtually no bumpsteer.

The key is moving the rack and the end of the steering arm ( outer steering pivot) in the same plane. Move the rack forward, say 2 inches and you must move the outer steering pivot 2 inches. Move the rack down 2 inches and move the outer steering pivot down 2 inches.

On Dick’s car the rack was moved a dissimilar amount in relation to the outer steering pivot. I will fix that by finding the correct outer steering pivot point and relocating same.

As to how Pettit fits in….. I do not know who built Dick’s frame and modded his steering arms. I do know Cam Worth well and I place Cam in the Top Echelon category of the rotary community. He knows all there is to know about toe steer etc and I am confident that his product should work properly. I highly doubt that with ½ inch bumpsteer Dick’s subframe and steering arms came from Pettit.



There are a few other posts I would like to address:

“there really isn't a noticeable difference under 140mph, over that the car does get a little wigglely, but what street car doesn't at that speed. power steering”

first off…. manual steering is my favorite FD mod. Power steering has no place on a 2800 pound car including the 20b iteration. Power steering equals no road feel. You might as well be steering a video game. So I agree w 20b 3rd gen on this point. ( BTW, do not just remove the belt to give manual a try. You will then have the worst of 2 worlds… heavy effort and no road feel as all you are doing is pushing the hydraulic fluid around manually. you must gut everything to get manual steering.) as to bumpsteer… if you have it, it will be evident at all speeds. I track my car at Brainerd Intl Raceway. The front straight is entered around 80 mph and is 6000 feet long. Turn one is a wide radiused banked 90 degree turn that you can run thru over 175 mph. So I have lots of experience running 175-180. my car is completely calm… no wiggle no nothing. Suspension alignment and areo are the key. If anyone has problems above 100 it is setup not the way the car was designed etc. so bumpsteer is not something that emerges above 100 mph.

As to lowering the car creating bumpsteer or specifically the question:

“my question is, would a lowered Fd experience bumpsteer as well?”

The answer is NO.

As you lower the car the upper and lower A arms track with the steering links and bumpsteer is not effected. Do make sure that you readjust camber after lowering the car as, due to the fd’s racecar suspension, negative camber gain occurs on bump which of course is the same as lowering.

bottomline on bumpsteer.... if you have a relocated rack and your car doesn't turn or track as it did before you moved the rack.... get it measured and reset to being exactly zero. any bumpsteer is poison.

I hope this has been of some help.

howard coleman

Red-Rx7

Howard-

Thanks for the reply, but look at my post date! This was long before I ever ventured into the 20b realm. GT1-20b's suggestion is how my car was built. Remember in last years RR we talked all about it? I think this is the best way to go.

Unfortunately, I wont be able to go to RR this year. I have an IT project going to production the same weekend.

It was a great post though!

t-von

iTrader: (8)

Howard, I'm the one that sent you the PM. Also great post.

t-von

iTrader: (8)

I was under the Fd the other day and noticed that if you use the Fd's rear plate to mount the 20b in the stock subframe, wouldn't the lower half of the engine still hit the rack(excluding the oil pan)? As the 13b sits in the engine bay you can clearly see if the 13b was longer it would hit the rack. To me is seemed that a 20b mounted with the Fd rear plate would need to raised a couple inches to clear the rack. Is this why the LIM is cut so much? Or is it just tilted to clear the rack? Lastly does cutting the LIM move the torque curve?

Based of the info that Howard gave, I may build my own subframe and a custom oil pan as well. That huge hump under the oil pickup is the main reason the rack is lowered so much with aftermarket sub-frames. I figure a custom oil pan with a re-roughted oil pickup will keep the rack from being lowered too much. This will also give me the opportunity to build a higher capacity oil pan using billet aluminum which will add strength to the block.

Herblenny

iTrader: (13)

3" is alot!.. Is it going to affect the air flow of the engine?? Just wondering..

Point of using rear plate is so that you could use the fd mounts to the fd subframe.
Problem of doing that would be the 20B oil pan will not clear. So, you would need to make custom pan.

Far as cutting the LIM.. 20B complete manifolds sit lot higher than 13brew manifolds (lim and UIM). Hence you would lower the 20b lim so that the stock hood will clear the UIM.

Most aftermarket subframe lowers the engine and also clears the stock 20b oil pan.. hence you don't have to worry about cutting etc.. but there is an issue with bump steer, as you've read so far..

I'm trying to calculate how much I need to lower my LIM.. Mike, I see that your LIM was completely overhauled.. I've asked this question before, how much did you lower it?? by pics, it was about an inch I believe?

Howard, It was great meeting you last year.. But unfortunately, I'm also not going to make it to Rotary Revolution. I have a wedding to goto that weekend.

Hope you could make it to our event @ Deals Gap.

t-von

iTrader: (8)

I understand that point but go out side and look at the car. Have you already mounted your engine with the Fd's rear plate? From what I saw, even if there was no oil pan on the stock 13brew, and if the 20b was mounted using the Fd rear plate and subframe, you would still hit the rack. Go outside and look. Just imagine the 13b being longer and you would see what I mean. The lower half of the engine would hit it. I measured and the engine would need to be raised about 2" to fully clear. I believe this is why the LIM is chopped like it is with the stock subframe because the engine is raised or tilted upward to clear the rack.


I'm not to sure about the manifold being taller. I have a subframe that I purchased from Rx7 Specialties. The engine mount mounting surface is 1/2" lower compared to the engine mount mounting surface with the stock sub-frame. That differance is because the 20b's engine mounts are 1/2" thicker when compared to the Fd's mounts. If I used my custom subframe to mount my 20b, it would sit the engine at the same height and the 13b and the hood would still close without me chopping anything.

Aristo

iTrader: (8)

So then, are the people like Red RX-7 using the stock FD subframe and 13b-rew rear plate with a custom oil pan tilting the engine, to clear the rack and if so does the angle create problems for the driveline.? If possible, it seems the cleanest install would just involve dropping the 20b into the stock location with a custom oil pan and shortened intake, and just dropping the steering rack an inch and dialing out the bump steer. I have been looking into having aluminum side housings and rear plates custom cast which would save in the neighborhood of 60lbs off the shortblock, so all the hacking involved in lowering and moving the engine back for the purposes of stock weigh distrubuition could be avoided.

t-von

iTrader: (8)

I think more or less that his engine is raised or a combination of both. I would need to see a pic of his shifter to see how high or low it sits in the cabin because if it is tilited upward, the shifter would sit lower in the cabin. This means the PPF would need to be modified or eliminated or a tranny brace fabricated. Personally I would rather just build a stronger custom oil pan with out that huge hump and modify the oil pick-up, then just lower the rack enough to clear the new oil pan. Then I could lower the steering arms the same amount without moving the rack forward. Of course I would still need to build my own custom subframe which shouldn't be too hard.

Speedie7

Could you explain why you would need a custom subframe if you were to go the route youre mentioning?

t-von

iTrader: (8)

In some ways I've already explained this. If the 20b is mounted using the stock sub-frame, and it's mounted at the same height as the 13b, the engine block will still hit the rack. You can avoid this by lowering the rack or raising/tilting the engine and fabing a new oil pan(raising or tilting will require the LIM to be shortened for hood clearence). I don't want to put a heavier engine in my car and raise it to fit so it can clear the rack. This will change the cars center of gravity. I don't plan on using the Fd mounts and rear plate to mount my engine. The stock 20b mounts are a few inches forward by comparison. This will help distribute the weight of the engine(which is why Mazda put them in that spot in the first place). Now because I'm using the 20b mounts, I will need to fab my own custom subframe. Also I feel that using the Fd mounts will put more stress on the upper portion of the bellhousing & bolts that holds the engine to the tranny. This and the fact that there is one less bolt to hold things together up there.

Herblenny

iTrader: (13)

t-von,

I'm not trying to get into an argument because I haven't acturally try to mount the 20b.

But, I have talked with several regarding this and I've gotten the same answer.

With FD rear plate (where you could use your fd mounts, stock subframe, and tranny) all you have to do is modify your oil pan, lower the LIM, use FD alternator, and water pump, it will clear the hood and will work.

I on the other hand, instead of modifying the OIL pan, I'm most likely going with dry sump kit.
I'll just put a flat piece of metal under side of the engine and should clear the rack with out a problem. But all this is just my guess from what i've heard and spoken with few specialists. I'm in a process of building the engine and cutting, welding, polishing the LIM.

I'm guessing I'll be done sometime in Summer of 2007.. I'm being realist here.. its going to be a long process and i want to do it right (builing 2x 20B engines).

thetech

iTrader: (1)

As you can see from my other thread I am actually doing this exact same thing currently. The main difference is that I am using custom mounts rather than 13B mounts and rear plate. There is a powersteering fitting on the rack that can be relocated to get the engine down another 1.5" or so from what appears to be the lowest point. This combined with the LIM cutting will get the engine under the hood it appears.

Although the engine sits higher than the 13B and alters the center of gravity, this is a far superior situation (in my opinion) than creating adverse bumpsteer effects and destroying any akerman in the steering system by moving the rack down. That is not a compromise I was willing to make.