Thinking about installing an BMW M6 engine in my Z

[grannyknot]

Got into the rockers and the panel the gas tank filler hose goes through today, luckily I spotted one at a Z swapmeet last year and bought it cause I was sure I would never see one again. $50 was cheap compared to what I just paid Tabco for some very simple punch out pieces, 2 rockers, a right rear quarter and hatch deck panel.

 

[grannyknot]

This whole contraption I built to buy more room for the engine - complete garbage, should have done a bit more research before cutting. Can't just screw with steering geometry and expect to get away with it.

I will have to find that extra room some other way, I did find a 1/2" by removing the the top of the collars  on the steering rack, cutting some of the urethane and modifying the hold downs. Not pretty but I can clean it up later.

Got a great deal on a set of 16x8 Rota Grids with 225/50 BF Goodrich g-force Rivals, the tires still have 3-4 yrs life in them so maybe I'll get to smoke them a bit. The car isn't going to be this low when I get finished, just testing to see where it sits on bump stops with no springs.

[Namerow]

9 hours ago, grannyknot said:

This whole contraption I built to buy more room for the engine - complete garbage

Not 'complete garbage'.  Rather, 'failed experiment'!

Nice looking wheels, by the way.

[Captain Obvious]

Why were the crossmember mods garbage?  What went wrong?

[EuroDat]

You could lower the steering rack and correct the geometry by replacing the tie-rods ends with straight ones. The originals are bent downwards. That would give you some room. I have some photos of the straight type. Not original datsun of course.

Anyway, the tie-rod should be parralel with the lower control arm and the same length, othrwise they work on a diferent radius and your toe-in goes all over the place when the suspension moves up and down.

[grannyknot]

1 hour ago, Captain Obvious said:

Why were the crossmember mods garbage?  What went wrong?

Bumpsteer, from an average height of about 6" (a little higher then where the car would normally ride)

then dropping the car down 135mm simulating a high speed bump the wheels toed in a total of 26-30mm! (that's a combination of the front of the moving in and the back of the wheel moving out)

Then on rebound the wheels toed out even more.  By adding 25mm bumpsteer spacers that number was cut in half to 13-18mm of toe in for 135mm of travel.  The thing is I'm pretty sure that the wheels are suppose to toe out on bump which is what I found when I installed the unmodified crossmember and did the same test. I also have  3/4" spacers dropping the crossmember down from the frame just to really complicate things.

Chas, I'm finding out through experimenting that what you're saying is exactly right, any chance you have a parts# for those straight tie-rods?

[EuroDat]

6 hours ago, grannyknot said:

Chas, I'm finding out through experimenting that what you're saying is exactly right, any chance you have a parts# for those straight tie-rods?

Chris, I have some photos on an external hard drive. Ill dig it up tomorrow and post what I have. Unfortunatly the car (260Z) is no more. The owners son spun off the road and rolled it into a ditch.

The important thing is keeping the tie-rod parallel with the control arm and the same length as the original. I think Techno Toys make such a kit. Its been a while since Ive seen one.

I found this by googling "bump steer tie rod kit datsun". You need the type with the adjustable pivot bolt on the end so you have vertical adjustment.

 

 

[Captain Obvious]

Gotcha. Maybe you could use a BMW steering rack instead to compensate? 

[Namerow]

19 hours ago, grannyknot said:

The thing is I'm pretty sure that the wheels are suppose to toe out on bump which is what I found when I installed the unmodified crossmember and did the same test.

From Fred Puhn's book, 'How to Make Your Car Handle':

"Many elusive twitches and wiggles can be blamed on bump steer, and often it is the cause of high-speed stability problems [Hello, 240-Z!]...  Try really hard for zero bump steer at the front.  A car with bump steer in the front suspension will be unpredictable in a turn and unstable during braking.  The car is very sensitive to toe-in changes.  With bump steer, toe changes can happen with every dip in the road or when you hit the brakes.  Both toe-in and toe-out errors give terrible handling and should be avoided in the front suspension".

The basic goal:  On each side of the car, keep the tie rod parallel with the lower suspension arm throughout the arm's full travel (or, if not, at least for all but the last inch of droop and/or compression). 

Bump steer's evil twin is something called Roll Steer.  It's the same thing, but experienced during cornering.  From a design point of view, you still want zero roll steer and bump steer at the front.  Roll steer seems to be of greatest interest at the rear.  For cars with IRS, the goal is to use roll steer to encourage understeer when the roll angle becomes extreme.

To relieve bump steer at the front of the car, one solution is to raise or lower the steering rack (which changes the angle of the tie rods).  In your case, though, this has become a problem rather than a solution, so you need to work with the other fixes to try to un-do the result.

One of the alternative fixes consists of bump-steer spacers.  You've tried these and experienced a positive (but incomplete) result.  Maybe you could fab additional spacers using your new/old lathe?

Another fix is precisely the type of spherical rod-end tie rod set-up that Chas has illustrated.  Puhn says, "This allows shims to be used between the steering arm and the tie rod end for small bump-steer adjustments.  Large changes cannot be made this way, but it is a good way of getting an extremely precise small adjustment."  Hmmm.  Maybe not enough to solve your situation?

Interestingly, Puhn's book talks about yet another technique for alleviating bump steer and uses a Datsun 510 to illustrate the point.  It consists of bending the steering arms.  Puhn cautions that this is a job best left to specialists because: 1) you may crack the arm casting if you heat/bend incorrectly, and; 2) the arms will need to be crack-checked and then heat treated afterwards.

I wonder if a less painful way for reaching your final goal would be to use a suspension geometry algorithm (I guess they're called 'apps' now) to investigate different combinations and settings before you actually do any more tweaking/cutting/welding/bending (and purchasing).  There's got to be an on-line, free-for-use app on a racers' site somewhere.  Maybe one of the other CZCC members can help out with this so that you don't have to go searching (and if you find one, please let the rest of us know so that we can play with it too ). 

Among other things, it would be interesting to see whether the stock Nissan set-up really does achieve zero bump steer over the full suspension travel (unless somebody else has already investigated this?)

[EuroDat]

I found the photo's and a brief description of what was done in dutch. The car is lowered about 1-1/2" and the lower control arm anchor point was raised to bring it back to hroizontal.  The distance between the steering arm and the ball joint on the tie-rod is 36mm. The bolt is an M14x1.5mm high tensile grade 10.9.

Bump steer and toe was checked using a wooden panel in the last photo. The car had coil overs and they were screwed all the way down to get free travel.

If you look in the first photo, you can see the lower control arm pivot point in the crossmember is higher. Its about 30mm higher. The reason for the 6mm difference  (36 spacer in tie-rod end and 30mm in control arm pivot) is because the new arm is straight compared to the original has a slight bent just after where it connects to the rack.

Like Namerow said, you can find plenty to read about this one the internet. Enough to drive you nutters. If you experiment a bit and test like in photo 3 you will get there. I have faith

 

[grannyknot]

Thanks guys, wrapping my head around this.  So I will not be using the modified crossmember at all, I have sunk the steering rack down in the rack cradles as far as I'm willing to go. I certainly want the engine to fit under the hood but not if the handling of the car is compromised, now I just have to determine how much my 3/4"crossmember spacers are changing things. I have set up the front end with a stock crossmember lowered 3/4" and installed a 25mm bumpsteer spacer, here are some pics, I'll start doing some bumpsteer measurements tomorrow to see how this setup pans out. Also because of the lowered crossmember I'm finding the R/L alingment of the crossmember to be very important, I'm going to have to come up with some way to secure the crossmember other then just bolts, perhaps locator pins of some kind.

 

[EuroDat]

If I understand your problem, its the steering rack thats giving you most of the clearance issues?

If that is the case then I would leave the cross member where it is and lower the rack first. Buy a set of techno toy tie-rods (see link) and use then to correct the tie-rod height. You will need to work the cross member for the steering column shaff clearance.

I wouldn't use the bump steer spacer if posible. I have read on this forum and others that they can make steering worse.  You should be able to get 25mm by driling the cross member and repositioning the lower control arm pivit point.

https://technotoytuning.com/nissan/240z/outer-tie-rods-datsun-240z-260z-and-280z

Here is an article on redrilling the anchor point for the LCA to improve the geometry on a lowered 280Z.

http://www.zcar.com/forum/10-70-83-tech-discussion-forum/285255-drilling-new-holes-control-arm.html

I have seen this done on Zeds and I was involved with one a long time ago (back in the eighties). We basically did the following:

Removed the spring to make moving the suspension easier. Cut a bolt so it would fit between the cross member faces. This bolt would fit the inner pivot on the LCA and you could clamp it in place with a G-clamp on the outside of the cross member. That would give you all the movement needed to test toe-in with the suspension in all its travel. Once you found the best position, you drill the cross member and reinforce it with an extra plate or washer on the outside.

The new hole doesn't necessarily need to be directly above the original hole. It depends on the radius of LCA versus steering arm. The one we did, it moved out about 5mm further out.

 

 

Edited August 28, 2016 by EuroDat
added link & tekst