Rear subframe brace into a '90
 

My '90 has no rear subframe brace or provisions to fit one. What are you guys doing for bracing? I assume a stock brace fits OK with a 7.5" diff and Martin's mounting hardware? I'll be dropping my subframe to mount my diff this week and if I need to weld on any brackets or bolts etc now will be the time to do it.
Thanks,
Brad

Assuming you have the stock braces to match up to, it should be pretty straightforward. Keep in mind your projected exhaust routing.

Mike

Researching this a bit, apparently Mazda made or makes a tab mounting kit for the earlier subframes to accept a later brace. I'm currently in the hunt for said kit.
Anyone here work at a Mazda dealership? I have a part number.

BTW Mike, I visited Pete with the P1800 a few weeks back. Very nice subframe you provided. That is one extensive undertaking he is doing.

Any chance of finding a newer sub in a bone yard with the stuff already there? Actually all they are is a couple of tabs with nuts welded on them and they would be easy to make.

Mine is braced every which way but sunday but I just got carried away playing with the welder...
http://i59.tinypic.com/v5fhxs.jpg
http://i59.tinypic.com/14w3a77.jpg

Did you see this post? [NA] Rear subframe brace w/weld on kit - MX-5 Miata Forum

By the time i saw it last night, he already had a buyer. Plus not a great deal. The tab kits are only $100 from the dealer....cheaper if you look around, and used braces are practically free.
That's ok I may have one coming from a forum member here for a good deal.

I am not using any type of rear brace, only poly bushings. The door bars helped the most.

I saw a post on grassroots where Keith (from FM) claims he can tell when a a tech has left the brace off the rear. I don't take every word I read on the internet as Gospel, but having met him, he doesn't come off a BS'r when it comes to these things. ...wants to sell their products, yes. But I see no $$ to be earned in the comment.

Several hrs of Internet University has led me to that conclusion.

I'm bringing this back, as i have freaking HORRIBLE wheel hop and my subframe has no OE mounting tabs for my oem diff brace.. Thinking about ordering the FM diff brace and welding in some 1" tubing...

I'm on Koni race struts and 325lb rear springs. Stock rear bar... My rear hops with even the SLIGHTEST big of throttle. HELLLLLP

I'm bringing this back, as i have freaking HORRIBLE wheel hop and my subframe has no OE mounting tabs for my oem diff brace.. Thinking about ordering the FM diff brace and welding in some 1" tubing...

I'm on Koni race struts and 325lb rear springs. Stock rear bar... My rear hops with even the SLIGHTEST big of throttle. HELLLLLP

What is the condition of your control arm bushings?

Pics would be helpful. You need to create a box around the diff from control arm to control arm across the width. A cobra style diff housing cover that bolts to a bracket connected to a solid mount is also an excellent add on.

Upgrade control arm bushings noted above are also a good upgrade, along w/poly diff bushings.

Another thing that I've shared over the years are welding 1/8" thick plates to fill the space between the lower control arm arms. This helps to keep the stock arms from twisting.

An extreme mod would be to tie the lower control arms to the frame rails w/ a pipe (heim joints @ each end) to keep the LCA from twisting while allowing normal suspension travel.

That ^ is the way Mr Shelby did it back in the day on his Mustangs...

No question you have something pretty wrong back there. I've only ever gotten wheel hop once and that was braking. I'm not sure how much good those frames will do you since they were designed for 1/4 of the tq. I have no idea if all the crap I welded in back there makes any difference but it is certainly sold in all directions back there and I get no hop. Here is a look at all that crap if you want to see it. Jim and Jenna build diary, Miata LS1 Conversion: The 3rd half of the back half, 8.8 Ford axles in a Miata.

Do you have your trans brace supported through the floor at the edge of the foot wells? I have no idea why but Jason says that makes a big difference. Maybe not so much on LS cars as Fords but it is an easy measure to add. Basically you just run a bolt though the floor and your trans brace at the edge of the tunnel where it is about 3 layers thick. More footage of that here if you want to see the concept. Jim and Jenna build diary, Miata LS1 Conversion: The mother of all transmission mounts!

Other than that +1 on the bushings. Poly makes a big difference. That being said mine are OEMs with 200,000 miles on them but I used poly in other cars and it will help. I would at least make sure they are good.

What tires and diff mount are you using and what are you diff bushings like? Does it wheel hop breaking at all?

I already have all urethane bushings for all control arms. I've been looking at adding this. apparently the CTSv guys are loving it.

Attachment 6786

I think it's a band-aide. You have a problem somewhere. I have the same diff and 430whp on 225/45/15 RA1's and zero wheel hop. IMO you either have a bushing that's allowing movement or its possible not having the rear brace is an issue -that's the only structural difference we have. ...that said...that brace isn't expensive and less labor than tearing down the rear so why not. If it helps I'd be curious to know as I'd be willing to add some bracing to mine too just to have it.

titangts you still haven't said what set up you are running and conditions it's happening so there is a whole lot of speculation going on here.

he's got 225/45/15 RS3's, a cts diff, Koni race struts with GC coils, and poly bushings all around.

....there's a parallel conversation going on on FB.... :ughug:

What OEM mounting tabs are you talking about? The two that mount the drive shaft side of the differential housing to the unibody? If this is what you mean, it explains a lot. If that side of the diff housing is just hanging there, it's never gonna be correct. Those mounting points must be used.

The pinion support brace that you posted will do the same thing as a Cobra style cover & should be used. However, you need to post a couple of pics (as has been requested). If your diff frame is poorly laid out (needs to be boxed & tied to the unibody),it will need more structure, as noted earlier. With pics, this can be resolved much sooner than later.

Here you go guys!

What I see from the video is a diff bolted to very thin brackets. Under load the diff is probably flexing those mounting points. Can't see much of the brackets that hold the ears of the diff, but caught a glimpse as you passed one near the end of the video. All look very thin. Those brackets should be @ least 1/4" thick or shore up the existing brackets w/ a perpendicular rib. I'd also use that diff brace you posted earlier. A higher durometer bushing wouldn't hurt either...the red bushings are usually in the 90-100 durometer range.

That is the minimum & should take care of the immediate problem. Next would be the box & then plate between the control arm legs. If you weld the 1/8" plate between the lower control arm legs from the arc to the bushings, this will keep you from needing the missing cross piece that would have run tab to tab & will help resist twist.

maybe this will help?

Attachment 6783

That bracket will deflect. I'd use thicker material or at least gusset the bends.

Great video and your exhaust is not half as bad as mine!

The tabs you zero in on at 1:10 and talk about look like you could get a washer or 2 in them to tighten things up. Anything you can do to tighten up slop in the diff mounts will help more than bracing the subframes together IMO.

I don't know that the tabs are in your photo of post 22 ^^ because I am not very familiar with this set up but I know wheel hop well enough to know that is a bad deal. I can piss more support than that! I also really don't like that there is no support at all on the back of the diff and twisting front to back is what you are trying to stop.

I know when T bird guys have wheel hop problems bolting the rear diff mount solid and makes a big difference for them. Those 2 horizontal bolts in the side of it stop all movement of the caged rubber in the mount.
Attachment 6782
I know it's not directly applicable but works to the theory that you don't want any rotation between diff and chassis at all. Rubber in mounts is not your friend and if you can eliminate diff movement you will eliminate your hop.

^^^thanks! well i checked tire pressure...both ( actually all) tires were at a 20psi...I've been told to run around 26 in the rear..

Is it safe to say that at 20 psi its definitely not helping?

I see our conversation paid off.... too much tire patch on the ground is no good with wheel hop. I have little Idea of how the CTSV rear bolts into the subframe as I did the Cobra mount on Nadine. I will say that it looks like they used a mount from the factory studs to a cross bar and then tie into the CTS diff with one bolt going front to back. On the front you also have just one bolt going front to back. I would add that other mount if it supports the front of the pinion. At least then you are mounted in three places and the diff cant rotate and rock as easily across two axis.
Did you buy the pinion support or is that just a link? I am wondering how it attaches to the car end... through the factory rubber bushing but with a longer bolt?

I'm back.... I looked into some of the issues with the CTSV diff and talked to a couple friends that have the cars. One has the supercharged 6.2 in his and says that he had to replace the front diff bushing almost immediately after buying his car. He went with the race (Red) bushing from Creative-steel.com It was $49.95 plus shipping. They rent a tool set for $30 with purchase ($250 deposit) for seven days after you receive the tool. You ship it back and they don't charge for the shipping days if you provide them tracking. With their tools this is supposed to be a 1 hour swap and you can do the job in the car!
Lucky for me I have the diff still on the bench and that new-to-me 12 ton press in the garage. Looks like I am doing the red bushing before it gets bolted together!

Once a builder upgrades to rear tires capable of using the "under the curve" HP/TQ, the importance of boxing & stiffening lower control arms will become apparent. Until then, wheel spin will occur before flexing & wheel hop (related to the flex created by improperly framed & stiffened LCA's). Near solidly mounted diff housing is effective, but is only part of the complete solution.

The site could use a couple of minutes after posting for errors to be corrected before the site adds the edited note.

Ah ha! Validated! Here I thought all that crap I welded under there was just because it was fun to weld crap under there!

I think i might have found a possible culprit....


Yes, It came out that easy with my hands Pre-video.

My dad has my upper control arms which he is completely boxing in. I ordered the 90-91 OBX (exact same as FM) differential brace.

Waiting to for my Revshift (green) bushing to come in so i can put it in and see what all happens!. wish me luck.

so yea this happened. more pics of other stuff coming tomorrow..

Attachment 6699

LOL loved the video and yeah that bushing was toast for sure! Looks like one heck of a set up you have cooked up there ^^. I'm looking forward to how this comes out but I have no doubt it will be much improved!

You guys thought I was playing about fixing this wheel hop issue

Attachment 6694

Attachment 6669

Looks familier. :)

Of course, now FFR is coming out with a new fabricated arm that should solve all the problems.

Isn't there a deal where guys drill holes in them and weld tubes inside to add rigidly?