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Project V8 RX-7: Part 17 – Eimer Engineering Arms, Wheel Spacers and the Hunt for Perfect Stance

by Mike Kojima

 

In our last Project V8 RX-7 article, we left you with photos of our new SSR Professor SP4 wheels and Toyo R888 tires looking flush but we weren’t ready to explain how we accomplished that look yet. With our Rocket Bunny aero kit giving us a huge space in the wheel wells to fill, we have to take advantage of all of that.

Typically, our show car brethren would do something like getting a 275 rear tire and 245-255 front tire, stretching them on 10-12" wide wheels and using a lot of spacers to fill the extra wheel well space to make it all flush, but us being us, that option would not fly.

Our car had to use all of the space for wide wall-to-wall rubber. More rubber means more grip, and with our LS3 providing the torque and power to use it all, we were obligated to do so. However, just putting wide rubber on a car not designed for it creates a few technical issues that, since we are MotoIQ guys, have to be engineered out of the equation. This means the typical spacer to fit and beat to match approach was not going to be a solution.

 

We spent a lot of time coming out with decent close to center offsets that would not stress out our wheel bearings too much by cantilevering the loads on them but still have plenty of badass looking lip. 

As you might recall, we got custom SSR SP4 wheels with an 18x11" with a 37mm offset front wheel and an 18x13" rear wheel with a -23mm offset. This huge combination would help us meet our goals with some help. 

 

For tires, we chose Toyo R888's, a 295/30-18 for the front and a 335/30-18 for the rear. That is a ton of rubber for this small, lightweight car!  Even with the wide wheels and huge rubber, we still had a lot of room between the tire and the front fender lips with our modest wheel bearing spacing offset. The tires also rubbed the frame rails at full lock.

We stated that we didn’t want to make up the nearly 2-inch gap between our front wheels and fenders with spacers or a wheel with a drastically different offset, so the only other way to move the wheel outboard is by modifying the suspension components as some of you already guessed in the comments section of the last article.  

If we used a more negative offset or spacers to fill the gap as what is done typically, we would have run into some issues. The first is that offsetting the wheel would cause it to swing in a larger arc as the steering was cycled. This would use up fore and aft space in our wheelwell, cause the tires to hit pretty solid, and require modifying stuff like the chassis rail ends as the steering was cycled.

Next, with a huge tire spaced outwards, the wide wheel would put a lot of cantilevered leverage on the hubs and wheel bearings. This would lead to rapid bearing wear and since we now had fixed caliper racing brakes, we would potentially have issues with brake pad knockback. This would give an inconsistent pedal on the track. 

With a large offset, the front tires would be pushed sideways as the steering wheel was turned. When this is done to a large extent as in this case--a super negative offset--the car can have odd turn in characteristics, tramline badly and have weird reactions when braking on split mu surfaces. The car would also heavily jack as the steering wheel is turned as the wideness levered outward exaggerates the effects of caster and king pin angle. This can really affect the corner weights and the balance of the car. 

Finally, we would like to get the front track as wide as possible. as this helps reduce understeer.

 

The stock lower control arms were nice forged aluminum units, and at first, we had considered modifying them to be longer. However, considering that we were going to be close to 2" outboard and that we wanted to have the abilty to fine tune the alignment, that was ruled out pretty quickly. 
 
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Comments
Dan DeRosia
Dan DeRosialink
Tuesday, August 01, 2017 7:18 AM
Let me be the first to make some sort of rod end in bending comment... no, just kidding, sorry. Great looking work - do you have any numbers on what you ended up with as far as scrub, out of curiosity?
Mike Kojima
Mike Kojimalink
Tuesday, August 01, 2017 11:30 AM
Well, we use the same size rod end for off road racing and for FD competition. Both pretty brutal and the strength of the rod end is several times stronger than the maximum calculated load. Don't be an FSAE geek! :)
Maytag
Maytaglink
Tuesday, August 01, 2017 11:57 AM
Can you connect the bottom of the lower spherical bearing bolt and the bottom of the steering bolt together and double sheer both of them simultaneously?
Mike Kojima
Mike Kojimalink
Tuesday, August 01, 2017 12:13 PM
No, because the bolt needs to articulate and it would bind solid. Not being in double shear on everything particularly outboard stuff is not an issue if it's designed right no matter why you got points docked on your FASE car!
Mike Kojima
Mike Kojimalink
Tuesday, August 01, 2017 12:15 PM
Dan, I think the scrub was something like 18mm as I recall.
Truckinduc
Truckinduclink
Tuesday, August 01, 2017 1:05 PM
actually you could double shear the tie rod and lower ball joint bolts together as both bolts are stationary to the knuckle.
Mike Kojima
Mike Kojimalink
Tuesday, August 01, 2017 1:37 PM
Truckinduc, yeah, we would have to re-design the knuckle though, we did this on our drift car, it came out pretty cool.
Mike Kojima
Mike Kojimalink
Tuesday, August 01, 2017 1:39 PM
Maytag, I think I miss understood what you wrote and Truckinduc is right if you are talking about the same thing.
Maytag
Maytaglink
Tuesday, August 01, 2017 1:43 PM
Yes, that was what I was talking about. Even connecting them together with a simple bracket would make them both stronger; if you can tie that bracket into another part of the knuckle, even better.
Mike Kojima
Mike Kojimalink
Tuesday, August 01, 2017 2:22 PM
yeah for some reason I was thinking connecting the bearing to the bolt and was like huh?
Maytag
Maytaglink
Tuesday, August 01, 2017 2:33 PM
Just like an engineer... Can't think outside the box

I've been dabbling in off-road suspension, and everything is in as many shears as possible, otherwise, it usually won't survive.

Dan DeRosia
Dan DeRosialink
Tuesday, August 01, 2017 3:29 PM
18mm seems pretty good considering the difference in wheel width from stock. I'm doing a tube chassis design and fighting for every bit of scrub, so it's on my mind a lot lately.
Mike Kojima
Mike Kojimalink
Tuesday, August 01, 2017 4:06 PM
I wanted to have enough scrub for feedback.
jeffreyball610
jeffreyball610link
Wednesday, August 02, 2017 8:27 AM
This is my favorite MotoIQ project. It's an extreme build using the proper parts and setup in an "attainable" project car. It does use some exotic parts, but they seem to be things that a regular person can get, make, or have made.

A huge bonus is Mike's excellent explanations, making it seem even more attainable.
RX-tacy
RX-tacy link
Wednesday, August 02, 2017 8:43 AM
How much for a set of those upper and lower arms?
warmmilk
warmmilklink
Thursday, August 03, 2017 5:58 PM
am I the only one surprised by how tiny the RX-7 knuckles are? My NC Miata knuckles look beefier...
Supercharged111
Supercharged111link
Thursday, August 03, 2017 7:24 PM
Any reason you didn't move the lower shock mount outboard to help bring the wheel rates back down some prior to revalving and respringing?
Dan DeRosia
Dan DeRosialink
Friday, August 04, 2017 6:44 AM
The NC MX-5 uprights *are* beefier, and I reckon have the potential to use a better bearing too. I wonder what the ND ones look like.
engineered
engineeredlink
Sunday, August 06, 2017 9:05 PM
Love this project. Keep us posted!
ginsu
ginsulink
Monday, August 07, 2017 4:31 PM
People put off mods like this because of NVH. But the handling gains are so worth it. If you are seriously worried about NVH, you can just replace that 'lateral links' with rod ends, and keep the 'vertically loaded' links rubber (usually just the mounting points for the struts).

Most of the NVH feedback, comes from the primary loads on the suspension, and not lateral loads from cornering. I've found that this is a good compromise for a street car. Although, there are some significant handling gains to be found by removing rubber from the strut mounts, you CAN build a great handling street car, and not sacrifice NVH.
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