Narrowing the front end of a mk4 to accept a 289 street slabside body?

[Jimtmich]

I would really like to build a Factory Five mk4 and put a slabside 289 street body on it. So being an engineer I want to plan it all out ahead of time. My concern is I don't want to mess up the front end suspension geometry as driving a good performing car is a very important part of the objective.

So, Mr Bruce indicates the 289 street body is 61-62 inches wide in front. The mk4 is 67 1/8 according to a drawing I found. So I need to figure out how to remove 6.125" if the above information is correct.

Pindrive lower control arms are 1.625" narrower than standard. Once again I read this on this site somewhere. That would be 3.25" closer to the goal. 2.875" still to reduce.

Standard 15" Hildebrand are 15 X 8 on the front with 5" backspacing leaving 3" from brake rotor face to the outside of rim. Dayton bolt on wire wheels look like they are available in a beadlace in 15x6 with 4 backspace leaving 2" from brake rotor face to outer wheel edge. 1" narrower per side so 2" closer to the goal with only 0.875" remaining. The Dayton catalog is not very clear so I'm not positive I got that right and even less sure the wire wheels would look 60'so British sports car.

I would greatly appreciate any input pointing out errors in the numbers, logic, wheel selection, where to find the last 0.875"..

Thanks,
Jim

[CraigS]

You are aware that FFR sells a 289 version right?
https://www.factoryfive.com/roadster/289-usrrc/
Wouldn't it be easier to start w/ this?

[Jeff Kleiner]

Send Jon ("jolsen42") a message https://thefactoryfiveforum.com/private.php?do=newpm&u=108

https://thefactoryfiveforum.com/showthread.php?28920-The-first-FF-based-Slabside

Good luck,
Jeff

[mike223]

My concern is I don't want to mess up the front end suspension geometry as driving a good performing car is a very important part of the objective.




I think you should figure out what tires you might find acceptable - to my understanding the selection of available 15" performance tires is pretty abysmal (and getting worse all the time).

[Jimtmich]

You are aware that FFR sells a 289 version right?
https://www.factoryfive.com/roadster/289-usrrc/
Wouldn't it be easier to start w/ this?

Yes, thanks for pointing that out. The USRRC is the same frame as the MK4. I would do a USRRC if the 289 street doesn't look promising for good front end geometry. I prefer the street car look, but the USRRC is a close second.

[Jimtmich]

Send Jon ("jolsen42") a message https://thefactoryfiveforum.com/private.php?do=newpm&u=108

https://thefactoryfiveforum.com/showthread.php?28920-The-first-FF-based-Slabside

Good luck,
Jeff

Jeff, thanks for showing me Jon's build. I did communicate with Jon and he started with an earlier frame and did move the suspension mounting points on the frame. I would like to get further details from Jon but I need to better understand the evolution of the FFR frame and suspension as there appear to be improvements over the life of the roadster that my make what Jon did not really applicable to a MK4 frame, or it may be totally applicable. I need to read more to understand.

[Jimtmich]

I think you should figure out what tires you might find acceptable - to my understanding the selection of available 15" performance tires is pretty abysmal (and getting worse all the time).

Thanks for that note. Yes, a car with 205/70/15 BFG would not be competitive in autosports, but could be predictable. What I don't want to end up with is a lot of bump steer. I had that on an old Triumph TR4 SCCA racer that wasn't set up well and it was miserable to drive in certain environments.

[Double A]

Have you checked out the ERA slabside kit? $22,900. base kit

[CraigS]

First I would want to confirm the dimensions you have. Losing that much width will be tough. At the front it's easy enough to lose an inch per side if you can cut and weld suspension pickup points on the frame. Taking the entire spindle and upper and lower control arms and moving them inward would maintain the geometry. Moving the suspension in one inch would work w/ power steering by not running the extenders on the rack. The extenders are usually installed to help bump steer. I run rear 10.5 wheels on the front for autocross and going to a 6.8 or 6.9 back space (10.5 wheels) from the usual 6.0 (9.0 width wheels) makes for a larger turning circle as the tires hit the F panels. So that makes doing both narrower suspension and more back space wheels a no go. But you could go from 9 in wide wheels (W/ the standard 6.0 BS) to 7s and lose 2 inches per side on the outside of the wheel.

[Jimtmich]

First I would want to confirm the dimensions you have. Losing that much width will be tough. At the front it's easy enough to lose an inch per side if you can cut and weld suspension pickup points on the frame. Taking the entire spindle and upper and lower control arms and moving them inward would maintain the geometry. Moving the suspension in one inch would work w/ power steering by not running the extenders on the rack. The extenders are usually installed to help bump steer. I run rear 10.5 wheels on the front for autocross and going to a 6.8 or 6.9 back space (10.5 wheels) from the usual 6.0 (9.0 width wheels) makes for a larger turning circle as the tires hit the F panels. So that makes doing both narrower suspension and more back space wheels a no go. But you could go from 9 in wide wheels (W/ the standard 6.0 BS) to 7s and lose 2 inches per side on the outside of the wheel.

Craigs, Great information! I appreciate the comments. I did not realize there were rack extenders in the PS rack. That sounds like a perfect way to reduce width without changing geometry in conjunction with relocating mounting points.

[Jimtmich]

I just talked with Bruce and he indicates that the 289 street body is designed around the MK4 frame with pin drive width LCA. He went through all the numbers with me to help me understand. Looks like the front suspension is nothing to worry about. Thanks for helping me understand everything guys.

Jim

[CraigS]

To clarify the rack extenders. They are kind of an option. I think both FFR and Breeze have them and supply them but I am not sure whether they are standard or not. This is one of the mods developed years ago that have been more and more often incorporated by some suppliers. I didn't want you to think that every PS rack will have them to be removed for your use. Good to hear that MrB has the mkIV in mind. You might want to check w/ FFR getting specifics on what they have now for pin drive width. There used to be short arms but I 'think' they may have been replaced by an alternate set of LCA mounting holes in the frame. I am not sure how the UCA is moved or shortened. Also ask about pin drive for the 2015 IRS. Again I 'think' I have read that there is no such option which might force you to go 3 link solid axle.

[Jimtmich]

Thanks Craig, ffr tech indicated that their racks are now wider so I should go with a fox width rack from stock parts store for power steering or flaming river fox width for manual steering. He also confirmed using pin drive narrower lower control arms and the inner frame holes designed for later stock mustang lca. Ideally upper control arm mount would be moved same amount as lca.

[CraigS]

Just so you know what you are getting into.
https://farm1.staticflickr.com/839/43603074672_be5b01598b_z.jpg (https://flic.kr/p/29r49Zw)MkIV IFS (https://flic.kr/p/29r49Zw) by craig stuard (https://www.flickr.com/photos/152454123@N04/), on Flickr
The ffr frame offers two mounts for the UCA. This picture shows FFR spindles so the UCA is mounted to the vertical surface. The horizontal surface mount (green circles) was used w/ Fox spindles because they are taller. So, to try to move the UCA inward:
1- If using the horizontal surface, it's easy to drill two new holes toward the center of the car. But you can only go so far because the UCA cross shaft will hit under the shock mounting ears. My guess is you would be limited to about 3/4 inch
2- If using the vertical surface,, the cross shaft 'might' be able to be bolted on behind it but it's a moot point. See the red arrow. The rear pivot of the cross shaft only clears that 3/4x3/4 tube by about 1/4 inch.
My experience checking bump steer on two MkIVs is that it is near perfect w/ the FFR front suspension shown here if you run power steering so you can get 6.5 deg or so caster.
The other option of course is too shorten the UCA tubes. There is a limit to how short they can go though because there needs to be room for the spring to fit inside. So maybe a combination of a shorter arm moved in as far as you can get it will work.

BTW I wish I could remember whose build thread I borrowed this from but I closed the second browser as soon as I copied it. Whoever recognizes this picture, thank you very much.

[CobraboyDR]

Have you checked out the ERA slabside kit? $22,900. base kitI understand ERA now has a two-year waiting list.

[j.miller]

All right, I'm just going to come out and say it....Mr. Bruce's bodies are crap. Have had my hands on enough to say "run away" or spend big money painting a body that is going to be problematic...….for years to come. Not attacking the man (don't know him) just the body.

[Double A]

J, are they as bad as my mk1 body? It is a total POS (no offense to ffr as I don't believe they were making their own bodies back then ) I want a slabside and Mr. Bruce seems to be the only affordable option. God only knows why Dave Smith won't make one, I mean no one else makes "other bodies" for FFRs' other cars. Anyway with as much work as this mk1 body is going to take, would a Mr. Bruce slabeside make sense since a FFR slabside is what I so desperately want. (Hello Dave Smith).

[cnutting]

I'm guessing that the reason is economics. The cost to design, prototype, tool and produce one (with at least one design iteration spin, probably more) outweighs the return. The vast majority want the MKIV, not many of us chose the FIA. Maybe the market is bigger for a slab side? I'm sure Dave has run the numbers.

[j.miller]

J, are they as bad as my mk1 body? It is a total POS (no offense to ffr as I don't believe they were making their own bodies back then ) I want a slabside and Mr. Bruce seems to be the only affordable option. God only knows why Dave Smith won't make one, I mean no one else makes "other bodies" for FFRs' other cars. Anyway with as much work as this mk1 body is going to take, would a Mr. Bruce slabeside make sense since a FFR slabside is what I so desperately want. (Hello Dave Smith).


I said what I had to say once....didn't like saying it …..needed to be said.....won't say it again. As far as FFR bodies go, yeah there have been a few bumps in the road but none that make me curl up in a corner of the shop and start rocking.....not "rocking out". But hey ! There's an idea.....I , CAN'T , DRIVE,,,,,,55 (Sammy Hagar ….The Red Rocker) anyone, anyone.....da Bat

[Avalanche325]

God only knows why Dave Smith won't make one.

Because one is about how many he would sell.

[BAUTO]

I beg to differ on that , the slabside seems to be the most sought after replica on the market right now and only 2 companies are marketing them now superformance and era

[frankeeski]

Because one is about how many he would sell.

Got to agree with you on that one. I can't tell you how many people "said" they'd build an FIA if only F5R would make them available. It's their biggest seller today..............NOT! It's all about the bottom line $$$. They have to concentrate on those models that sell and leave the rest.

[Jimtmich]

I talked one of the FFR employees at EAA last summer about FFR offering a slabside and he shared the number of FIAs sold.....that put the slabside project on the back burner for FFR.