What is your solution for the Willwood Clutch pedal/frame interference problem?

[Jazzman]

For those of you that have already dealt with the problem of the Willwood clutch pedal hitting the 3/4" square frame member, what solution did you choose?

1. Notch the Willwood Clutch pedal
2. Remove a section of the frame and weld or bolt a new structural piece to the outside of the 3/4" frame member.
3. A alternative solution. (Please post your solutions to this problem.)

Thank you for your opinions.

[karlos]

Both options 1 & 2 will reduce the capability of the otherwise unmodified structure. The trick is in determining how much. The loads and stresses acting on the pedal are relatively straightforward and easy to calculate. The loads and stresses acting on the frame are not. So for me this becomes an exercise in trading the known for the unknown. Either option is probably fine. But I want some assurance and I only know how to get there for the modified pedal option.

At the risk of straying into TMI territory, here are some numbers for the modified pedal. This is what I mean by modified.


pedal_modification.jpg


Lower flange has been removed (one side only) just shy of the web. This has the effect of altering (reducing) the section properties of the pedal cross section. By putting dimensions into my handy section property calculator I can calculate by how much.


section_properties.JPG


For the modified pedal the critical parameter is 'Ix', which is the moment of inertia about the x-axis of the cross section. This is critical because the pedal is loaded in bending at the notch. In other words, a force applied to the footpad of the pedal creates a bending moment at the notch which is resisted by the I-beam shape of the pedal's cross section. The moment of inertia is a measurement of the cross section's ability to react that bending load.

As shown by the graphic, Ix for the notched pedal is calculated to be 0.0532 in^4. This compares to a value of 0.0594 in^4 for the unmodified cross section. This is a reduction of 10.4%, so we'd expect that the capability of the pedal would be reduced by that same amount, and this turns out to be close to the final reduction.

I can now relate the bending moment, the inertia, and the strength of the material to come up with the max pedal force that the pedal can withstand. I don't know with certainty what the specific material is but I know it's some type of forged aluminum, probably 6061. 6061 doesn't have great strength among the various aluminum alloys, so the assumption of 6061 will make for a conservative answer. To be even more conservative I'll use the compressive yield strength in the calculations, which represents the stress level at which the material begins to experience permanent deformation (conservative because the stress at which the material actually fails is a bit higher).

The table shown below pulls everything together. The maximum pedal force (applied at the footpad) that the pedal can withstand is 370 lb for the un-notched pedal vs. 308 lb for the notched pedal, a 16.5% reduction.


margin_calcs.JPG


The final thing we need to know in order to interpret this result is how much force can reasonably be applied to the pedal. From a 1971 Air Force study that is summarized graphically in the figure below, we can see that even young, strong Air Force pilots could only achieve a peak pedal force of about 190 lb. So 308 lb compared to 190 lb results in a margin of safety of 0.62, or 62%. This means that the notched pedal has 62% more capability above the peak value of 190 lb. Also remember that 308 lb represents the value at which the pedal begins to deform, not fail, so there's actually even more capability above 308 lb.

Finally, it's also worth stating that at the location of the notch the cross section is loaded in compression. This is important because modifications that increase stress levels can also result in reduced fatigue or cycle life. Not an issue in this case because the notch is in compression. I would think twice about making a modification like this if the stresses increased on the tension side of the neutral axis. Fortunately not an issue in this specific case.

So...after all that I vote for notching the pedal



pedal_forces.JPG

[oldguy668]

That's exactly why I notched my pedal shaft exactly as shown in your photograph. However, since I don't have the capability of performing an analysis as you have, I concluded that bolting a short piece of hollow tubing over a cutout section accomplishes nothing at all, and notching the pedal was easier anyway.

[edwardb]

I concluded that bolting a short piece of hollow tubing over a cutout section accomplishes nothing at all...

Except that none of us in the frame notching camp did that. Bolted a solid piece of metal across the notch with plugs in the open ends of the chassis tube. It's interesting that the head engineer at FF said in another thread it was OK to notch the inside of the frame enough to clear the pedal. He suggested filling the cutout by welding a piece back in, but the reason cited was to prevent water from going down into the chassis tube. Not for strength reasons. Finally though a real live engineering analysis of notching the pedal. Way above my pay grade. But definitely shows that's a completely acceptable option. FF certainly has the capability to do the same analysis with the frame mod approach with Solidworks. Or maybe just fix the problem and make it go away. OK, there's a novel concept.

[chopthebass]

I notched the pedal. Doesn't require much and it won't weaken it IMO

[oldguy668]

Edward, I'm not going to belabor the point, but there's a thread running elsewhere about this subject that carries a photograph showing a 3-inch section of frame cut right out and another piece bolted on top of it. Probably the worst approach but who am I to judge? I agree with you that cutting a small notch in the frame and welding in a patch is probably the best approach.

[David Hodgkins]

I notched the frame without cutting it all the way through. I fact I only notched less than half of the tube. The notch now acts as a backstop.

[Jazzman]

Karlos, that is an astoundingly detailed explanation of the math behind your decision. I really appreciate the level of detail. I certainly could not have done that math if my life depended upon it!! Now for something really difficult: Can you mathematically explain the phenomenon that any bolt, nut, fastener, etc that is dropped from any point around the perimeter of the rectangular space of the frame always comes to rest exactly at the center of that rectangular space, equidistant from any access location and about 1" farther than my arm will reach?!!

[edwardb]

Edward, I'm not going to belabor the point, but there's a thread running elsewhere about this subject that carries a photograph showing a 3-inch section of frame cut right out and another piece bolted on top of it. Probably the worst approach but who am I to judge? I agree with you that cutting a small notch in the frame and welding in a patch is probably the best approach.

I'm not going to belabor the point either. Just looking for accuracy. (1) If the picture you're referencing is the one below, it's mine, and that's my paw in the foreground. That's a solid piece of 3/4 inch square steel (not a "short piece of hollow tubing") and there are 5/8 inch solid steel plugs in the open ends of the frame tube that the bolts are going through. I've done two that way. I would challenge that it's every bit as strong as the original hollow tube frame in that area. (2) I didn't say cutting a notch in the frame and welding in a patch is the best approach. I only cited it as what FF engineering said, specifically in the context of strength of the frame.

I think any of the three approaches, done properly, are OK. I've personally had a concern with cutting the pedal arm just on pure principle. The frame interference exists because the pedal arm was made bigger and beefier to prevent breaking. To then turn around and cut it just didn't feel right to me. The engineering analysis makes a strong case that it's OK to make the necessary cut, as many also have successfully done.

The best solution approach would be for it to be fixed by a design change and leave the factory ready to use.

[christenfreedman]

Notched the pedal...Didn't do the math but was pretty obvious that it would be fine...

[2bking]

For the modified pedal the critical parameter is 'Ix', which is the moment of inertia about the x-axis of the cross section. This is critical because the pedal is loaded in bending at the notch. In other words, a force applied to the footpad of the pedal creates a bending moment at the notch which is resisted by the I-beam shape of the pedal's cross section. The moment of inertia is a measurement of the cross section's ability to react that bending load.

So...after all that I vote for notching the pedal

I did a similar stress analysis and basically concur with your numbers but used a lower quality aluminum for the basis of compression limits. Don't forget the side load that can happen ('Iy') in a panic situation or a non straight push on the pedal. I think I found the notched pedal arm was good for about 100 lb side load at the pedal pad which is way more than can reasonably be applied.

[NICK C]

I did the same as you Edwardb and feel,comfortable with that. Nick

[ArcticAZ]

I notched the tube and welded the notch closed with a 16 ga steel patch. Seemed to me to be the best solution, although welding while standing on my head was a bit awkward...

[pcww]

Notched the pedal in about ten minutes. It really does not take a bunch of grinding to clear the frame. I vote for the pedal fix. But I do agree with Paul that fixing this at FFR should take about ten minutes. It does not make them appear too user friendly to keep requiring this thread. For what it is worth , if I owned the company I would fix it.

Henry

[Raceral]

I cut the frame, wish I had cut the pedal....

[BULLITT428]

My end result to what I did was the same as Dave Hodgkins. I notched the frame and after doing so realised I had cut too much out so I put 5/8" peace inside the 3/4" tube Drilled plug weld holes in the 3/4" tube and welded it up. I then cut a small peace out if the peace of 3/4" I cut out and welded it in place. Cut a small piece of rubber and put it on the end as a stop for the clutch pedal. End result is exactly what Dave's looks like.

14.jpg15.jpg18.jpg20.jpg21.jpg23.jpg22.jpg24.jpg25.jpg30.jpg

Sorry for the photos the way they are I'm not sure how to do it any other way!

Ed.

[smithbks]

I did the same as David Hodgkins as well. A little notch in the frame (using a Dremel cutoff wheel of all things) and a little notch in the pedal. Seems to be fine and minimized the modification in both parts.

[David Hodgkins]

I did the same as David Hodgkins as well. A little notch in the frame (using a Dremel cutoff wheel of all things) and a little notch in the pedal. Seems to be fine and minimized the modification in both parts.

Yep, did mine with a Dremel too. I got really lucky and the shape in ended up with matches the end result of shot #4 in Ed's post. You really don't remove a lot of metal, when you take into account the angle of the pedal arm. Plus, that section of tubing is isolated, being welded on both sides. I'm very comfortable with this approach, and it's certainly not the only "adjustment" I made in the build! Oh, and BTW I was going to notch the pedal, and actually started as you can see in the pic above, but decided to go with the frame notch instead.

[RRussellTx]

I notched the pedal as a lesser of 2 evils.

My considerations were more risk based.
1. What's the worst thing that could happen if it failed?
And
2. How hard would it be to fix correctly if it failed?

Plus it was much easier to notch the pedal.

[pattratt1]

Does anyone know if FF has corrected the problem yet? It is my understanding they are now using the Willwood pedals in all Mk 4 kits now. Mine is suppsed to be shipped the 23 of January! I msy make a call to FF on this.

[Jeff Kleiner]

Does anyone know if FF has corrected the problem yet?.

FFR has made no changes and in fact the following is the response Jim made on the subject just last week:

Wilwood made a running change to the pedals, going from a cast T shape to a forged H shape which reduced the clearance to the chassis. Because there is still enough throw in the pedal for good clutch disengagement we haven't made any updates around the change. No issues with making a small notch to get extra adjustment on the clutch pedal, just make sure to seal the tube back up so it doesn't create a water trap where it angles down to the front suspension.

Jeff

[j.miller]

The cars I build I cut the tube and put in a cripple....in addition, I upgrade to a Ford OEM or BBK (OEM standard) clutch cable, Cut a hole through the motor mount to give the cable the smoothest path and then add heat wrap to protect from the header......that's just me....da Bat

[Raceral]

FFR does know about the issue. They should make the correction at the Factory...

[russelljones48]

I use what appears to be a new approach. I cut the 3/4" tube just behind the footbox wall, bent the tube towards the outside so that an extension of 5/8' bar inserted into the tube will be parallel to the forward part of the 3/4' tube, inserted another piece of 5/8" bar into the forward tube and bolted them together. It's reasonably well documented in my "Rustlesroadster" thread. I know of another builder who welded it all together rather than bolting.

[Avalanche325]

I consider notching the frame and adding the solid bar and inserts the lesser of two evils. It is likely to be the strongest point o the 3/4" tube. Taking material off of the pedal makes it weaker no matter how you look at it. I haven't heard of anyone breakjng one, so it is probably fine.

Doing the whole web like Karlos's picture is much better than a notch. A notch makes a stress riser (a point of concentrated stress - AKA "break here"). If you did a notch, I would suggest going back and at least filing it to a U shape.

[dougski]

I did the same as russelljones48. Here is a picture of completed mod.

[Al_C]

Thanks for this thread! For me, it is most timely and extremely detailed. Timely because this is about where I am. Now that I'm off the pain meds, it might even make sense! (new shoulder - just need to get thru PT). I was going to do the frame mod - but I like David Hodgkins's approach best. If that doesn't work, I'll notch the pedal.

[SSNK4US]

Ok, my first real post. And it's to something that's a year and a half from the last post lol. And I don't even have a car yet! (My wonderful wife says March order, but I need to get the garage ready) So I've been reading everybody's builds, old and new, to see what I might run into and ideas of things I'd like to do, or build on. I would like to thank EVERONE for their hard work on detailed builds....I will follow in your footsteps.
Anyway......clutch pedal issue that has been beaten to death. Something that I haven't read about.... Everyone talking about the chassis mod is always talking about it being stronger than the original, but has anybody gone the "karlos" route of TMI on chassis mod? Sure edwardb's mod is probably stronger. But should it be? Nobody has said anything about "crumple zones". Is that part of something that should collapse? Bend? Or something in a collision? I'm not an engineer, maybe karlos is? Could edwardb's bolts shear in a collision when they shouldn't? (Sorry to use you as an example but I like the way you did it, your thought process and frankly your build) I'm probably as (if not more) OCD as you guys lol So I way overthink things, like FFR has known of this issue for a while, but hasn't corrected it... Is there a reason? Crumple zone issue? I don't know, maybe only the engineers that actually designed the chassis do? Hopefully nobody ever has to find out.... But it does put a ? in my head lol
Thanks for listening to my OCD and for all you guys do!!! I bought the PDF build manua last weekl and am soooo glad this forum is here!!!!

Kurt

[boat737]

The original builder that started my kit notched the 3/4" frame. When I took over, I inserted a piece of bar stock inside the 3/4" tube to beef it up. http://thefactoryfiveforum.com/showt...terference-Fix

[edwardb]

Since you used me as an example (it's all good ) a couple of responses from my side: (1) Unless something has changed, you won't get an answer from FF about whether it's a crush zone, what strength the tube plays in the chassis strength, etc. because the official party line is there isn't a clutch interference problem. They say you should have adequate clutch pedal movement before the arm interferes with the chassis. I don't agree (and neither do too many others from what I've observed) but there you go. (2) I'm not engineer, but I really can't believe that tube has much to do with crash strength, crush zone, whatever. It's 3/4-inch thinwall tubing (.050 wall thickness, just measured) and would fold up like origami in any kind of hard collision. The strength (and side impact crash resistance) in that area is the 1-1/2-inch horizontal and vertical tubes around the perimeter of the footbox, the x-frame along the side of the footbox, the 2-inch square vertical tube the door is hanging on, the 2-inch horizontal tube across the lower part of the dash, the 2-inch horizontal tube under your feet and under the seat, and probably a couple more I'm forgetting about. Those are all heavy duty thick wall (some DOM) tubing pieces and tie back to the main 4-inch chassis tube. I believe the frame mod I did (which you referenced, and many have done something similar) is as strong or stronger than the unmodified tube. But does it matter for crash resistance? I don't think so. Does it matter whether the bolts would shear or not? I don't think so. (3) I don't know if you've run across it yet, but a full engineering analysis was done on the pedal mod, and the actual data indicates the amount of material removed to clear the interference does not weaken the pedal anywhere near to a breaking point in actual use. Conclusion IMO: Either mod in their various flavors will be fine. Lots of things to obsess over with these builds (and I do my share) but this isn't one of them. Do what you're most comfortable with, including nothing if you can get it to work that way. Then move on.

[SSNK4US]

Example.... YES! Your frame mod is a great looking and a well thought out solution (to something that doesn't exist lol) Your build threads are great. I saw the data posted on the pedal mod, amazing how someone can come up with that stuff. I've just never seen anything written about the frame mod being too strong. What you say makes total sense. Like I said, my OCD kicking in. I value your opinions, I value the info on your builds and enjoy reading all your threads.
Thanks for your input and everything you've done.

[ckrueger]

Did the pedal mod. Quick and easy with a dremel. I am using the Forte external slave cylinder on a coyote. Not much load on the pedal to activate the clutch. I was more comfortable with that than cutting the frame. Folks much smarter than me have calculated the stresses and the frame repairs. If you follow one of their examples, either would be fine.

[RickP]

I notched the pedal. Didn't seem like a viable choice to modify the frame. Pedals are replaceable, frames ... not so much. Do whatever your comfortable with.

[karlos]

Should probably just let this old thread die, but...

It's important to understand that none of the modifications that have been posted will result in better or even equal capability as compared to the basic unmodified tube. For example, the graphic below shows what happens when a portion of the tube is cut out and reinforced with a solid piece bolted to the outside surface.





The repair forces the load to go out and around the cut-out, which induces a bending moment in the tube. The bending moment causes a deflection, and the deflection in turn produces a bending stress that's not present in the unmodified tube. Stresses are low where the tube is reinforced, but notice what happens where the tube transitions back to the un-reinforced section (where the arrow is pointing). Stresses at this location are increased by 42%, even with the repair, as compared to the unmodified tube. This reduces the load carrying capability of the tube by 30%, an even bigger hit than the 16.5% reduction for the pedal mod. Which brings me to my primary motivation for posting this:

Any fix we the end user can make is a band-aid that results in some loss of the original strength. Unacceptable in my opinion given the length of time that this has been a known issue. FFR should make a modification to preclude the problem in the first place and not force its customers to choose between a small handful of not-so-great options. And I'm not buying the explanation that there is enough pedal travel prior to the parts contacting one another. There simply isn't.

In any event, either fix will get the job done. The frame mod carries a bigger strength hit than the pedal mod, but the frame probably has way more reserve strength to begin with. Six of one, half-dozen of the other.

[SSNK4US]

I guess I opened a 1 1/2 year old can of worms lol Sorry for that... But that's me lol Thank you karlos. Are you some kind of engineer? That's what I was looking for, a viable numbers answer. But as edwardb said, no real structural / safety issue.
And as you put it, six of one, half-dozen of the other. Thank you all for putting up with, and answering my crazy question. I'm sure it won't be my last. And who knows, when I get mine it may not even be an issue.
Thank you all for everything!