Early Trailing Arm Geometry for Widebody

[Jandrews]

There is a lot of discussion material available on this topic, but after extensive searches, I could not find definitive answers to my questions.

There are many early cars that have been retro-fitted with RSR and turbo-bodied flares. As you know the trailing pickup points on the early cars result in a situation where the wheel/tire sit way inboard of the 11" wide flares. Many (most) people compensate for this with super-wide spacers/adapters. Others, go all the way and relocate the pickup points and go with coilover suspensions.

For those with large 3"+ spacers/adapters, many say that is an unsafe or sub-optimal arrangement. The arguement being that the wider stance put substantial load on the bearing in the trailing arm, as well as creating a "weak" spot at the spacer/adapter/longer lug bolts.

I guess my questions are these: Do the adapters truly represent a weaker junction than what a deep-dish offset wheel would? Does the deep-dish offset wheel "solve" this problem? In that case, the location of the wheel bearing and the location of the tire are no different than they are with adapters. Then, as a follow-on, is re-locating the trailing arm pickup points and converting to coilovers the only "real", "valid" solution to this problem? Then, finally, if so, what can be done with the front to deal with the "revised" track width front to back?

Thanks in advance,

JA

[304065]

Well, Porsche used a spacer program for years so I'm not sure what the specific concern is.

That said, the huge adapters don't seem like a good idea. Here's my layperson understanding of the forces involved. Ideally you don't want the wheel bolts loaded in shear-- you want them loaded in TENSION, holding the wheel so tightly to the hub that it can't move relative to the wheel. That said, some portion of the load on the bolts is experienced in shear, or perpendicular to the bolt axis, particularly on acceleration and deceleration.

The longer you make the bolt the more force you put on it-- remember Newton's law: Load arm X load force = effort arm X effort force. A force applied at the end of a long bolt will bend it more than the same force applied closer to the hub. So you ideally want the bolts to be as short as possible while still maintaining proper thread engagement. This is the big drawback of the spacer.

I can hear the pundits now. . . "Porsche did this on the RSR bla bla bla" Porsche probably threw the bolts out after every race.

[gestalt1]

or you use low offset wheels with no spacers an the wheel fits the flare. the goal is to get more tire, not so much to widen the track.

[Jandrews]

Thanks John and gestalt1.

With regard to the "shear vs tension" issue, with adapters, the lug bolts really aren't any longer. For example, it seems a three inch adapter would effectively "widen the hub", as opposed to increase shear on lug bolts. But, I'm no engineer. And to gestalt1's point, I agree, more tire is the goal, but does it matter if you put a 315mm tire on a deep-dish offset wheel like shown in your picture.... or a late offset wheel with a large spacer? Is there any safety/stress differences other than aesthetics? By the way, gestalt, that's a nice looking project you have there.

Thanks,

JA

[fastbacker]

Originally Posted by gestalt1

the goal is to get more tire, not so much to widen the track.

The car with a wider track and the same size tires as another is going to handle better.

[Bill Verburg]

When you cahnge the O/s of the wheel you change bearing load and scrub radius, scrub radius isn't important in back but bearing life and reliability is.

Turbo rear suspension has always used wider track trailing arms, spacers and beefed up bearings compared to n/a 911.

The safest thing to do is use 930 suspension pieces and pickup points w/ wheels that have appropriate o/s and width.

[Jandrews]

Ahh...that's the answer I was looking for Bill. The wheel bearing is under additional load with a late offset wheel and spacer vs a deep-early offset.

[klaucke]

John, I think you misinterpreted Bill (although maybe I did the misinterpreting -- I won't speak for anyone here but myself). The Turbo trailing arms have a wider track than stock trailing arms, even w/o spacers. Porsche widened this further on the Turbo with the addition of spacers. Therefore, if we compare widebody cars with narrow body suspension vs. Turbo suspension, for the same size wheel, bearing loads are increased on the narrow body suspension when compared with the wide body. The narrow body wheel will require less backspacing compared with the Turbo wheel.

As far as the bearing loads are concerned, a wheel with a lot of backspacing used in conjunction with a spacer will apply the same loads as a wheel with no spacer, but backspacing equal to the 1st wheel w/ a spacer. In order to minimize bearing loads in the hub, the required backspacing to the hub face should be maximized.

That said, many people choose to run narrow body suspension for widebodies with wide wheels and tires. This does increase bearing load, but it is not catastrophic. Bearings are a wear item anyway and these owners must simply account for it. Actual bearing life will depend on a lot of variables.

[Bill Verburg]

the constraints are where the outer edge of the wheel sits, where the hub sits and the amount of back space room.

for 930 flares on a 930 suspension
an ET+15 9" wheel needs a 28mm spacer
an ET+12 11" wheel needs no spacer.
Bearing load is the same for either

but if the 911 suspension is used it places the hub ~1.3" further inboard. To do that w/ an 11" wheel you either need a 1.3" spacer or ET-21 wheel
neither increases the bearing load on a large 930 bearing over the other but both increase the load that a 911 bearing would see.

or you could use a 13" wheel but there isn't enough back space room so an o/s that increases bearing load is needed.

[Jandrews]

Now I am a little confused... Let's take the 930 out of it for now. Take an early car with 11" rear flares. Put a deep-dish offset wheel on it like the Lindsey's, with little or no spacer. Then, take that same car, and put a late offset wheel on it like a Cup wheel, with about a 3" spacer to properly locate the tire in the fender well. Which one creates more load on the early trailing arm/bearing?

Thanks,

JA

[gestalt1]

Originally Posted by Jandrews

Now I am a little confused... Let's take the 930 out of it for now. Take an early car with 11" rear flares. Put a deep-dish offset wheel on it like the Lindsey's, with little or no spacer. Then, take that same car, and put a late offset wheel on it like a Cup wheel, with about a 3" spacer to properly locate the tire in the fender well. Which one creates more load on the early trailing arm/bearing?

Thanks,

JA

both examples would have the same effect - no difference. think of the length from the outside edge of the wheel to the wheel bearing as a lever. the longer the lever the more leverage the forces acting on the wheel have on the wheel bearing. moving the wheel bearing out (widening the track) reduces the lever length and the forces acting on the wheel bearing. think of the spacer as just a part of the wheel. if the distance from the outside edge of the wheel to the wheel bearing is not changed then weather there is a spacer or not does not matter.

as a side note about positive offset vs. negative offset it is important to think about the scrub radius, or where across the tread is the pivot point of the suspension geometry. this is not that important on a semi trailing arm rear suspension as it is on a front suspension where the wheels are also steering.
-matt

[Jandrews]

Ok, now that is clear....thanks gestalt1. That's the way I saw it initially...the adapter is part of the wheel.

Now that we have established (I think) the bearing load issue, i.e. there is none, let's move on to the safety question. In the event of super-long studs to accomodate standard flat spacers, I would see an increased risk due to the shear stress on the lug bolts. However, in the case of say a 3" adapter, where there is no additional length to the studs, just an additional set of bolts and nuts, are there any strength/safety issues over just a deep-dish offset wheel with no adapter?


JA

[Bill Verburg]

Originally Posted by gestalt1

both examples would have the same effect - no difference. think of the length from the outside edge of the wheel to the wheel bearing as a lever. the longer the lever the more leverage the forces acting on the wheel have on the wheel bearing. moving the wheel bearing out (widening the track) reduces the lever length and the forces acting on the wheel bearing. think of the spacer as just a part of the wheel. if the distance from the outside edge of the wheel to the wheel bearing is not changed then weather there is a spacer or not does not matter.

That's not true. The lever is created by the wheel o/s. A zero o/s wheel has 1/2 it's width inboard of the hub and 1/2 outboard. There is no leverage.

In the case of 2 wheels of the same width, 1 w/ a built in +24.5mm o/s and one w/ +49mm o/s. A 24.5mm spacer used w/ the latter wheel results in a net +24.5mm o/s and the same leverage on the bearing.

Note these are all back of the envelope calculations, but are representative

A 9x16 Fuchs on a stock 911 rear hub has an o/s of +15 and mostly fills a 9" SC/Carrera flare. Add 2" to that flare and you have the 11" 930 flare. To fill that you need a ~2" spacer or a ~2" wider wheel, all on the outboard side. Or some combination of spacer and wider wheel. When width is added to the outboard side it reduces the o/s. A 9" ET15 wheel w/ a ~2 " spacer to fill a 930 flare from a 911 hub has a net o/s of -45. It places the outer edge of the wheel where the same wheel w/ a 930 28mm spacer would place it. But in doing so drastically increases the bearing load. o/s goes form +15 on a 911 to -45 w/ 930 flares.

What most would want is to use an 11" wheel which needs -21mm o/s for the same wheel placement in a 930 flared 911. This still increases the bearing load a lot.

Alternatively a 13" wheel w/ +5ET does the same but w/ less bearing stress

[gestalt1]

you are right bill, however you did confirm my point about adding width to the outside of the wheel only increases the load on the wheel bearing (not to mention the extra grim from the wider tire). keeping the hub mounting surface as near to the the middle of the wheel width is the goal. i'm a little confused about the 13" wheel idea, i'm assuming you are adding the width to both the inside and the outside of the wheel so the hub surface is still in the mid point of the wheel width. unfourtunatly a 13" wheel will not fit 11" flares.

[Bill Verburg]

Here is the data

the 2 most critical criteria that are fixed by the chassis/fenderstrailing arms are front space wrt hub and backspace wrt hub. These 2#s determine what fits(not counting brakes and non stock suspension pieces. The former determines how the wheel sits wrt the fender lip, the latter wrt to the inner wall(trailing arm bolts and oil lines are the usual limits)

note: that stock front space wrt hub for stock 9" fitment is ~5.3" so if you wish to have a wheel look like stock wrt to the lip that is the # to look at.

note: when stock 911 trailing arms are used the hub sits ~1.3" further inboard than for 930 arms. To get the same appearance wrt the lip the front space wrt hub must be ~6.6"

note: none of the back space #s come close to the limit imposed by trailing arm bolts.


Assumption: 930 trailing arm o/s is ~1.3"

as you can see a 13" wheel will fit when a 911 suspension is used.

[Jandrews]

Well, conventional, logical wisdom be damned! I truly understand Bill's point. He is saying that the axis of the wheel in relation to the hub determines the leverage, not just the position of the outside edge of the wheel.

Bill, do you have that data for any other wheels? I can tell you that a 10x17 Cup wheel sits just about perfect with a 3" spacer, but that puts almost the entire wheel/tire assembly OUTBOARD of the actual hub! Must be something like -65 o/s!!

My next question was going to be, "do you think this is significant", but I think anyone can see from the data that it is. The additional load put on the wheel bearing by effectively lengthening the lever 3+ inches HAS to be substantial, maybe exponential. I think the conclusion here is to try to locate a wheel fitment that approaches 0 or slightly positive o/s, similar to what Bill shows in the table with the 11" or 13" specs.

Bill, which specific 11x wheel is your table referring to that produces the 21mm o/s? Also, have you ever seen a 13" wheel that would work in this application, preferably 17" to enable tire choices?

Thanks for the incredible lesson here...exactly what I was looking for. This thread will go in my personal archives.

JA

[Chuck Moreland]

That's not true. The lever is created by the wheel o/s. A zero o/s wheel has 1/2 it's width inboard of the hub and 1/2 outboard. There is no leverage.

True if the bearing is in the same plane as the hub. In practice, the bearing is inboard of the hub. The true "lever arm" is the distance between wheel center and bearing center.

[Bill Verburg]

Originally Posted by Chuck Moreland

True if the bearing is in the same plane as the hub. In practice, the bearing is inboard of the hub. The true "lever arm" is the distance between wheel center and bearing center.

Yeah, I know. I Thought about clarifying that but didn't want to muddy the water any more. It should more accurately read, o/s increases or decreases leverage on the bearings by the amount of o/s. os > 0 will allways increase bearing load o/s < 0 will decrease the bearing load down to the depth of the bearing then increase it beyond that

the only real wheels there are the 9x16 Fuchs and the 10x17 Ruf. The rest are all hypothetical customs w/ any o/s you desire. The reality is that o/s and width are only available incrementally. You make your choice from the menu they offer.

the other thing is tire selection, some people may want +/- o/s to fit specific tires.

[BRAIDusa]

Originally Posted by Bill Verburg

Here is the data

the 2 most critical criteria that are fixed by the chassis/fenderstrailing arms are front space wrt hub and backspace wrt hub. These 2#s determine what fits(not counting brakes and non stock suspension pieces. The former determines how the wheel sits wrt the fender lip, the latter wrt to the inner wall(trailing arm bolts and oil lines are the usual limits)

note: that stock front space wrt hub for stock 9" fitment is ~5.3" so if you wish to have a wheel look like stock wrt to the lip that is the # to look at.

note: when stock 911 trailing arms are used the hub sits ~1.3" further inboard than for 930 arms. To get the same appearance wrt the lip the front space wrt hub must be ~6.6"

note: none of the back space #s come close to the limit imposed by trailing arm bolts.


Assumption: 930 trailing arm o/s is ~1.3"

as you can see a 13" wheel will fit when a 911 suspension is used.

Bill, I hope you are still subscribed to this thread. Can you confirm that rear 930 wheel mounting plate is 1.3" further outboard than 911?

Cheers

[Bill Verburg]

Originally Posted by BRAIDusa

Bill, I hope you are still subscribed to this thread. Can you confirm that rear 930 wheel mounting plate is 1.3" further outboard than 911?

Cheers

Yes, each side of the rear 930 pushes it's wheel out 1.3"(33mm) more than a 911.

Then they added 28mm spacers to that so that they could use the same wheels as were used on the 911.

in the front of a 930 they use a 21mm spacer, till '80 it was a removable spacer, from '80 they increased the hub o/s 21mm