Differential Rotation (under car video)
Thread Starter
Le Mans Master
Joined: Dec 2003
Posts: 5,768
Likes: 33
From: Van Steel Corvettes
St. Jude Donor '12-'13
Yep, new camera location to watch the C-beam is already in the plans. Have to rotate the tires saturday before sunday's event and while I'm under there was going to see if there is a good place for the GoPro. This car has thrown me (and my very good C4 mechanic) for a loop many times over...this just tops the cake so far.
On a side note, Paul from van steel is thinking about having me ditch the lower heim joint trailing arms for a set of STOCK rubber bushing arms to see how the car reacts on the autocross (especially since I tend to run on loose tracks). That was BEFORE this video. We'll see how it goes.
On a side note, Paul from van steel is thinking about having me ditch the lower heim joint trailing arms for a set of STOCK rubber bushing arms to see how the car reacts on the autocross (especially since I tend to run on loose tracks). That was BEFORE this video. We'll see how it goes.
Last edited by shakedown067; Apr 11, 2013 at 12:33 PM.
Thread Starter
Le Mans Master
Joined: Dec 2003
Posts: 5,768
Likes: 33
From: Van Steel Corvettes
St. Jude Donor '12-'13
I'm trying to hunt down a good A4 c-beam locally just in case....
Damn, totally forgot to check out the motor mounts yesterday. I'll have to watch the motor when it's on the dyno Saturday.
I have this awesome vision of a custom built, massive billet aluminum C-beam...and the vision of an empty bank account.
Damn, totally forgot to check out the motor mounts yesterday. I'll have to watch the motor when it's on the dyno Saturday.
I have this awesome vision of a custom built, massive billet aluminum C-beam...and the vision of an empty bank account.
Last edited by shakedown067; Apr 11, 2013 at 12:46 PM.
Race Director
Joined: Apr 2003
Posts: 17,700
Likes: 1,667
From: moraga ca
Hello shakedown067,
Since the rear end is basically attached to the car by the bushings in the end of the batwing and the C-beam it would seem that your flex would be caused by either a possible loose bolt(s) on the c-beam or worn bushings on the batwing. There is the possibility that the c-beam is flexing or could have a crack in it. By design, I do not think you can eliminate all the flex without making some kind of mod to the setup. Maybe even solid bushings in the batwing.
I am wondering if the addition of a lower support from the bottom of the diff to the frame would help to keep it in place and alleviate the flex you are seeing.
See the attached pic for a simple fix. I know it is not a C4 frame but you get the idea. I have also seen where the c-beam gets additional gussets added to help stiffen it up to reduce flex.
Anyway, just some thoughts for you.
Since the rear end is basically attached to the car by the bushings in the end of the batwing and the C-beam it would seem that your flex would be caused by either a possible loose bolt(s) on the c-beam or worn bushings on the batwing. There is the possibility that the c-beam is flexing or could have a crack in it. By design, I do not think you can eliminate all the flex without making some kind of mod to the setup. Maybe even solid bushings in the batwing.
I am wondering if the addition of a lower support from the bottom of the diff to the frame would help to keep it in place and alleviate the flex you are seeing.
See the attached pic for a simple fix. I know it is not a C4 frame but you get the idea. I have also seen where the c-beam gets additional gussets added to help stiffen it up to reduce flex.
Anyway, just some thoughts for you.
Burning Brakes
Joined: Jan 2012
Posts: 1,191
Likes: 153
From: Gilbert AZ
2018 C1 of Year Finalis4
Hello 383vett
Yes, that is true. I was just offering a suggestion using that picture as an example. I think you might still be able to come up with a Y shaped support that could tie to the frame though that would spread the attachment points out quite a bit and there would also be some inherent flex with longer mounting points . There is also the exhaust to content with when adding a lower sport brace.
It is possible to have extra pieces of aluminum welded to the C-Beam to stiffen it up and that would allow for the rest of the system to re-main as the GM engineers designed it.
What are you running in your race car?
Do you still have the batwing and c-beam for your rear end setup?
Yes, that is true. I was just offering a suggestion using that picture as an example. I think you might still be able to come up with a Y shaped support that could tie to the frame though that would spread the attachment points out quite a bit and there would also be some inherent flex with longer mounting points . There is also the exhaust to content with when adding a lower sport brace.
It is possible to have extra pieces of aluminum welded to the C-Beam to stiffen it up and that would allow for the rest of the system to re-main as the GM engineers designed it.
What are you running in your race car?
Do you still have the batwing and c-beam for your rear end setup?
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
whats hard about adding a cross-bar type bracket between the rails ahead of the diff-carrier?
NONE....then the link ( J-link) is cake. AND a solution as stated previously.
A properly designed link would act as a lever that would push up and forward against a cross brace as the diff tried to rotate. In other words, if designed correctly it would be pushing against the car. I doubt the diff would win that arguement.
This could also be done with a stiff hyd dampnener to act as a shock absorber. Allowing some degree of movement but making it controllable...predictable.
I see many possibilities in the "Cola" design.
NONE....then the link ( J-link) is cake. AND a solution as stated previously.
A properly designed link would act as a lever that would push up and forward against a cross brace as the diff tried to rotate. In other words, if designed correctly it would be pushing against the car. I doubt the diff would win that arguement.
This could also be done with a stiff hyd dampnener to act as a shock absorber. Allowing some degree of movement but making it controllable...predictable.
I see many possibilities in the "Cola" design.
Thread Starter
Le Mans Master
Joined: Dec 2003
Posts: 5,768
Likes: 33
From: Van Steel Corvettes
St. Jude Donor '12-'13
So what I'll be exploring this weekend.
Motor mounts: watch engine for movement under hard acceleration on dyno saturday (won't surprise me if this is it)
C-beam: find location for camera to watch for flex in the support beam. If replacing, upgrading to beam-plates.
Batwing poly bushings: install last incase problem is elsewhere. Don't want that to be more rigid right now as the old rubber bushings should help with relieving the impact of the deflection, assuming it's not the bushings themselves causing it.
Motor mounts: watch engine for movement under hard acceleration on dyno saturday (won't surprise me if this is it)
C-beam: find location for camera to watch for flex in the support beam. If replacing, upgrading to beam-plates.
Batwing poly bushings: install last incase problem is elsewhere. Don't want that to be more rigid right now as the old rubber bushings should help with relieving the impact of the deflection, assuming it's not the bushings themselves causing it.
Drifting
Joined: Jun 2001
Posts: 1,704
Likes: 459
From: Mass Mass
I stared at your video for awhile (to long ) and it appears the bottom of the differential is rotating forward, or swinging forward pivoting on the Bat Wing ears.
If the drivers side engine mount was torn, the engine would exert a counter clockwise rotation on the Torque Tube, and the "Bat Wing" looks to be designed to resist this rotation.
However, I don't see any way that force would try to rotate the differential forward, unless the torque tube twisted up badly (and shortened), and I don't think you have the power for that yet.
The bottom of your differential is rotating forward (in the plane of the wheels) and the Batwing design has no real resistance to that rotation (just the two rubber mounts).
The only part resisting that rotation is the Torque Tube.
Either your lifting the entire weight of the engine and transmission off the engine mounts, which I doubt even if they are worm out - or torn.
Or my guess, something is wrong with the Torque Tube to Differential attachment joint.
Either the bolts are not tight, or maybe a crack in the case you can't see until power is applied.
Just my opinion
Interested to see what it turns out to be.
And you have to love these new camera's.
) and it appears the bottom of the differential is rotating forward, or swinging forward pivoting on the Bat Wing ears.If the drivers side engine mount was torn, the engine would exert a counter clockwise rotation on the Torque Tube, and the "Bat Wing" looks to be designed to resist this rotation.
However, I don't see any way that force would try to rotate the differential forward, unless the torque tube twisted up badly (and shortened), and I don't think you have the power for that yet.
The bottom of your differential is rotating forward (in the plane of the wheels) and the Batwing design has no real resistance to that rotation (just the two rubber mounts).
The only part resisting that rotation is the Torque Tube.
Either your lifting the entire weight of the engine and transmission off the engine mounts, which I doubt even if they are worm out - or torn.
Or my guess, something is wrong with the Torque Tube to Differential attachment joint.
Either the bolts are not tight, or maybe a crack in the case you can't see until power is applied.
Just my opinion
Interested to see what it turns out to be.
And you have to love these new camera's.
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Thread Starter
Le Mans Master
Joined: Dec 2003
Posts: 5,768
Likes: 33
From: Van Steel Corvettes
St. Jude Donor '12-'13
Nice info SuperL98! I'm still curious how I sheared the top of the bolts off where the C-beam connects to the transmission last summer. Nuts and top of the bolts where just gone. I've managed to do all kinds of unique things with this car.
Good thing I'm not into drag racing....
Good thing I'm not into drag racing....
Race Director
Joined: Apr 2003
Posts: 17,700
Likes: 1,667
From: moraga ca
Hello 383vett
Yes, that is true. I was just offering a suggestion using that picture as an example. I think you might still be able to come up with a Y shaped support that could tie to the frame though that would spread the attachment points out quite a bit and there would also be some inherent flex with longer mounting points . There is also the exhaust to content with when adding a lower sport brace.
It is possible to have extra pieces of aluminum welded to the C-Beam to stiffen it up and that would allow for the rest of the system to re-main as the GM engineers designed it.
What are you running in your race car?
Do you still have the batwing and c-beam for your rear end setup?
Yes, that is true. I was just offering a suggestion using that picture as an example. I think you might still be able to come up with a Y shaped support that could tie to the frame though that would spread the attachment points out quite a bit and there would also be some inherent flex with longer mounting points . There is also the exhaust to content with when adding a lower sport brace.
It is possible to have extra pieces of aluminum welded to the C-Beam to stiffen it up and that would allow for the rest of the system to re-main as the GM engineers designed it.
What are you running in your race car?
Do you still have the batwing and c-beam for your rear end setup?
Last edited by 383vett; Apr 12, 2013 at 03:20 PM.
Drifting
Joined: Jun 2001
Posts: 1,704
Likes: 459
From: Mass Mass
This is all interesting stuff. 
Cast aluminum fails easier in tension, and this looks like a tension break to me.
Look at how deformed the Batwing arms are.
Looks like you literally twisted the rear Batwing clockwise ... Impressive
Cast aluminum fails easier in tension, and this looks like a tension break to me.
Look at how deformed the Batwing arms are.
Looks like you literally twisted the rear Batwing clockwise ... Impressive
Melting Slicks
Joined: Mar 2008
Posts: 2,141
Likes: 76
From: Santa Maria, CA
I'll lay odds there's a buildup of tolerances from elongated C Beam attachment holes and worn batwing bushings. I can't imagine the C Beam flexing as much as it would have to for the differential to rotate that much. The fact there has been a history of sheared C Beam bolts says there is lateral movement at the connect points.
Melting Slicks
Joined: Mar 2008
Posts: 2,141
Likes: 76
From: Santa Maria, CA
It could be that simple. The plates should help increase the clamping area. The extra material should prevent elongation in the future. Also, GM used what I believe was epoxy to fill gaps between the C beam and the attach points. You might want to consider that as well.
Burning Brakes
Joined: Jan 2012
Posts: 1,191
Likes: 153
From: Gilbert AZ
2018 C1 of Year Finalis4
383vett,
I see. That would make sence as the stock c-beam would not hold up to the amount of torque you have to be putting out to get the launches you are making. I assume the batwing is gone and it’s a solid axel since it’s a 12 bolt.
Your torque arm set up is interesting. Thanks for sharing.
I see. That would make sence as the stock c-beam would not hold up to the amount of torque you have to be putting out to get the launches you are making. I assume the batwing is gone and it’s a solid axel since it’s a 12 bolt.
Your torque arm set up is interesting. Thanks for sharing.
Melting Slicks
Joined: Jun 2011
Posts: 2,384
Likes: 385
From: Leland , North Carolina
I'll lay odds there's a buildup of tolerances from elongated C Beam attachment holes and worn batwing bushings. I can't imagine the C Beam flexing as much as it would have to for the differential to rotate that much. The fact there has been a history of sheared C Beam bolts says there is lateral movement at the connect points.
Race Director
Joined: Apr 2003
Posts: 17,700
Likes: 1,667
From: moraga ca
383vett,
I see. That would make sence as the stock c-beam would not hold up to the amount of torque you have to be putting out to get the launches you are making. I assume the batwing is gone and it’s a solid axel since it’s a 12 bolt.
Your torque arm set up is interesting. Thanks for sharing.
I see. That would make sence as the stock c-beam would not hold up to the amount of torque you have to be putting out to get the launches you are making. I assume the batwing is gone and it’s a solid axel since it’s a 12 bolt.
Your torque arm set up is interesting. Thanks for sharing.
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
Something I forgot about...might/probably don't matter but I wanted to mention it so you can continue to collect info to hopefully solve this problem...
I took a broadside hit a few yrs ago at about 35mph. The brunt of the impact was on top of the rt rear wheel. This was somewhat dampened by the door and the rear door jam that absorbed a LOT of the impact forces...destroyed the damn door and the jam had to be rebuilt...a slow and painstaking process to lay glass and shape those sharp angles,...but it got done.
Point being, there was suspension damage that was unique to the Corvette and
the C-beam...
The 2 control rods were slightly bent. The side impact with good, tight bushings and all was in nice tight shape...they did not have a lot of "give" to move sideways. The batwing mounts were bent about 1/4" along with the rear ends fender supports slightly out of place. Body seams were all OK, but the underside needed work.
The most interesting part of this was the transmission...the rear case was broken...where the C-Beam had TRIED to move sideways...it could not because it was a solid attachment from diff to tranny...so the trans case broke. Broke BIG too...not cracked...broke off. Big mess.
So, that C-beam WILL transmit quite a lot of force before it deflects. I would have thought that the beam would have bent...but no. It simply transmitted the forces to the weakest part...the trans case. The C-beam was completely uneffected by the incident and bolted up to the new parts exactly as it should...not pulling pushing or prying to get bolts to line up...it was where it needed to be..straight and undamaged.
I took a broadside hit a few yrs ago at about 35mph. The brunt of the impact was on top of the rt rear wheel. This was somewhat dampened by the door and the rear door jam that absorbed a LOT of the impact forces...destroyed the damn door and the jam had to be rebuilt...a slow and painstaking process to lay glass and shape those sharp angles,...but it got done.
Point being, there was suspension damage that was unique to the Corvette and
the C-beam...
The 2 control rods were slightly bent. The side impact with good, tight bushings and all was in nice tight shape...they did not have a lot of "give" to move sideways. The batwing mounts were bent about 1/4" along with the rear ends fender supports slightly out of place. Body seams were all OK, but the underside needed work.
The most interesting part of this was the transmission...the rear case was broken...where the C-Beam had TRIED to move sideways...it could not because it was a solid attachment from diff to tranny...so the trans case broke. Broke BIG too...not cracked...broke off. Big mess.
So, that C-beam WILL transmit quite a lot of force before it deflects. I would have thought that the beam would have bent...but no. It simply transmitted the forces to the weakest part...the trans case. The C-beam was completely uneffected by the incident and bolted up to the new parts exactly as it should...not pulling pushing or prying to get bolts to line up...it was where it needed to be..straight and undamaged.
Race Director
Joined: Apr 2003
Posts: 17,700
Likes: 1,667
From: moraga ca
Something I forgot about...might/probably don't matter but I wanted to mention it so you can continue to collect info to hopefully solve this problem...
I took a broadside hit a few yrs ago at about 35mph. The brunt of the impact was on top of the rt rear wheel. This was somewhat dampened by the door and the rear door jam that absorbed a LOT of the impact forces...destroyed the damn door and the jam had to be rebuilt...a slow and painstaking process to lay glass and shape those sharp angles,...but it got done.
Point being, there was suspension damage that was unique to the Corvette and
the C-beam...
The 2 control rods were slightly bent. The side impact with good, tight bushings and all was in nice tight shape...they did not have a lot of "give" to move sideways. The batwing mounts were bent about 1/4" along with the rear ends fender supports slightly out of place. Body seams were all OK, but the underside needed work.
The most interesting part of this was the transmission...the rear case was broken...where the C-Beam had TRIED to move sideways...it could not because it was a solid attachment from diff to tranny...so the trans case broke. Broke BIG too...not cracked...broke off. Big mess.
So, that C-beam WILL transmit quite a lot of force before it deflects. I would have thought that the beam would have bent...but no. It simply transmitted the forces to the weakest part...the trans case. The C-beam was completely uneffected by the incident and bolted up to the new parts exactly as it should...not pulling pushing or prying to get bolts to line up...it was where it needed to be..straight and undamaged.
I took a broadside hit a few yrs ago at about 35mph. The brunt of the impact was on top of the rt rear wheel. This was somewhat dampened by the door and the rear door jam that absorbed a LOT of the impact forces...destroyed the damn door and the jam had to be rebuilt...a slow and painstaking process to lay glass and shape those sharp angles,...but it got done.
Point being, there was suspension damage that was unique to the Corvette and
the C-beam...
The 2 control rods were slightly bent. The side impact with good, tight bushings and all was in nice tight shape...they did not have a lot of "give" to move sideways. The batwing mounts were bent about 1/4" along with the rear ends fender supports slightly out of place. Body seams were all OK, but the underside needed work.
The most interesting part of this was the transmission...the rear case was broken...where the C-Beam had TRIED to move sideways...it could not because it was a solid attachment from diff to tranny...so the trans case broke. Broke BIG too...not cracked...broke off. Big mess.
So, that C-beam WILL transmit quite a lot of force before it deflects. I would have thought that the beam would have bent...but no. It simply transmitted the forces to the weakest part...the trans case. The C-beam was completely uneffected by the incident and bolted up to the new parts exactly as it should...not pulling pushing or prying to get bolts to line up...it was where it needed to be..straight and undamaged.