Register now to get rid of these ads!

Technical Engine cross-member fabrication, engine mount angles?

Discussion in 'The Hokey Ass Message Board' started by BitsAndPieces, Mar 12, 2018.

  1. BitsAndPieces
    Joined: Mar 12, 2018
    Posts: 3

    BitsAndPieces

    Hi guys,

    1) I am wondering why most engine cross-members for RWD engines have the engine mounts at approximately 45-degrees on either side...? Is this due to rigidity of cross-members's connection/etc(?), or does it come down to the arc that the engine wants to push/pull in and is required to damped and isolate that movement and NVH??

    2) I understand that there are differing styles and materials of engine mounts... What would be important to consider if I fabricated my engine mounts to sit either at 0 or 180-degrees (vertical/horizontal) to my engine? I think it's safe to assume that if these typical 45-degree angles aren't used on either side there would be different loads on the engine mounts and they could have compromised longevity or different NVH damping?

    Backstory: My build has hit a roadblock and I've deiced to fabricate an engine cross-member in order to gain extra header clearance for the V8 I'm putting in.
     
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 39,752

    squirrel
    ALLIANCE MEMBER

    If the mounts are at 45 degree angle, the weight of the engine is supported in both the vertical and horizontal directions, but it also allows the engine to twist a little and the mounts absorb torsional vibrations. It works really well.

    Newer cars seem to have more of a tube in a tube, with rubber between them, mounted front to back. This eliminates the problem of a laminated flat mount coming apart and letting the engine move far enough to affect the throttle linkage (this was a big problem for GM in the 1960s, recalls and lawsuits, etc)

    If you make a sketch of what you intend to do, we might be able to offer more useful suggestions.
     
    Hnstray likes this.
  3. porknbeaner
    Joined: Sep 12, 2003
    Posts: 40,351

    porknbeaner
    Member

    Bisquit mounts used a horizontal mount, but most side moutns are built like they are for the exact reason that Jim ( @squirrel ) described.

    I suppose that if the mount cam out perpendicular to the mount on the motor and was properly gusseted that it work just fine.
     
  4. Fordors
    Joined: Sep 22, 2016
    Posts: 1,127

    Fordors
    Member

    For me it was a function of how everything fits together more than anything else. If you are building a hot rod there will be the height of the frame rails to consider (after the suspension is set up), the floor/trans hump and also the aesthetics. I built the frame for my channeled coupe with the engine and trans relatively low so that I could have a flat floor and a small hump over the bell housing. The mounts on my frame are only about 10° from horizontal and I have stock type SBC rubber mounts with the engine low in the chassis. I have adequate oil pan clearance but then again my frame is not as low slung as others have built.
    I wanted to be able to drive long distances (think leg room in a channeled coupe) and I also considered being able to negotiate bumps, the occasional pot hole and steep driveways.
    The first photo shows the top of the T-10 trans level with the top of the frame and also how small the bell housing hump would need to be, and the 2nd and 3rd show the mounts at a 10° angle.
    I tried to tighten up the post but I'm not computer literate enough to eliminate the wasted space.


    Scan_0063.jpg Scan_0064.jpg Scan_0065.jpg
     
    Register now to get rid of these ads!

  5. BitsAndPieces
    Joined: Mar 12, 2018
    Posts: 3

    BitsAndPieces

    Hey guys, thanks for your excellent responses.

    Although expensive, I'm considering buying a large sheet of think plate and getting the profile laser cut and then having my local machine shop with a very large press bend it in 2 locations on each end.

    What are you thoughts on how well the engine mounts will last and their vibration damping when positioned horizontal like this on each side??

    Cheers!
     

    Attached Files:

  6. squirrel
    Joined: Sep 23, 2004
    Posts: 39,752

    squirrel
    ALLIANCE MEMBER

    that does not look very strong....

    here is a pic of the mounts I made for this little Chevy, it has a blown 427 and has run high 9s and driven 20,000 miles. There was another piece added to each side, to fill in the opening inside. It's all just 1/8" x 2" mild steel.The materials cost about five bucks, I think.

    If you arrange the geometry right, you can make a very strong structure with very little material.

    hint: flat plates are not very strong when laid flat, and asked to support weight across a long span

    progress07.jpg
     
  7. BitsAndPieces
    Joined: Mar 12, 2018
    Posts: 3

    BitsAndPieces

    I need something that's going to give me the most cubic area under the exhaust ports and between the block and the rail - that's pretty much the main design criteria.

    I think that if something like my picture above was attempted, it could certainly be done with plate say 1/4" thick plate... Obviously it would be heavy and there is little way of getting around that, but portions from the center of the cross-member could be cut out at the same time the template is cut.

    What do you think about the positioning of the engine mounts? All else disregarded, do you think they are compromised if I used a design where they are positioned like this??
     
  8. Hnstray
    Joined: Aug 23, 2009
    Posts: 9,488

    Hnstray
    ALLIANCE MEMBER
    from Quincy, IL

    The mount brackets and insulators are the least of the problem with this design. That setup would probably work okay, even if not optimal. It is the ‘engine cross member’ tha is problematic. The cross section of the cross member is probably about 1/4” to 3/8” by 2” or 2.5”. No matter, the point is....that piece of material would be the strongest in this application with the 2” side vertical ....and weakest with the 1/4” side carrying the load, as you have pictured.

    And the farther the mounts are from the frame rail the worse the bending ‘moment’. The most extreme example being if there were a single engine mount in the center of the engine cross member. A better choice for the cross member material would be square or rectangular tube ...for example, a 1.5” x 1.5” with a .125” wall thickness. There are other dimensions that would work as well, or better, but this should serve to illustrate the point.

    The other consideration would be to NOT use a cross member, but short brackets from each side of the frame rail to support the mounts, as in the photo Squirrel included. However, that would depend on frame rail configuration, i.e., ‘C’ channel or box section, the distance from the nearest crossmember, the length of the bracket, the weight of the engine, etc.

    Ray
     
  9. I think the mount and biscuits are fine,
    The plain rubber ones are cheap and easy and clean looking. There's others that are a similar concept but hydraulic used in some high end luxury cars. They are not as clean looking but far superior in dampening.

    You need better support them in the item marked you have crossmember.
    Here's a few ideas to start your wheels turning
    image.jpeg image.jpeg image.jpeg

    There are others too. Cumbersome but great dampening
    image.jpeg
     
    Last edited: Mar 14, 2018
  10. indyjps
    Joined: Feb 21, 2007
    Posts: 3,415

    indyjps
    Member

    As others have stated, think tube, box, c channel, I beam, H beam type cross sections. They will support torsional stress much better than flat plates, with thinner material. Adding a gusset across the back of the proposed engine mount would help, but it will still rock back and forth.
     
  11. squirrel
    Joined: Sep 23, 2004
    Posts: 39,752

    squirrel
    ALLIANCE MEMBER

    I was assuming the chassis rails are a rectangular tube, as shown in his drawing. If they are actually rectangular tube, then they will easily support the cantilevered type mount.

    Perhaps if we could see more info about the exhaust system...maybe it's some kind of forward facing headers, for a turbo application? :)

    Anyways, yeah, a crossmember needs vertical height to resist bending load. It's a pretty basic engineering principle.
     
  12. 73RR
    Joined: Jan 29, 2007
    Posts: 6,102

    73RR
    Member

    I'll take the OP side on this question and suggest that his design is not terrible and the flat cross-member will work fine but it will need to be a pretty fat flat bar....½ x 3 ?
    I know that the biscuits will be fine, ask the flathead guys. They also hold up the married-transfer case on one of my 4x4 trucks and it is a long distance to the front mounts.
    My concern for the design is how the crossmember attaches to the rails. The rails will definately require a tube welded between the walls for the bolt.
    Perhaps one of the younger guys with an FEA programme will take a look.

    .
     
  13. DDDenny
    Joined: Feb 6, 2015
    Posts: 9,421

    DDDenny
    Member
    from oregon

    A somewhat more modern version, lightweight and strong.
    These are on my aluminum small block but I've seen this design used on many big blocks.
    Could increase tubing diameter if it was a concern.
    20161106_120810.jpg
     
    Hnstray likes this.
  14. RR
    Joined: Nov 30, 2008
    Posts: 75

    RR
    Member

    Even if flat bar supports the engine in the static state, it will allow it to dance around while it is running and worse, under load. The cross member needs some depth to it to stiffen it.
     
    Hnstray likes this.
  15. 73RR
    Joined: Jan 29, 2007
    Posts: 6,102

    73RR
    Member

    Apparently, you really don't know just how stout a piece of ½ x 3 bar stock is. Buy a piece and play with it for awhile....'dance' it will not....
    For those without any engineering background we are dealing with something called 'moment of inertia'. You can develop the same 'beam' qualities (deflection and such) by having something tall and skinny or, something short and quite fat. Yes, there are inumerable better shapes for a cross member than a solid rectangle but everything is based on design requirements and space availability. In this case form needs to follow function.
    Your concern about the flat bar whilst the engine is under load is certainly a non-issue since the leverage is then (case #1) between the two engine mounting points (12" maybe?) and (case #2) the distance from the mounting point to the connection to the frame rail which will be likely be less than the former.

    .
     
    Hnstray likes this.

Share This Page

Register now to get rid of these ads!

Archive

Copyright © 1995-2013 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.