Transverse mounted fwd engine motor mounts

[ejm211]

I'm new here so stop me if I'm asking an obscene question.

I hear all the time how guys in a front engine rear drive setup are relocated or modifying the motor mounts to put the engine a few inches further rearward in the engine bay. This is mainly done to improve front/rear weight distribution. My question to you, oh knowledgeable ones, is:

What kind of problems will I face designing new engine/trans mounts to move a transversely mounted fwd engine 3-5 inches rearward. This car will see both drag strip and autocross and is a general street fun car. For drag racing, moving this particular setup rearward would actually concentrate the weight of the drivetrain over the front tires, improving traction. For autocross, this will greatly improve the 65/35 f/r weight distribution; with a 2300lb car and the engine and transmission combo weighing right around 400lb, that's a nice chunk of mass.

The few problems I can think of would be:

Halfshaft angle: Obviously, putting the transmission 3-5 inches further rearward will increase the halfshaft angle between the trans and the front hubs. How will this affect performance and wear?

Shift linkage: This car uses two bars, one being a shift rod and the other being a stabilizer. Being a hollow tube with relatively generic ends, remaking these slightly shorter shouldn't be too much of a problem.

Thank you for your time and I appreciate any opinions/ideas you may have to offer. I look forward to your replies.

Eric Mills

 

[NormPeterson]

All else equal, the limiting acceleration potential of a FWD is better at 63/35 weight distribution than it would be at, say, 62/38. So unless there's also a commensurate drop in CG height, dragstrip performance will most likely suffer. That's regardless of the location of the powertrain, BTW (and I think it's safe to say at least for now that any dynamic effect coming from changes in the sprung mass pitch moment of inertia that result from moving the powertrain will be minute). I say 'most likely' because the (in) ability of the powerplant to overcome the available grip could cause the engine to 'bog', in which case the ability to spin the wheels would be to advantage.

If this powertrain relocation involves rotating it in any way as seen in side view, there will be fluid level consequences both in the engine sump (including the pickup) and in the transaxle (especially if automatic). Further, and particularly if it's a V-type engine, oil drainback from the valve/cam cover volume(s) may not be as good under certain conditions.


Norm

 

[Fabrico]

As far as drag racing, moving the weight backwards to over the front wheels will only decrease traction up front. All other things remaining the same, it will only increase weight transfer to the rear.

Those are pretty short half shafts, so you will be adding substantially to the basic angles of the CV joints. It might live if your engine is still basically stock and all you do is race ocassionally. If you have added power or plan on many street miles it will simply wear them out or break. Once you figure out what the new angle will be on the outer CV's, as viewed from above, you can immediately subtract it from your available steering angle in each direction. Don't forget to readjust the stops!;-)

You may soon find you car is no longer so "fun".

 

[GregLocock]

You should be moving your engine forward, I'd have thunk. Easier to move the rear wheels back.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NormPeterson]

"As far as drag racing, moving the weight backwards to over the front wheels will only decrease traction up front. All other things remaining the same, it will only increase weight transfer to the rear."

Ummm, less traction = less acceleration = less rearward load transfer (still assuming the same CG height). The "new" vertical rear wheel loading under acceleration will be greater than the "old", if that's what you had in mind.


Norm

 

[ejm211]

You have certainly answered my questions. However, the only genre mentioned was the drag racing portion. What are your (this community) thoughts on how this will affect a car's autocross performance.

Sidenote: I did plan on designing these mounts to lower the engine a couple inches. I only say a couple because the car itself is barely streetable up here on the wonderful roads of Michigan as it is.

Eric Mills

 

[patprimmer]

Drag race modifications to increase straight line acceleration traction and straight line stability inevitably reduce performance where maximum manoeuvrability are paramount.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[NormPeterson]

Just what car is involved here, and what autocross class is envisioned? There's considerable range of what's class-legal among Street Modified, D/E Prepared, X Prepared, D/E Modified, and A Modified, those being the auto-x classes that this thread suggests it might possibly fit within.

Some things might actually tend to favor both auto-x and drag in a FWD car, such as a slightly smaller rear wheel/tire package that that used up front. Not as small as a dedicated dragstrip car, but perhaps 20mm narrower. A 2200 lb car with less than 40% rear weight may not heat up competitive tires (in let's say 225mm section) soon enough to be useful over much of the run, and the understeer/oversteer balance will shift as the fronts do. Especially on cooler days. Some folks are already experimenting with 205's on the rear only in a class that permits 225.

Depending on whether you're a follower of the so-called "soft spring/big bar" approach to suspension tuning or oriented more toward using the bars more for fine tuning probably has greater influence on how you decide to handle that will than a couple or three percentage points of weight distribution will (obviously I haven't run any numbers here). So might the difference between being a "point and shoot" driver or one who more smoothly carries momentum through the course elements. There is even some disagreement among FWD auto-x'ers regarding front vs rear spring rates and getting the car to rotate. Anyway, my initial thoughts in light of your comment about "a couple of inches lower engine placement" (or complete sprung mass lowering) is generally that the stiff spring/light bar(s) plan may be preferable, with ride comfort essentially being ignored.


Norm

 

[MikeHalloran]

Fabrico's point about the extra angulation of the half-shafts decreasing the steering angle by the same amount was a real forehead- slapper for me. You'll break a half-shaft, or run out of steering lock, on the first turn.

If it brought any advantage in drag racing, and I can't see that it would, it might be worthwhile, but it will absolutely kill the car for autocross. You might as well drive a bus.






Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

Great catch. A typical rzuppa has a 47 degree articulation limit, a typical tripot is 25 degrees. Typical inner wheel cut angle is say 40 degrees, so at best you could manage 7 degrees of plan view halfshaft articulation. In practice you'll need some margin.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

Norm

As well as not specifying the class of X cross racing, he does not specify location, so unknown local rules might apply.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ejm211]

I reside in Michigan and I follow the STS class but I'm thinking about moving. I pilot a 1998 Escort ZX2 with a 5spd manual and a 2.0L DOHC inline four cylinder.

This car is really more of a street car/fun weekend car than a dedicated track car. I would venture to say the time in this car is 60% street, 25% drag, and 15% autox.

Eric Mills

 

[evelrod]

Hi Eric.
This is just my opinion, but dealing with this car and it's intended use...Unless you have some masochistic leaning that your not telling us...I would consult with some of the purveyors of Ford mod parts, Bat (British American Transfer) Ent., Esslinger Engineering, etc.(probably many more that I am not up on), to get a feel of what you can do to improve the performance of your car within the accepted norm...Again, JMO, there is enough things you can do to alter the performance of a FWD car as to 'boggle the mind'...without all the hassel you suggest.
An Escort is not my first choice to through money at, but, 'what ever trips your trigger'. I worked over a Fiesta once and folks thought I was nuts, too...so enjoy.

Merry Christmas

Rod

 

[MikeHalloran]

If you're willing to move the engine back a foot or more, you could move the diff with it, but leave the halfshafts where they are, connected to some stub shafts, driven by a pair of _serious_ timing belts.

You'd probably need outboard bearings on the diff output shafts to deal with the belt tension.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

For a street car? Also I think you'll find the diff tends to be behind the trans, so there is no empty space.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ejm211]

Evelrod, I own two of these cars. One of them wears a custom equal length turbo manifold, t3/t4 57trim turbo with a .63 a/r, 3" exhaust, pnp head, turbo cams, lightweight valvetrain, 6 puck clutch, and limited slip differential. I can run consistent 12.8's in the quarter mile with 2.0 60fts. This car has a completely custom rear suspension setup using billet aluminum lateral links, trailing arms, and subframe. The front has chromoly lower control arms and a tube frame front subframe.

The topic at hand is for my other zx2. I use these cars because they're not generic. I have muscle and import guys alike coming up to me at the drag strip applauding my efforts. It's a passion for me and I'm always looking for something else to modify/fabricate.

Mr. Halloran,
I see where you're coming from but I'm trying to steer clear of additional complexity and packaging would mostly likely be a prominent issue in your idea, as this is quite a small car.

Eric Mills

 

[patprimmer]

Just my opinion, but to really improve a FWD drag race car.

Turn the seat 180 degrees.
Lock the front wheel steering straight ahead.
Move Steering wheel, pedals dash etc to the rear, facing the driver as he faces the rear.
Install rear wheel steering that operates via the steering wheel and disconect steering wheel from front wheels.
Flip the Crown wheel over so that gear box output is reversed in direction of rotation.
Weight transfer now becomes your friend instead of your enemy.
Don't drive at night as the head lights are really bad and the reversing lights are blinding to following traffic.


;-)

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evelrod]

Pat---

You know the Mini is my first effort (absent the abortive Fiesta) at racing a FWD. It's---well---different! It's taken me five years to get down to class record times and feel confident in my ability (such as it is, these days) to compete (at least 'keep up') with the younger crowd. So, don't give up so easily. Having watched a couple of the Speed Channel shows on FWD drag cars...I'm amazed. These guys are turning times that would have smoked a AAF car of my drag days!

Rod