Frames - pan vs tube

I bet you guys grit your teeth when a neebie pops up with a topic like this, but some input would be appreciated. Is there going to be that much appreciable strength difference between the 2 types of frames? What effect on performance, short term/long term driveability? How difficult to get a tube car Ca registered? Again, thanks to all who respond with info.

Yes, I am considering buying one, and am just doing the research. Great comments, thanks.

I have driven both pan-based and tube frame replicas and of the cars I drove the tube frame is definitely more rigid and provides better handling. A tubular frame may also offer better protection in an accident, especially a side impact as the longitudinal frame member would absorb some of the energy.

I have a CMC with a pan chassis which definitly has some flex to it. I am in the process of retro fitting a partial tube frame to stiffen it up. The new tube chassis will tie in to the bodies structural frame. Once in place the body cannot be removed.

Theron,
What year was this "newer design" introduced (VS/JPS)?
Thanks.~R
(Message Edited 7/10/2003 1:00:22 PM)

The newer VS design (which is JPS as well) uses a very sturdy tube frame over the VW pan, granted, it's not as complex as the Intermec solid tube frame and I'm sure it flexes much more. But i have not seen the door-closing problem that some manufacturers claim this design has. However the Intermec does have a nice cage design and pushes the engine slightly further forward.

Then you just have the reg problem without a VW pan. Many with tubeframes reg them as Porsche Speedsters, however, one smart Highway patrol and you are going to have a nasty ticket for falsifying documents and possibly jail time for smog fraud.
-=theron
(Message Edited 7/10/2003 2:56:23 PM)

Dale,
Very thought provoking questions which I assume means that you are considering a Speedster replica and trying to decide which is right for you, correct? I have a pan-based car with a swing axle and a mild 1776cc type 1. It suits my purposes just fine. IMHO, a tube frame car would be superior, as I believe that anything which is purpose built is inherently better than an adaptation. In terms of long term driveability, there are not that many members who have had their cars long enough that there would be a representative sample, that is if you mean long term as many, many years. Pan based cars seem to be most often registered by the chassis year of the VW from which they were constructed. Most tube based cars, I beleive, and some pan based cars as well are registered through title services; In this case, the car is ususally titled as a Porsche rather than a VW. I'm sure others will be glad to share their views, right guys?
John H.

(Message Edited 7/9/2003 5:40:21 PM)
(Message Edited 7/10/2003 3:51:25 PM)

AS the owner of a JPS pan based speedster let me sight an example of how rigid a properly construcated pan chasis can be. When I jackup the front corner of the car, (either corner) the whole front of the car rises off the ground, both wheels. When I jack up the rear of the car, the whole side (front and back) comes up off the grund. I was impressed. The sub frame consists of 2X4 square steel tubing, which is pretty stiff stuff!) bolted to the pan around the passenger compartment and 2X2 tubing around the cowl and engine compartment.

As a former design enginer, I have no doubt a properly designed tube chassis could duplicate these feats with less weight, but if we are only talking rigid, and not going to worry about a few extra pounds, a pan chassis CAN be very rigid.

David , I agree the pan is ridged however any body parts extending over the pan tends to flex. On my CMC flareside you can push down on the front or rear and get a lot of flex. My fiberglass gealcoat is covered with cracks in these areas. I want to support the body before I paint it in the fall. So what I have done so far is fabricated a front structure to supprt the nose and act as a crash berrier. For the rear I will be fabricating a brace that will bolt up to the two side door posts. It will wrap around the rear seat( over the transaxle) and will create additional rear body support. I will connect two vertical supports from this cross brace down to the engine cradle. Two tangential bars will be connected to the rear side 2" tube in the engine compartment. Rubber mount will be added to reduce the rattle.

Tube frame subject.....

We have an unpainted tube frame as purchased new from Thunder Ranch see www.thunderranch.com that we may sell

Anyone seriously intereested ,email directly "drclock@hotmail.com

Alan Merklin

If you have a CMC car there are three deficientcies in their body mounting, the first and most serious is they use sheet metal from the sub-frame to the body along the foot wells, the best is to replace this with 1/4 inch plate steel.
Second the front body mount must be installed as you releive some of the front droop. After that when you install the bumper brackets through the body fill the area with silicone or urethane to use these for more front support.
The worst area is the rear, the tail section of the body is attached to the sub frame with a few pop rivets, remove these and jack up the body at the rear and close your door gaps then urethane between the panel and the subframe, install some self tapping # 8 tek screws and then drill and bolt the panel at the rear with no less than a 5/16 th bolt.
when this is done make sure the body is bolted down wwith 3/8 bolts and that the rear mounting point to the shock tower is shimmed, don't put in a bolt and pull the body down! When we whim it we lift up the rear of the car and add a thin shim to releive any excess stress on the sub-frame. If you chassis is welded correctly it will be stronger than the sub-frame but the combination of the two will be stronger than a single tube chassis.

Tube frames are great. IM's have tube frames and the JPS and Vintage cars have a tube frame inside the glass body work, also very stiff. I'm not sure which car is better in that regard but I have no problem jacking up my Vintage at one corner and still being able to open/close the doors. Somehow the JPS and Vintage cars are about three hundred pounds lighter than the IM cars as notified by another person here on this sight. Maybe the IM need all the extra support they can get? I'm not worried about how stiff the car is for handling as the swing axle design limits how good the handling will be. I love how the little 5" tires react to hard cornering (if you could call it that), more fun at lower speeds.
As far as getting into an accident, any which way you slice it the accident will be bad in one of these cars. Just my nickles worth.

J-P

Jean-Paul, "light" is good as long as it is also STRONG. The first determinant is how strong does the chassis have to be to provide adequate support, safety, and rigidity? This will imply a "given" weight depending on the chassis type, and anything lighter than that may not be desirable.

It is not good engineering to use fibreglass structures to reinforce a flexible chassis. Chevrolet found this out many times over when, after various approaches to the Corvette flexy-flier chassis cars, they finally began using a full steel subframe (was that in 1988?).

John , thanks for the additional info. Our frame design will stiffen up the chassis , support the rear body, and some safety. We have use some of our old sprint car designs as references. Out fabricated roll bar will be welded in place rather than bolted. I will post some picures soon. For the question regarding chassis stiffeness .. you want a very stiff chassis . A soft chassis will flex and twist which affects the tire contact creating traction loss.We had thoughs of fabricating a full chassis which would of made the doors inoperatable. My wife will not drive it if we go that route.