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IRS chassis - cad, 3d ..and moving on to discuss chassis stiffness


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Anyone have a CAD drawing of the IRS chassis - autocad .dwg .dxf , in 3d Rhino or whatever format,  please ?

I have paper copy of the chassis dimension drawing, also on-line at poor resolution, but one drawn in a computer aided design program would be useful to me. 

Thanks,  Peter

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4 hours ago, stuart said:

The drawing in the workshop manual gives you all the dimensions you would ever need.

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14 hours ago, Bfg said:

I have paper copy of the chassis dimension drawing

Thanks Stuart,

In fact it doesn't  The dimensions on the workshop manual drawings are simply for checking mount positions (..body tub, suspension, engine mounts, etc)  and otherwise gives very few dimensions to create an accurate drawing ..let alone for making or modifying chassis sections.   

Nevertheless if one accept inevitable inaccuracy of scaling off a distorted and highly pixelated image then, together with the dimensions given, it can be traced ..roughly.

I've now mostly done that myself.  And it ought (when finished) be sufficient for my needs until I can measure a chassis in person.

So..  Thanks.  Now sorted.  B)

943733409_acaddrawing-small.thumb.jpg.580c311608fcedb50acbd203aeab3224.jpg

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Peter,

you make difficult look so easy:)

It would be interesting to see how stiffness can be increased where needed, and at the same time some weight reduction achieved to lower stressed area’s.

I see our next generation (and possibly also some from our) doing great things with Inventor and similar software.

Waldi

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16 hours ago, Bfg said:

Anyone have a CAD drawing of the IRS chassis - autocad .dwg .dxf , in 3d Rhino or whatever format,  please ?

I have paper copy of the chassis dimension drawing, also on-line at poor resolution, but one drawn in a computer aided design program would be useful to me. 

Thanks,  Peter

 

1 hour ago, Waldi said:

Peter,

you make difficult look so easy:)

It would be interesting to see how stiffness can be increased where needed, and at the same time some weight reduction achieved to lower stressed area’s.

I see our next generation (and possibly also some from our) doing great things with Inventor and similar software.

Waldi

I’m in the process of building up a Solidworks model of the chassis but am not 100% confident of the dimensions until I get the tub off it and check it over. 

My paper drawing is a little flaky  on some of the dims so want to check with the physical thing. 

How long can you wait?

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22 hours ago, Waldi said:

Peter,

you make difficult look so easy:)

It would be interesting to see how stiffness can be increased where needed, and at the same time some weight reduction achieved to lower stressed area’s.

I see our next generation (and possibly also some from our) doing great things with Inventor and similar software.

Waldi

Thanks,  it used to be my profession (design engineer) so as a dyslexic ; draughting and design thinking are easier than English spelling and grammar   :blink:

Chassis Weight :  Is not an issue (to my way of thinking)  because at around 95kg it's really not very heavy when laying flat under the car  (v. low c. of g).   Whereas it taking the stresses means the body panels are lighter in weight and very conveniently bolt-on.  On a TR4 the chassis weight is just 10% of the total .. so any saving will be proportionately small.  There are (imo) much more worthwhile places to save weight. 

Stiffness The most obvious issue regarding torsional  stiffness (chassis twist which effects handling, steering, and chassis life) with the IRS chassis is that for almost the total  length ..between the front (suspension) turrets to the rear suspension mounts,   one side of the chassis is only joined to the other side by a bit of pressed tin plate and the almost-in-line bolts of the gearbox mount. 

Conversely, the TR4 chassis has its central cruciform structure.  ie.  from a further forward position, close to the forward turrets diagonal bracing  (compare the foot well bolt holes in the illustration below) the central rails lead directly to the cross-braced box to link one side of the chassis to the other  (..in addition to the gearbox mount).   And then of course the rails go out again to just where the suspension spring and damper are mounted.  The additional transverse (cross) braces (two on either side) support mid-span of each quarter rail (..which is why they can be lighter weight) and at the same time tie-in to the body-tub mounts (..so the outer rail is in bend rather than being twisted) 

287768049_TR4andTR4Achassissidebyside.thumb.jpg.996b99e609df3ef2937c7559340dfddd.jpg

The  TR4A - TR6  chassis is (to my way of thinking) structurally the equivalent of two side rails,  almost independent of the other side.  Whereas the TR4 much better ties together four rails with cross bracing, closer to the mid-position between front and rear suspension.   As as result the IRS chassis frame load path goes in n' out,  and then up n' down the spring hanger, then again around the back to the damper mounts  ..perhaps as an afterthought.?   With such twist, the set-up of the suspension is compromised (difficult to predict and varying with different road surfaces)  and is constantly fighting at critical points of its mountings.   The result is well know in terms of localised stress cracking on brackets which proved satisfactory on competition Tr3's and TR4's.  

Much of the stress  damage however is not immediately obvious ..insomuch as localised stressing induces corrosion (through the metal and their joints being repeatedly flexed).  This may be seen,  for example, when one removes the pressed tin-work bridging one side of the chassis to the other..   lo n' behold the rail underneath that plate has rust hole through it.     

Of course chassis cars were expected to twist,  and so their suspension design allowed for that.   So, if all of a sudden the chassis is made super-rigid then the suspension will need to be altered to suit,  and it may not have the range of adjustment or design to fully achieve that.   I'm certain that some cars ..so braced-up,  are now both worse in ride and handling than the original.   Its a balancing act which takes a little thought,  assessment,  and judgement.

I'll not go so far as to say it's wrong,  but it does seem to me that some of the extra bracing I've seen is an attempt to cure a symptom rather than the cause. 

I hope that helps a little   ..or at least starts a constructive conversation..  ;)

Pete

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Hi Pete,

you have some good fresh thoughts:) Weight saving on the chassis indeed does not bring much, as you clearly show.

Re the overall design: It is much easier to assess a design which has been in operation for several decades, and where all weak spots are known to folks like us. No FEA needed for that:) We have actual results /proof of the weak areas. The original designers did not have that feedback nor FEA tools.

Changing aspects like (torsional) stiffness could indeed affect handling and suspension/steering characteristics, but I think in a positive way: more responsive handling, similar to other improvements (?) like poly bushes, modern shocks etc. But being a car enthusiast does not make me an automotive engineer, I know my limitations.

Some thoughts though:

Would “bonding” chassis to body not increase torsional stiffness a lot? The current rubber pads/bolted joints allow relative movement and with that no/limited added stiffness. 1+1 = 2 where it could be 3.

Fitting a stiff fire wall in front of the fuel tank welded by a strong adhesive will probably give great improvement in torsional stiffness??

Just some ideas, as you indicate, for a constructive conversation.

Cheers,

Waldi

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The chassis design was probably the best that could be achieve in terms of what the engineers were allowed to spend at the time and evolved from what went before, I think trying to radically alter it now would simply bring up the next weak spot the main ones of which are known as said through long term ownership and can be addressed. Major alterations take away too much of the originality and some of the charm of owning one of these cars for some.

If you want to improve it and make a modern chassis I'd scrap the old one altogether and consider using front and rear sub frames with dbl wish bone multi link suspension built around two ladder frames and a torque tube linking the gearbox to the diff. Only problem is IMO is that its no longer a TR but more of a kit car.

Simple things I've done to improve the current chassis and tub stiffness is to seam weld some of the seams, seam welded the tub mounting brackets, and extended the downward return flange on the sills up to meet the inner sills ie sandwiched between the outer/inner sill so you have effectively two tubes down each side. I also plan while beefing up the steering column mount is to weld a brace across bulkhead to cut down scuttle shake all of which will be invisible.

In the end each to there own!

Andy 

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3 hours ago, Waldi said:

Hi Pete,

you have some good fresh thoughts:) Weight saving on the chassis indeed does not bring much, as you clearly show.

Re the overall design: It is much easier to assess a design which has been in operation for several decades, and where all weak spots are known to folks like us. No FEA needed for that:) We have actual results /proof of the weak areas. The original designers did not have that feedback nor FEA tools.

        in practice FEA and any other design aid is a useful datum for comparison in the theoretical sense.  But the proof of the pudding ..when it comes down to automotive design is to build it and test it under all conditions.   MIRA  (Motor Industry Research Association) provide the facility to accelerate fatigue and to test cars under extremes of speed, cornering, braking, and a host of other tests ..either individually or all together.   The equivalent of hundreds of thousand miles could be racked up in a relatively short amount of time and with no risk to the general public.   But back then it was also OK  for a car manufacturer to take a prototype car and blast across Europe.  Amazing very few accidents were (officially) recorded. ! 

But of course we are now talking about our TR's ..built and have been stressed, abused, neglected, and rusting internally for several decades now.  And that's not so easy to replicate. 

Changing aspects like (torsional) stiffness could indeed affect handling and suspension/steering characteristics, but I think in a positive way: more responsive handling, similar to other improvements (?) like poly bushes, modern shocks etc. But being a car enthusiast does not make me an automotive engineer, I know my limitations. There's some very experienced guys race prep'ing their cars,  who frequent this website .. and they still have to change one thing at a time and then go out onto the track to see if it really does make the difference they are after.   They then have to change settings &/or components (not least of all tyres) according to the track, the amount of fuel they are carrying, and the weather conditions.    As as designer I would often ask the guys on the shop floor, but as a newbie - I watch out for certain individual's posts, and otherwise ask the more experienced guys.    But even then I have to make the decision from a choice of opinion,  put my hand in my pocket spend lots of money,  and then suck it n' see.   I know I will make mistakes ..so I'll try not to burn bridges as I go.

Some thoughts though:

Would “bonding” chassis to body not increase torsional stiffness a lot? The current rubber pads/bolted joints allow relative movement and with that no/limited added stiffness. 1+1 = 2 where it could be 3. 

Fitting a stiff fire wall in front of the fuel tank welded by a strong adhesive will probably give great improvement in torsional stiffness??

Door cutouts and not having a roof means the resultant 'benefits' of having the body more secure and/or stiffening the body tub - may be localised, but overall the chassis and sills (mid section) is the skinny bit which will bend.   Jack the car up in one corner and see if both doors shut as well !    Yes, bonding the body onto the chassis would seem to offer the benefit of combined structure, but least so with the IRS chassis and the ribbed floors of these body tubs ..which barely touch each other.  

Enough goo like Sikaflex would certainly change the feel of your car, but your first impression might be to wonder if something has damaged your hearing.. because 90% the creaks, rattles, and some amount of mechanical, tyre and wind noise will have gone. :rolleyes:    

Just some ideas, as you indicate, for a constructive conversation.

Cheers,

Waldi

 

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17 hours ago, PodOne said:

The chassis design was probably the best that could be achieve in terms of what the engineers were allowed to spend at the time and evolved from what went before,

By the mid 1960's there were all sorts of frames from Lotus super seven to their Elite.  The American car giants were having marketing wars between perimeter frames and X-frames.  The E-type and others used sub-frames, which had both advantages and disadvantages..  "what compromise would you like to accept Sir ?"    Companies like Rover and Bristol persevered with chassis frames because they could isolate noise and vibration better than with unitary bodies.   And then companies like Citroen and VW had their own take on things.   Bottom line : the knowledge was there,  the testing procedures were real use,  and..  even today cars are recalled because the CAD jockies get it wrong !

What is less clear is..  why on earth did Triumph so radically change the chassis between the Tr4 and 4A  et all. ?   For any other company the design development would have been progressive,  or else they might have dropped the sit-on-top ladder chassis altogether and take the first steps towards unitary.  If there had been an advantage, with the new chassis,  such as moving to a backbone design (as they did with the Triumph Spitfire - introduced at the same time) or else perimeter rails with a central spine  ..so the foot wells might be dropped (like the MGA) - then that would have been a marketing gold star,  but simply to change it for sake of rear suspension mounts doesn't make much sense to me, especially as the new chassis has to be more expensive in its press tooling costs.

I think trying to radically alter it now would simply bring up the next weak spot the main ones of which are known as said through long term ownership and can be addressed. Major alterations take away too much of the originality and some of the charm of owning one of these cars for some.

I have to agree with you wholeheartedly ..in principle :rolleyes:   but I know me..  I do change things.

If you want to improve it and make a modern chassis I'd scrap the old one altogether and consider using front and rear sub frames with dbl wish bone multi link suspension built around two ladder frames and a torque tube linking the gearbox to the diff. Only problem is IMO is that its no longer a TR but more of a kit car.

Simple things I've done to improve the current chassis and tub stiffness is to seam weld some of the seams, seam welded the tub mounting brackets, and extended the downward return flange on the sills up to meet the inner sills ie sandwiched between the outer/inner sill so you have effectively two tubes down each side. I also plan while beefing up the steering column mount is to weld a brace across bulkhead to cut down scuttle shake all of which will be invisible.

In the end each to there own!

Andy 

Again I'd agree with the route you've taken. weld some seams and brackets more securely,  extend the odd flange here and there, and also to add in a few more gusset (triangular corner)  and boxing in plates..   Under the dash - might you find a way to triangulate a brace across bulkhead ?   As suggested in my reply to Waldi, I doubt if that will do much for the chassis stiffness ..relative to suspension and steering effect, but it might well make the car feel generally more solid. 

I'm also very likely to add in an additional cross brace or two, similar to the  TR4 chassis,  and again (as I discussed before)  an addition bridge or two from one side of the chassis to the other (easier done without having an overdrive unit and twin exhaust pipes in the way ..but I'm sure I'll find a way around.!   And then I was wondering about welding the prop-shaft tunnel in, together with a few stiffening rings ?   

I would like more space for my knees and feet, but that I think will be a 'stage two' development.    

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Pete and Andy,

Your remark about the prop shaft tunnel triggered me.

IF the GB tunnel would be solidly connected to the tunnel and bulkhead, and be made from a stiffer design (like 1mm steel), now that would certainly aid to torsional stiffness. Preferably welded to floor, tunnel and bulkhead. But gear box jobs become more difficult by that.

Waldi

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Hi Waldi

Suppose you could make it so it bolts in.

Not quite as stiff but at least removable only thing is the floor panels are like "wet rags" even with the corrugations.

Or why not brace the bulk head and triangulate from there to meet at the metal tunnel attached to a welded in half hoop, just make the brace a bolt in affair but again the floor pans are still able to flex!

Where do you stop?

If I had the space I'd buy another MK1 MX5 put in the V6 lump it was designed for and watch it run rings round a 6 but then I'd be divorced after promising not to mod anymore cars and have to be happy with Nissan Micra!

Just no fun around here:( Besides the old girl needs to be kept old.

Andy

  

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I was only proposing welding the rear section of the tunnel (with additional stiffening hoops) to the chassis,  so the gearbox cover remains and its rear mounting / drive shaft flange remains accessible .  This tunnel would extend right the way back and be welded to the rear suspension's spring hanger T pressing.  

But..  the TR6 does not have short tunnel over the rear drive-shaft flange as the Tr4A does.  Instead it has an almost flat pressed metal plate welded onto the chassis with the driveshaft being above this.    But why does that pressed metal plate not continued forward to the gearbox mount ? And if this was repeated underside of the chassis also to box that in (perhaps bolted every 65mm along its length so it may be removable for exhaust fitting) then the two mid rails would (structurally) act as a single backbone rectangular beam with great resistance to twist.    1 + 1 = 4

Unquestionably the H frame supporting the dashboard helps minimise scuttle shake,  but I wonder if an additional and more securely bolted steel hoop under the gearbox might not cross brace the chassis itself.   I don't know about gearbox removable but perhaps that cross beam might be welded in place too ?

Add in a rear cross beam to support the outer rails near the rear suspension mounts  ..and then one has a tortionally much stiffer chassis  (in my opinion).   Roughly like this . . .

55132248_IMG_0930crossbraced.thumb.jpg.e4566bca05f11de0828c7b2c7c62bff9.jpg

just for conversation like  ;)

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Hi Pete,

as you know I’m sure, creating a box with as much material on an as large radius/distance is what improves stiffness, either torsional or plain bending (sorry, I do not know correct English terminology).

Sandwiching in front of the T-shirt as you show in your picture above (wish I could do that by the way) would clearly improve both. Even more when it is extended to the outer legs.

“Interruptions” in stiffness should be avoided, otherwise all movement will concentrate there, and increase localized stresses to high values since that weak area will start acting as a hinge/pivot point. And as we know, movement equals stress per Hookes law.

cheers,

Waldi

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Waldi,

While the radius/distance is a crucial factor, we are limited in the TR chassis by practicalities of the car floor being directly above the chassis and limited ground clearance below it.   But the factor you seem to be missing..  is that of  Length of the rail (or chassis assembly) being twisted and its direct relation to the angle of twist . . 

IMG_6598-e12938033429641.jpg

^ If in this example the beam were half as long - it would take twice the effort (force) to twist it to the same angle.  

So what I've done in my sketch proposal is to reduce the unsupported lengths of the inside rails (those down the centre of the car) to less than half their original length ..by adding ties to adjacent structure.   Think of it the other way around : for the same twisting force (the reaction of the suspension going over a bump) the resultant angle of twist would be halved. !

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Hi Pete,

I (think I) understand what you mean and understand the principle of torsion.

What I meant is if some area’s of the chassis are very stiff, and movement is forced to the chassis, the more flexible part will absorb that movement, the energy needs to go somewhere. Yes, overall the chassis will be stiffer.

Think we should gather for a beer in the pub, makes this sort of conversations so much easier:)

maybe one day.

Cheers,

Waldi

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  • 1 month later...

Peter.

I was sorting out the TR6 showroom sales brochures I've collected with their different reprint dates and found in the box a chassis dimension data drawing. It would appear to be the same as you've copied . It has 74 measurements, most with two (presumably permitted tolerance), a few with just one. All in inches though! It also has an un-dimensioned figure 2 "enabling spot checking for alignment" by checking diagonals.

The source is a supplement to "Motor Trader" 24/31 December 1969 headed "Service Data No. 487a".

The next two pages show the 95 body parts including 21 for the hardtop.

The last page details how to remove the body (refit is a reversal!) and gives a list of sealing compounds and their manufacturer and approved sealing materials for an unpainted body.

Regards

Bill 

 

 

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6 hours ago, Bill944T said:

I was sorting out the TR6 showroom sales brochures I've collected with their different reprint dates and found in the box a chassis dimension data drawing. It would appear to be the same as you've copied . It has 74 measurements, most with two (presumably permitted tolerance), a few with just one. All in inches though! It also has an un-dimensioned figure 2 "enabling spot checking for alignment" by checking diagonals.

The source is a supplement to "Motor Trader" 24/31 December 1969 headed "Service Data No. 487a".

Yes, a copy from the factory workshop manual.  I have another workshop manual for the TR4 and then another for the TR5 /250 but I haven't compared them, as I now have the information I required to pipe-dream modifications.

1890048503_chassisacad.thumb.jpg.8c5e92d8b7a08a21713ac8fdde363152.jpg

With a little more done ..re. the IRS spring hangers, I passed a copy onto InfinityJon  as requested.  It will be interesting to see what he comes up with. 

 

3 hours ago, Andy Moltu said:

Put a full roll cage in?

For myself ..being so tall,  I'll need to push the seat further back than usual (rear wheel inner arches indented etc.) and I suspect a full roll cage would restrict that movement and shoulder room.  Aside from which I tend to be more concerned about being run into from behind and my head then smashing into the roll cage behind ..rather than my overturning a car.  And as the usual roll-cage has to be narrow enough for the hood frame to clear ..then think it looks rather odd. 

If I were to have one then I prefer it to be affixed to the chassis and equally to look like an integral part of the car's design (perhaps like the Lancia Beta Spider) rather than having such an ill-fitting accessory.  But of course such a style (as the Spider) wouldn't look right with the Michelotti body style.  A steel Surrey-top would be the better option if I could go that route.

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Stag type roll bar, if the hood somehow could be incorporated to keep it 'open air'?  Failing that the factory steel hard top is one solution off the shelf that provides extra bracing.

Alan   

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B)

That ought to keep you busy and out of trouble for a few hours.  Might I ask what you hope to use the model for ..aside from just the enjoyment of modelling it in 3d ? 

ie., are you exploring design changes ?

.

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Surely the contribution of the body to stiffness can be improved. Bolt body to chassis without rubber washers, or even weld ? If a full roll cage is ruled out.....tubular x frame welded to A anf B posts? ( or weld doors shut !) Another x between B posts.  Brace top of front suspension turret to top of A post. Join tops of Aposts with tube, connect centre zone of this to top of turrets. Fit f'glass panels to compensat for weight gain

Racers will have been here before .

Peter

 

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Maybe a consideration to increase torsional stiffness, and possibly reduce the dreaded derrière droop:

An owner recently mentioned that he had braced between the rear spring tower crossmember and the trailing arm outriggers. (Some also brace between the same crossmember and the diff rear crossmember but not sure if he did this). The forward brace is at roughly 45 degrees, is inboard of the inner wheel arches, and required boxing in the rear shelf front panel sheet metal around the braces.  It did not seem to limit seat adjustment, but if taller drivers have managed to get their seat back further it might be an issue.  The brace attachments would likely would likely be fairly high stress so doubler plates or gussets might in in order.

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