17 replies to this topic

[blueprint2002]

Most, but not all, racing Bugattis were and are renowned for outstanding handling and roadholding qualities, matched by very few of their contemporaries. Practically every author waxes lyrical on the subject, but I have never seen any explanation, not even rudimentary, of just how this was achieved.

On the other hand, much is usually made of certain design features, which may conceivably have contributed to this happy result. Here are some that come to mind:

1. Reversed quarter-elliptic springs to suspend the live rear axle (in contrast with the widespread use of half-elliptic springs by most others).
2. Half-elliptic springs passing through forged rectangular openings in the tubular front axle (as against being bolted above or below the axle, whether tubular or I-beam).
3. In the case of the Type 59, the front axle, again tubular, is in left and right halves, coupled at the centre by a kind of sleeve joint, which seems to permit relative rotation, and possibly some variation of length, but no other motion of one half with respect to the other. (A “degree of independence” is sometimes attributed to this, without explanation).
4. The two main chassis members vary in depth along their length, very deep around the middle, tapering progressively towards either end, this being logical as far as bending moments are concerned. (More-or-less constant depth along the length, except at the very ends, is more usual).

Does anyone know of any published work that explains precisely how each feature plays a part in the end result (assuming that they do)? And, can anyone add to the above list of possible contributions?

Weight distribution, and the partially-related polar moments of inertia, are known to have an effect, but most Bugattis don’t seem, superficially at least, to differ much from contemporary practice in this regard. On the other hand, the Type 54, with its very heavy 4.9 litre engine, was reputed to have handled poorly, unlike the otherwise rather similar Types 35 and 51. And the Type 32, which was radical in this regard, begat no successors, so I assume it was deficient. Conclusions?

P.S. The contribution of two Bugatti features seems so clear as to need no verification.

First, the unique lightweight wheels, of two types, cast aluminium/ radial wire-spoked (T59), which of course kept unsprung weight unusually low.

Second, the rigid mounting of the engine in the chassis, which must have substantially stiffened the frame in torsion, in contrast with the lack of cross-bracing, in competitors such as the Delage V12 and the Talbot straight-8.

 

[Charlieman]

Hugh Conway in the book "Bugatti" includes some road reports from sporting motorists. They tend to be favourable but Conway is honest enough to record that some drivers objected to the gear change. Dick Seaman briefly owned a T37 which he strongly disliked.

 

The T59 front axle design was atypical. The hollow forged single piece design (T35 and many others) was very clever. Along with the wheels, this helped with rigidity and reduced unsprung weight.

 

The tapered chassis members and rigid engine mounting could be considered. The front of the chassis is rigid in torsion and bending, but the rear, connected by tubular cross members, is quite the opposite. Rear spring stiffness needs to be considered in this context.

 

Brake design deserves reflection. The actuating mechanisms were light and rigid, and the mechanical front brakes (Bugatti was a late adopter) would have been helped by the axle design.

[Charlieman]

Two other considerations:

* Driving style. The "classic" Bugatti designs succeeded when cornering technique comprised lift off/brake/coast. Racing motorists such as S C H Davis discussed this technique well into the 1930s but the top tier drivers had learned from the mid 1920s to control lateral forces using the throttle. I'm thinking of Nuvolari and compatriots, with busy driving. Would a Bugatti be as well suited?

 

* Purchase and running costs. I'll try to dig out some comparisons shortly. The "classic" Bugattis were designed to be driven by amateurs as well as big stars. Some racing models were intended for extended road usage, others were more highly stressed. Bugatti considered the limitations of customer ability and wallet size. Companies like Delage or Alfa Romeo built GP cars which would be driven by the best, perhaps sold on to committed drivers trying to make their name. Bugattis had to be less fragile or tricky, keeping the driver alive to spend money on spares and upgrades. In the 1950s and 1960s, original Bugatti spares were rare but owners were still able to compete in cars which were less than prime.

[Michael Ferner]

" the top tier drivers had learned from the mid 1920s to control lateral forces using the throttle"

 

Have you ever seen footage of racing before the Great War? The "top tier drivers" certainly knew how to steer with the throttle some twenty years earlier than you seem to think!

[Lee Nicolle]

Hugh Conway in the book "Bugatti" includes some road reports from sporting motorists. They tend to be favourable but Conway is honest enough to record that some drivers objected to the gear change. Dick Seaman briefly owned a T37 which he strongly disliked.

 

The T59 front axle design was atypical. The hollow forged single piece design (T35 and many others) was very clever. Along with the wheels, this helped with rigidity and reduced unsprung weight.

 

The tapered chassis members and rigid engine mounting could be considered. The front of the chassis is rigid in torsion and bending, but the rear, connected by tubular cross members, is quite the opposite. Rear spring stiffness needs to be considered in this context.

 

Brake design deserves reflection. The actuating mechanisms were light and rigid, and the mechanical front brakes (Bugatti was a late adopter) would have been helped by the axle design.

Off subject but how do you forge a hollow axle. I understand the process of forging but unless you machine a bloody great hole through it should be solid.

[Charlieman]

From memory so apologies for vagueness. You take a hot workpiece and press a tapered mandrel through each end whilst pressing the outer form of the axle beam in a die. This gives you a hollow axle with greater beam strength in the centre with two blobs at the end. The blobs are then pressed to create the vertical pivot etc closing off the hollow beam. The advantages are no hidden sharp edges of course. The disadvantage is that you need access to the sort of press used in the railway industry at that time.

 

I've seen cross section drawings for the axle in several books. Nowadays it would be much easier to manufacture...

[10kDA]

Off subject but how do you forge a hollow axle. I understand the process of forging but unless you machine a bloody great hole through it should be solid.

As Charlieman states, the tubular piece is formed over a mandrel. This process has been used for firearms barrels for a long, long time. A huge specialized press is not always needed, a forging hammer with easily-configurable tooling can be used.

[Doug Nye]

Over the past six weeks or so I have just been working with a rather wonderful and uniquely important Bugatti Type 57S - see: 

 

https://www.bonhams...._release/31422/

 

Its front axle is suspended upon two leaf springs, each passing through the axle tube in typical Bugatti fashion.  However, the Type 57S front axle was built up from two halves, with a centreline joint featuring external left- and right-hand threads supported internally by a double-ended tapering mandrel. These two ends are then joined externally by a double-threaded collar which permits a limited degree of independent rotation as the springs flex.  

 

This feature permits a subtle degree of differential movement between the two half axles, the effect being to minimise front-axle tramp under braking which gave the front tyres at least a chance of avoiding hop, losing adhesion, and the brakes locking or grabbing.  

 

Once properly set up, the system worked well and the T57S was noted for its accurate steering and pleasant handling (by the standards of the time).  With an unblown 3.3-litre straight-8 engine the model offered something approaching Jaguar XK120 performance, ten years ahead of time...  Certainly the ability to cruise at around 100mph up the A1 in 1937 made the Bugatti's owner-driver absolutely the king of the kids, since 'normal' production cars at the time were already unreliably hard-pressed at around half that speed.

 

DCN

[Charlieman]

"This feature permits a subtle degree of differential movement between the two half axles, the effect being to minimise front-axle tramp under braking which gave the front tyres at least a chance of avoiding hop, losing adhesion, and the brakes locking or grabbing."

 

Fairies, Doug. Like the Matra 4WD F1 racer worked when power to the front wheels was 20%

[blueprint2002]

 

 

This feature permits a subtle degree of differential movement between the two half axles, the effect being to minimise front-axle tramp under braking which gave the front tyres at least a chance of avoiding hop, losing adhesion, and the brakes locking or grabbing.  

 

DCN

In a rear-wheel-drive car, even one with a rigid front axle, the front wheels are already capable of an unlimited degree of rotation, about their own axes, with respect to one another. Or, put it another way, unlimited "differential action" is possible. This is how they are able to go around a bend in the road, one rotating at a much higher speed than the other.

In any case, axle tramp is associated with gyroscopic action, under certain circumstances that have nothing to do with the action of the brakes. 

[Doug Nye]

Naturally one bows to expertise.

 

Which is why I passed on the considered opinion expressed to me just last week - in just the terms quoted - by one who grew up surrounded by Bugattis, who has worked upon, rebuilt, restored and driven Bugattis all his working life.  He is hands-on where these particular cars are concerned. Fine meaning of commonly expressed terms - tramp, patter - may vary between practical horny-handed blokes on the shop floor who may have forgotten more about these cars than some of us, myself included, will ever learn.  

 

DCN

[mariner]

A while back I was able to go on tour of Ivan Dutton's workshops near Thame. I am not a great Bugatti fan but it was really interesting. The Dutton's have very balanced view of Bugatti's - they appreciate the beauty etc but will  willingly criticise the silly design features like a oiling system which requires a crank re-build every 7,000 miles or so. Good for their business though, people fly the cars into them for re-builds 

 

As part of the business they have had many parts re manufactured and the new front axles as being discussed, here are one of their proudest achiievments .

 

The Bugatti axles is hollow, curved, has kingpin forks at the ends and, most difficult to make , the square spring slots . A specialist forging company in England made a few  at  great expense but use one  to display at forging industry exhibitions. Apparently even today making one is cutting edge stuff.

[Doug Nye]

Funny you should mention the Duttons...    

 

DCN

[plannerpower]

Le Patron's rotating joint, whilst not ideal in the light of today's knowledge, does show that he was thinking about the problems associated with beam axles.

 

Others at the time were also thinking along the lines of allowing one end of the axle some small movement in relation to the other end and Mercedes - Benz continued this line of thought into their independent suspensions.

 

We had a discussion of this not very long ago;

 

https://forums.autos...le#entry9146787

 

Apropos the front axle with its differing IDs, Bugatti patented its construction in 1926;

 

https://www.freepate...com/1592755.pdf

 

Figures 2, 3 & 4 in the patent show the stages of forging the outer ends; they were probably die-formed, a technique that was well-known by that time.

 

 

We tend to think of a "blacksmith" as a brawny fellow who hammered on glowing horseshoes but the blacksmith's trade was at about its zenith at that time and its practitioners were highly-skilled metalworkers with considerable traditional knowledge, much of which is now lost.

[mariner]

Speaking of blacksmiths I once went on a visit to the Mil Green Forge near Hatfield as one of the blacksmiths is a 750 racer. Seeing the blacksmith's skills was great but they told us a funny story which might strike chord on the subject of hstoric car apparent authenticity.

a 

They do work for the film industry, things like swords and those metal lantern baskets you see in castles. They made up a  set for film and had them sent back as they were not covered in  pockmarks. The forge explained that there would have been no pockmarks in period as there wouldn't be 500 years of rust etc then.

 

Doesn't matter said the film people we want pockmarks to make it look "real" .. So the balcksmiths spent a happy time hammering in dents .

[blueprint2002]

Naturally one bows to expertise.

 

Which is why I passed on the considered opinion expressed to me just last week - in just the terms quoted - by one who grew up surrounded by Bugattis, who has worked upon, rebuilt, restored and driven Bugattis all his working life.  He is hands-on where these particular cars are concerned. Fine meaning of commonly expressed terms - tramp, patter - may vary between practical horny-handed blokes on the shop floor who may have forgotten more about these cars than some of us, myself included, will ever learn.  

 

DCN

The skills acquired by the operation and maintenance of any piece of hardware (even up to a complete rebuild), over however long a period, are essentially manual, though their level is certainly influenced by the intellect. Insights into the thinking behind the various features of that hardware are not an automatic outcome, requiring completely different skills, which are usually acquired by quite different means.

Such insights do not necessarily arise from contact with the people involved in the creation of that hardware either, not even with the designer himself. They will only come about if the seeker (for want of a better word) himself possesses complementary skills, in addition to the essential social skills. Put rather crudely, he has to know how to ask the right questions, and also to be able to sift out the relevant portions of the answers.  

And the rarity of such insights, in over a century of published automotive literature, shows clearly that writers possessing those skills are even rarer than designers of the calibre of Porsche or Issigonis, Jano or Chapman.

Edited by blueprint2002, 09 February 2021 - 01:35.

[Allan Lupton]

Questions about the effect of the Bugatti's various design details on handling should have been Hugh Conway's field as he was a well-qualified engineer as well as a Bugatti enthusiast/historian. I dare say the Bugatti Trust could suggest a current candidate but I'd agree that the Duttons would be worth asking - Ivan, despite seeming to be just anovver norf lunnon mo'or dealer, knew what did what when preparing and racing  Bugattis.

[cpbell]

Questions about the effect of the Bugatti's various design details on handling should have been Hugh Conway's field as he was a well-qualified engineer as well as a Bugatti enthusiast/historian. I dare say the Bugatti Trust could suggest a current candidate but I'd agree that the Duttons would be worth asking - Ivan, despite seeming to be just anovver norf lunnon mo'or dealer, knew what did what when preparing and racing  Bugattis.

Having re-read the relevant parts of "Grand Prix Bugatti", the view was that it was the plank stiffness of the chassis, the positive location of the axles and the resistance to lateral load of the rear suspnsion due to its mounting points, allied to a chassis that was flexible in torque but not to the extent of others of the period and the relative stiffness of the quarter-elliptic springs.
 

Edited by cpbell, 10 February 2021 - 11:35.