32 replies to this topic

[D-Type]

A trivial question that came up in a discussion elsewhere.  When did racing cars first have larger rear wheels than front ones?  

The prewar Mercedes and Auto Unions definitely had them while earlier cars like the Bugatti Type 35 which still sported a spare wheel obviously had four wheels the same.  Looking at period photos, some of the cars in the late twenties and early thirties appear to have larger rear wheels but not always.  Was this something that came in gradually and intermittently or did it come in with a bang with the 750kg formula?

 

[Charlieman]

Consider also the riding mechanic rules. If you had to carry a mechanic, you'd have a spare* wheel that worked on all four corners. 

 

*edit for "spare"

Edited by Charlieman, 11 August 2015 - 21:42.

[kayemod]

An interesting question, not something I've thought about, but I take it you mean the rim diameter, not just the tyre section?

[Ray Bell]

The horsepower gains of the early thirties would surely have led to this?

They were starting to rely on horsepower and so the front wheels became less important?

[63Corvette]

The heaviest end of the car traditionally carried the largest tire (Porsche). The car doesn't have to be rear or mid engined to have the largest percentage of weight on the rear (1963 thru 1967 Corvette - both 327 and 427). Also works for lighter cars with more horsepower and torque. Funniest thing is a ricer (front wheel drive japanese car) with bigger REAR wheels and tires.

[Lee Nicolle]

I sell wheels, and yes I get requests for the drag race look for front drivers. Some people really do not know, they just have to have the 'look' Recently I sold more normal 'stagger look' for a Camry. 4 cyl auto! 18x8 & 18x9 with one size larger tyres for the wider rims. You guessed it, the wide ones are on the rear!

The 'look' even on rear drive classics frequently annoys me. The American styles of drag race rims. Centre line, Convo, Weld etc. Often marked for drag race use only and use bias belt tyres only. So people fit them onto 4000 lb road cars with radials. Worse the moronic insistence on very skinny 'front runners'  That then use 135x15 Michelins designed for a 750 kilo on 4" ? rims Renault  and being used on 4000lb cars on 3" rims!  Bloody dangerous and stupid. Said tyres too also have about an 85mph speed rating on cars that do the 1/4 ending at 130mph! 

I do know a couple of street drag racers. 9" wide Centrelines and DOT [but not roadworthy] drag radials but do use 6" rims with a 205x65 front tyre that does at least have the load and nearly the speed rating required. Even they say the 'wannabes' are idiots. They do have street tyres that they sometimes use to drive sometimes on the street.

I also sell many 10" wide rims for people who want the 'look' of our GpC road arcers of the 70s. Though noone [except actual racers] use 10" on the front. Though so many have no real idea of the problems to fit the tyres involved and often end up with tyres for 7-9" wide rims on 10" rims. Illegal if generally not particularly unsafe. Though I have seen major surgery to the car bodies to fit the bigger tyres. I try not to sell them and cost myself a sale sometimes. These rims ofcourse are several inches over legal track limitations and I really pity the poor old axles they are attached too. On occasion the axles break off outside the bearing.  8" rims generally are the best compromise and even then generally overtrack, but not 2" a side more though!

 

We also see the 'drift' look with very narrow tyres on very wide rims. It makes some sense as they wish to 'unhook' the rear tyre and get as much lock as possible on the front. Though tyres do fall off the rims on occasion. Though really smaller front rims with big offsets make more sense on the front [as is starting to happen now] . And just using smaller tyres on smaller rims really makes more sense for the rear. Though none of this makes any sense on a street car where the extremes are usually pushed further.

[Catalina Park]

Oh god, make it stop.

[Rob G]

I'm guessing the 1894 Paris-Rouen is not the answer you're looking for.

[Roger Clark]

Pomeroy, in The Grand Prix Car, gave detailed specification of many cars. It seems that there was a fashion for larger rears before and for a few years after the First War: the 1913 Peugeot, the 1914 Mercedes, the 1920 Ballot, the 1922 Vauxhall, the 1922 Fiat all had larger rears, both diameter and section. After that, the 1927 Delage, the Type 35 Bugatti, and the Tipo B Alfa Romeo had the same size front and rear. The 1935 Mercedes had larger diameter but same section rears. The rears of the 1936 Auto-Union were larger in diameter and section.

[byrkus]

Maybe it was all practical reasons. Smaller front wheels are easier to steer.

[Lee Nicolle]

Go back to the 20s and before and you will see wider rears, often dual wheel rears for hillclimbs etc. 

By the 50s wider rear tyres were common place on race cars of many types, as were bigger dia tyres too

By the late 60s some very upmarket sports cars were using different front to rear and racecars almost all used them

Obviously getting power down and controlling over steer is the reason.

[f1steveuk]

If you're going back to Panhard Levassor's and the like at the turn of the 1800s into the 1900s, the big wheels were to get as much from what power was available as possible, forming an integral part of the gearing, such as it was. Having restored a few of the cars of that period, we did one that the larger, though driving wheels, were on the front. The bigger the wheel, the further thee distance covered in one revolution, little wheels for steering, to reduce the effort needed, and quicken the steering.

 

I'm guessing, but maybe the same philosophy struck a designer again later on, if there was a gearing problem? Though now, bigger wheel tyre, more rubber on the road, better traction?

 

(Over simplified to prevent me getting a headache)

Edited by f1steveuk, 12 August 2015 - 13:37.

[cpbell]

Interesting that Mercedes didn't apparently opt for larger diameter rears in '34, but did the following year.  I always reckon that the difference was most noticeable on the '38 W154s, especially in side view.

[D-Type]

Thanks for all the contributions, folks.  Like many simple questions there doesn't appear to be a simple answer.  Not even in the definition with wider rims and larger section tyres as well as larger diameter wheels confusing the issue.

Incidentally I have just remembered that the triumph Herald estate had larger section tyres than the saloon because of the greater load capacith.  To compensate, Triumph fitted a lower back axle ratio so that the estate wouldn't be faster than the saloon.

[GMACKIE]

Or the saloon slower than the estate, perhaps.

[kayemod]

Thanks for all the contributions, folks.  Like many simple questions there doesn't appear to be a simple answer.  Not even in the definition with wider rims and larger section tyres as well as larger diameter wheels confusing the issue.

Incidentally I have just remembered that the triumph Herald estate had larger section tyres than the saloon because of the greater load capacith.  To compensate, Triumph fitted a lower back axle ratio so that the estate wouldn't be faster than the saloon.

 

Not entirely relevant perhaps, but many modern road cars have a smaller diameter setup on the back than they do on the front, the reverse of the old Auto Unions etc. My own road car has 40 section fronts and 35 rear, both on 18" wheels. I'm not going out at this time of night with a tape measure,  but the diameter of the rears must be a bit smaller?

[Lee Nicolle]

Thanks for all the contributions, folks.  Like many simple questions there doesn't appear to be a simple answer.  Not even in the definition with wider rims and larger section tyres as well as larger diameter wheels confusing the issue.

Incidentally I have just remembered that the triumph Herald estate had larger section tyres than the saloon because of the greater load capacith.  To compensate, Triumph fitted a lower back axle ratio so that the estate wouldn't be faster than the saloon.

Load rating has always been the OEM reason for larger tyres on vehicles to carry more load. Though most 'normal' cars use them all round. 

In more modern times there is higher load vesrsions of the same size tyre,, which often end up slightly larger anyway. eg 215x60x16 in 95 load, normal passenger, 99 load for wagons and normal utes and 104 load for 1 tonne utes.

[bradbury west]

I always understood that the van/estate in some cases, versions had lower diff ratios  was because they were more likely to be running far more loaded for more of the time than the car version and top speed would not necessarily have been a key consideration. The classic example ISTR was the old Minor 1000 van used by the GPO which ran something like a 4.875:1 ratio and I think there was even a 5.125 or lower, later beloved by racers with very high revving engines. I have not checked, for the rivet counters, but the ratios may have been even lower. I should check, as I have half a dozen 4.22, 4.55 etc in the garage.

Roger Lund

Edited by bradbury west, 20 August 2015 - 10:34.

[Roger Clark]

Thanks for all the contributions, folks.  Like many simple questions there doesn't appear to be a simple answer.  Not even in the definition with wider rims and larger section tyres as well as larger diameter wheels confusing the issue.

Incidentally I have just remembered that the triumph Herald estate had larger section tyres than the saloon because of the greater load capacith.  To compensate, Triumph fitted a lower back axle ratio so that the estate wouldn't be faster than the saloon.

Why would larger section tyres be compensated by a lower axle ratio?  I could understand large diameter.

 

The thread has wandered a long way, but I thought that the original question had been more or less answered.  If not, what is the question?

[Allan Lupton]

Why would larger section tyres be compensated by a lower axle ratio?  I could understand large diameter.

Larger section on the same rim diameter does result in larger diameter.

[Roger Clark]

i thought section referred to the width!

[kayemod]

i thought section referred to the width!

 

You're right, and I'd guess that my front 40s probably have a similar rolling diameter to the wider 35s on the back, but I still can't be bothered to go outside to measure them.

[2F-001]

Section is normally the width, I think; the other dimension (50, 40, 35 etc) is sometimes referred to as 'aspect ratio' which is, in theory, a percentage of the width. However in practice, different manufacturers measure width differently (tread, shoulders, sidewall edges etc), so all the dimensions (particularly on road car tyres) are more-or-less arbitrary.

In Rob's case, I assume his 35 aspect rears are wider than the fronts and thus have a rolling circumference equal to, or greater than, the fronts.
Some race tyres (eg. Avons) specify an outside diameter, wheel size and tread width rather than an aspect ratio.

[Bob Riebe]

The heaviest end of the car traditionally carried the largest tire (Porsche). The car doesn't have to be rear or mid engined to have the largest percentage of weight on the rear (1963 thru 1967 Corvette - both 327 and 427). Also works for lighter cars with more horsepower and torque. Funniest thing is a ricer (front wheel drive japanese car) with bigger REAR wheels and tires.

You forgot the REAR wing and fart can.

 

Some early race cars ran duals on the rear.

Edited by Bob Riebe, 20 August 2015 - 15:55.

[Allan Lupton]

i thought section referred to the width!

With modern tyres effectively you have three dimensions stated: section width, section depth and rim diameter (the actual depth is hidden in the aspect ratio).
D type was referring to the Triumph Herald which dates from a time when only two dimensions were given or implied, so (e.g.) a 550 × 14 would have an overall diameter around 1" greater than a 500 × 14.

 

That's why you can use tyre section as a way of making small gearing changes.

 

ETA of course if you go a bit further back the two dimensions quoted are overall diameter and section so my 1912 car on 820×120 tyres is geared up 8% compared to the 760×90 tyres it had when new.

Edited by Allan Lupton, 20 August 2015 - 16:16.

[kayemod]

In Rob's case, I assume his 35 aspect rears are wider than the fronts and thus have a rolling circumference equal to, or greater than, the fronts.
Some race tyres (eg. Avons) specify an outside diameter, wheel size and tread width rather than an aspect ratio.

 

It's stopped raining, so I've just been outside to check, overall diameter is the same both ends, fronts 225 40 18" and rear 255 35 18". I checked the MB brochure for the first time since buying the thing, and no rim width is specified, I suspect it could be the same both ends. The spacesaver spare of course is like something off a BSA Bantam, wouldn't want to go very far on it.

 

To get back to somewhere near the original intention of this thread, didn't Jack Brabham use 13" front rims on his BT19 and 15" rears? I think it was to fit larger diameter rear discs, but later versions had 15" rims at both ends, as I think did the later BT20 & BT24.

[Charlieman]

To get back to somewhere near the original intention of this thread, didn't Jack Brabham use 13" front rims on his BT19 and 15" rears? I think it was to fit larger diameter rear discs, but later versions had 15" rims at both ends, as I think did the later BT20 & BT24.

That doesn't seem true for Tauranac/Brabham engineering practice. If you change the rear wheel diameter by two inches, is/was there enough adjustment to put the wheel back to conventional camber?

[Roger Clark]

I believe Kayemod is right. The BT19 was converted to 15" front wheels at some point in 1966 but I don't know when. The BT20 had 15" fronts from its introduction.

[Roger Clark]

With modern tyres effectively you have three dimensions stated: section width, section depth and rim diameter (the actual depth is hidden in the aspect ratio).
D type was referring to the Triumph Herald which dates from a time when only two dimensions were given or implied, so (e.g.) a 550 × 14 would have an overall diameter around 1" greater than a 500 × 14.

That's why you can use tyre section as a way of making small gearing changes.

ETA of course if you go a bit further back the two dimensions quoted are overall diameter and section so my 1912 car on 820×120 tyres is geared up 8% compared to the 760×90 tyres it had when new.

I thought the 500 in 500x14 referred to the tyre width. How does increasing that make changes to gearing?

[GMACKIE]

I thought the 500 in 500x14 referred to the tyre width. How does increasing that make changes to gearing?

Provided the aspect ratio remains the  same, the diameter will increase with the width.

[Roger Clark]

Obviously. But does the aspect ratio remain the same?

[Allan Lupton]

Obviously. But does the aspect ratio remain the same?

Yes and it did for most of the time tyre markings had the form [section] × [rim diameter]

I didn't think I'd have to put that detail in on this Forum - just shows how short memories can be

[Lee Nicolle]

I feel some of you blokes need to go to the haberdashery and buy yourself a dressmakers tape. Very handy for a small outlay. You can tape the diameter of the tyre and get true changes and or info.

 

Old 50s cars for instance used for instance a 5 25x16 on a Vanguard sedan and 600 16 on the utes. The wider tyre was also bigger in diameter. A good inch in height. Often compensated by a shorter diff ratio,

 

Modern cars are a bigger conunderum.  You have to look at the width, the aspect ratio and the load rating. Yet alone the speed rating.

For instance a 195x50x15 tyre  generally has a similar roll out to a 195x45x16. And generally the same load rating. Generally [and not always] the 45 aspect tyre has a higher speed rating.

Though different brands still will roll out slightly different too.

 

AWD/ 4WD cars especially with constant 4wd will never tolerate different diameters. If nothing else it will kill the centre diff and generally the transfer case and gearbox  too. With those vehicles be very carefull too. A highway pattern tyre is generally slightly smaller than an A/T tyre which is smaller than a mudder! Really the best [and safest] is the same brand and construction all way around.

THOUGH, some hi performance AWD cars use different tyres one end to the other. A tyre manuafcturer and the car manufacturer have done some very detail work to get [as above] a 35 aspect to marry up exactly with a 40 aspect. Or similar. And on occasion can cause grief with the relacements that are not quite the same size unless you use the OEM tyre,,,, and after a few years OEM do not make the tyres anymore. Ooops. Happens way too aften on very expensive low volume cars and older hi volume cars.  And the spare is probably about 3" wide,, but the same dia or very close. A sore point I know with many inc myself.

 

Ofcourse many [most]  2wd sporties use different size and sometimes aspect front to rear. Though again usually they are similar in dia. Whereas hotrodders just want the 'look' and have a dogs breakfast of sizing. Works ok usually though the spare can be a drama. With passenger on full size vehicles most tyres roll out at about 80". Go measure them!