Reduced safety margins due to car modifications

[AdamW]

Larger contact patch area = marginally more grip

If the car weighs the same?

Please explain Dave. Sorry for the stupid question.

[manilva15b]

The lack of a Low box is one of the reasons I don't like the Freelander. HDC is good, but you still don't have enough control at very low speeds without abusing the clutch.

I'd quite forgotten about the Frelander clutch - not so much a safety issue though. The company had a fleet of 17 from new, 3 of which we managed to burn out the clutches on - the clutch seemed hopelessly underengineered for the demands placed on them in a 2 tonne car. Reversing up a modest gradient was guaranteed to fill the cabin with clutch fumes. Maybe that is a safety issue after all?

[AdamW]

The lack of a Low box is one of the reasons I don't like the Freelander. HDC is good, but you still don't have enough control at very low speeds without abusing the clutch.

I'd quite forgotten about the Frelander clutch - not so much a safety issue though. The company had a fleet of 17 from new, 3 of which we managed to burn out the clutches on - the clutch seemed hopelessly underengineered for the demands placed on them in a 2 tonne car. Reversing up a modest gradient was guaranteed to fill the cabin with clutch fumes. Maybe that is a safety issue after all?

Just like the K Series engine that some had.

[Tav]

Larger contact patch area = marginally more grip

If the car weighs the same?

Please explain Dave. Sorry for the stupid question.

More rubber on the road = more grip. So moving from a 5.5 inch wheel to a 6 inch wheel gives you ~10% more rubber per wheel. So on the front axle you have ~20% more rubber in contact with the road surface. I've seen and felt a noticeable difference moving from 5.5 to 7 inch wide wheels on a friends car...as well as the benefits of choosing different compound/construction/pattern tyres.

I'm changing my car from a BBS RA 13x5.5J wheel to a BBS RA 14x6J wheel. The wheel looks the same with the exception of being an inch larger in diameter. I will be using lower profile tyres...but hardly what I would call rubber bands at a 45 or 50 profile. Should provide some of the performance increases I am looking for...whilst including a few compromises to boot.

Standard 1.0 MK3 Polo wheel is 4.5 inches wide. Change this to the G40 5.5 inch wide wheel and the difference is huge. 45bhp will quite happily break traction through the 4.5 inch wheel. The 5.5 wheel is used to cope with 115bhp and 150Nm....115bhp through 4.5 inch wheels would result in a lot of smoke and very little forward motion. 170bhp requires at least some increase in wheel/tyre package width and stickier rubber to work properly.

[Gromit37]

This is probably Stressed Daves realm of expertise...

The question is, does a 20% increase in rubber give a proportional increase in grip, or is it not as simple as that? On top of that, all other things being equal, increased friction = increased rolling resistance = bigger fuel bill?

I also have a question with regard to rotating mass and performance. While many young drivers add bigger alloy wheels with greater width to replace small steel wheels, is there a big enough reduction in rotating mass to warrant the expense?

[Gareth]

While many young drivers add bigger alloy wheels with greater width to replace small steel wheels, is there a big enough reduction in rotating mass to warrant the expense?

When I looked into this in detail for my car, I found that changing to larger wider wheels normally went the wrong way as far as rotating mass is concerned.

[hardboiled]

I recently carried out a repeat of an experiment in the Honda Revs (club) magazine where acceleration of a car was compared with 17" wheels and the standard 15" inch wheels. In my case my friends car was running 16" wheels and he swapped to 15".

Over a range of 40-70mph in third gear repeated three times from the same place with a very accurate data logging device, the smaller wheels gave a 0.2 seconds quicker acceleration time. This backed up the original findings although the difference was smaller (wheel size difference was less and car was more powerful).

Overall my 15" wheels are more than capable of generating enough grip should I need it. The change to a larger wheel will just adversely in my view affect the handling characteristics of the car and hence I'll stick to the standard alloys for the time being.

[Renny]

This is probably Stressed Daves realm of expertise...

The question is, does a 20% increase in rubber give a proportional increase in grip, or is it not as simple as that? On top of that, all other things being equal, increased friction = increased rolling resistance = bigger fuel bill?

I also have a question with regard to rotating mass and performance. While many young drivers add bigger alloy wheels with greater width to replace small steel wheels, is there a big enough reduction in rotating mass to warrant the expense?

Many lightweight high performance cars suffer badly when "over-tyred". Wider tyres produce more drag (rolling resistance and aeordynamic) so are less fuel efficient, larger wheels are required which are usually heavier resulting in more inertia (reduced acceleration) and higher unsprung weight (poorer wheel control). Have a look on the Mazda MX5 or Caterham bulletin boards for some discussions on this.

Its also worth remembering that manufacturers have a huge budget for development taking ride, handling and other parameters into consideration. However there are times that marketing pressures overtake engineering sensibilities.

[Big Err]

Many lightweight high performance cars suffer badly when "over-tyred".

Another issue with this has been demonstrated in the office here as one of our admin people who has a MG ZR has discovered the cost of replacing the 205/45/17 Z tyres, so has opted for a budget make.

Another article I was reading from Autoexpress was about braking distances and the exponential increase in wet road stopping distances once the tread depth goes below 3mm. Apparantly the wider the tyre the more critical the tread depth is for dispersing water, thus making the wider tyre less capable in wet conditions than a narrow tyre with same tread depth?

Eric.

[jont]

Another issue with this has been demonstrated in the office here as one of our admin people who has a MG ZR has discovered the cost of replacing the 205/45/17 Z tyres, so has opted for a budget make.

People seem to expect that because sports cars are getting cheap to buy, they will be cheap to run too. On the MR2 forum I frequent, cheap tyres are usually known as TDFs - Taiwanese Ditch Finders.

[hardboiled]

This is probably Stressed Daves realm of expertise...

The question is, does a 20% increase in rubber give a proportional increase in grip, or is it not as simple as that? On top of that, all other things being equal, increased friction = increased rolling resistance = bigger fuel bill?

I also have a question with regard to rotating mass and performance. While many young drivers add bigger alloy wheels with greater width to replace small steel wheels, is there a big enough reduction in rotating mass to warrant the expense?

Check out Don Palmers guide http://www.creativecarcontrol.co.uk/modelgrip.htm
Basically for a tyre under load then wider will produce more ultimate grip. However the reverse also applies under a light load (e.g. rear tyres under heavy braking) then the tyre has the same amount of weight to apply to the road but more tyre width to do it with so the actual force per square inch on the road is less than on a narrow tyre.

Oh and as Gareth pointed out bigger wheels (of same type/quality) are almost certainly heavier. Only going for very expensive alloys gets around this.

[ROG]

On the subject of tyre grip, did anyone else see the experiment that was carried out on TV, where vehicles were made to go around a set circle at ever increasing speeds until the rear wheels broke away?
It turned out that ALL the vehicles lost grip at the same speed even though each FELT different.
Must be something to do with physics!