ALLOY PROBLEMS
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andytheman108
- New
- Posts: 4
- Joined: Thu Apr 28, 2005 4:59 pm
ALLOY PROBLEMS
HI HAS ANYONE HAD ANY PROBLEMS WITH THERE GEAR BOX AFTER PYUTTIN 16 " OR 17" WHEELS JUST SOMEONE TOLD ME THAT THEY DID AND WANTED SOME MORE EVIDENCE B4 PUTTIN SOME ON MINE!
No, if you think about it it, increasing the size of the wheel increases the gearing, since larger wheels turn slower. Now, the torque is scaled inversley with the gear ratio, thus increasing the gearing, reduces the torque - which you already know - since to overtake, you reduce the gear ratio by changing down, and you accelerate faster.
What you say is correct about the speedo, since the wheel rotates slower for a given road speed. This is why the speedo reads wrong. You could say that it would increase the top speed, since at 5,800 RPM in 5th you would now be going faster proportional to how much larger your wheels are. But unfortunatley this isnt the case, because even with standard wheels all but the GTi are not capable of reaching maximum RPM in top gear, since the resistance becomes too great for the engine to overcome.
Sadly, when you increase the wheel size, you actually do lose top end speed as well, because the point at which the engine output equals the drag occures at a slower speed (the point at which you cannot accelerate any more), unless you increase the amount of power that the car produces. Physics dictates that power = torque * speed of rotation. So by changing one factor you affect the others. So you can see that if the speed of rotation drops (with larger wheels) and the power output remains the same, then the torque has to fall.
So to maintain the performance you have to ensure that either your rolling circumferenence remains the same, or you change the final drive ratio, or you put up with the reduced acceleration. But its not practical to keep the rolling circumferance the same when you put very large rims on because it would require a very low profile tyre.
One other point to note is that the rolling resistance of your car contributes more than you would expect to the top speed of the car, its about 20%. This resistance is caused by the width and type of tyre compound, and the pressure in your tyre. So soft tyres that are very wide will increase the drag significantly. More than you would think. Its all about trade offs.
What you say is correct about the speedo, since the wheel rotates slower for a given road speed. This is why the speedo reads wrong. You could say that it would increase the top speed, since at 5,800 RPM in 5th you would now be going faster proportional to how much larger your wheels are. But unfortunatley this isnt the case, because even with standard wheels all but the GTi are not capable of reaching maximum RPM in top gear, since the resistance becomes too great for the engine to overcome.
Sadly, when you increase the wheel size, you actually do lose top end speed as well, because the point at which the engine output equals the drag occures at a slower speed (the point at which you cannot accelerate any more), unless you increase the amount of power that the car produces. Physics dictates that power = torque * speed of rotation. So by changing one factor you affect the others. So you can see that if the speed of rotation drops (with larger wheels) and the power output remains the same, then the torque has to fall.
So to maintain the performance you have to ensure that either your rolling circumferenence remains the same, or you change the final drive ratio, or you put up with the reduced acceleration. But its not practical to keep the rolling circumferance the same when you put very large rims on because it would require a very low profile tyre.
One other point to note is that the rolling resistance of your car contributes more than you would expect to the top speed of the car, its about 20%. This resistance is caused by the width and type of tyre compound, and the pressure in your tyre. So soft tyres that are very wide will increase the drag significantly. More than you would think. Its all about trade offs.
wheels
correct!JWC wrote:No, if you think about it it, increasing the size of the wheel increases the gearing, since larger wheels turn slower. Now, the torque is scaled inversley with the gear ratio, thus increasing the gearing, reduces the torque - which you already know - since to overtake, you reduce the gear ratio by changing down, and you accelerate faster.
well explained!
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bstardchild
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Sorry, was a slow afternoon, been banging my head against the wall with some dudes from Aerosystems, and Brunel. I think I'm going to send them a couple of post-it-notes with a nice diagram marked 'arse' and 'elbow' !!!
The point I was trying to get across is that VW don't put on the wheel sizes that they do for fun, or to keep the cost down. They do it to keep the car in what they consider to be an 'acceptable performance envelope'. Larger wheels look nicer, but there is always a trade off, and going over 16" would make your 4th like your old 5th gear, which makes your 1st almost like your old 2nd, which isn't really a good thing.
There is legislation on the way in the UK, to prevent people fitting wheel and tyre combinations on cars registered after 2002 to a larger overall diameter without special approval, I think that means recalibration of your speedo will be madatory - but should be simple as they are all electronic. Fitting larger wheels also keeps your cars mileage down, which isn't legal either.
The point I was trying to get across is that VW don't put on the wheel sizes that they do for fun, or to keep the cost down. They do it to keep the car in what they consider to be an 'acceptable performance envelope'. Larger wheels look nicer, but there is always a trade off, and going over 16" would make your 4th like your old 5th gear, which makes your 1st almost like your old 2nd, which isn't really a good thing.
There is legislation on the way in the UK, to prevent people fitting wheel and tyre combinations on cars registered after 2002 to a larger overall diameter without special approval, I think that means recalibration of your speedo will be madatory - but should be simple as they are all electronic. Fitting larger wheels also keeps your cars mileage down, which isn't legal either.