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Old 8th November 2004, 03:52   #16
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But, you have to consider that most cars hit their top speed in 4th gear and the 5th gear actually drops the speed. So, for these cars, wheel size will make a diff.

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Old 8th November 2004, 03:55   #17
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No, see you're increasing engine power here. Increased engine power will give your car a higher top speed, But that is not what we're discussing here. *

By just changing the wheel diameter, top speed in 5th gear maynot be affected. But if a car hits top speed in 4th gear at redline, then it surely will.

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Old 8th November 2004, 04:00   #18
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Quote:
Originally Posted by [b
Quote[/b] (Shan2nu @ Nov. 08 2004,01:22)]So, for these cars, wheel size will make a diff.
ya probably would.

say a wagon R ... as per ACI.. top speed 151km/h at 6050rpm in 4th gear.
so a very slightly smaller tyre.. might take it to 151 @ 6300rpm....

OOPS... won't work on a wagon R!... as the BHP actually goes down.. after 6000 rpm!!

Anyways.. we were talking about tyre pressures!... if that would change the speeds in gears (don't think abt top speed as it will surely suffer b'coz of low pressure).
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Old 8th November 2004, 04:05   #19
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Quote:
Originally Posted by [b
Quote[/b] ]well... AW is not only dependent on the car but also the engine. The BHP it produces @ 6000 RPM is say 100..... so it can't cross 195km/h taking into consideration the shape. but if somehow.. its running at 6500rpm its producing 103bhp.... so it might be able to move forward say till 105bhp and 200km/h.. i.e. it hits its new AW equilibrium.
I'll tell you where you're getting confused. Ok, at 6000 rpm in 5 th gear, the vtec is producing 100bhp and the true speed at this point is 195kmph.

now you say that if i use a smaller wheel the rpm in 5th will rise to 6500rpm with the engine producing 105 bhp, right? But the true speed will still not increase here. Boc the smaller wheel will need to have a higher rpm than the bigger whel at 195kmph.

So, what speed the big wheels manage at 6000 rpm engine speed, the small wheels will need to hit 6500 rpm to do the same. Forget what the speedo is showing, coz, at 6500rpm in 5th, it is caliberated to show 200kmph, but that isn't the true speed. The car is still doing 195kmph.

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Old 8th November 2004, 04:10   #20
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Quote:
Originally Posted by [b
Quote[/b] ]say a wagon R ... as per ACI.. top speed 151km/h at 6050rpm in 4th gear.
so a very slightly smaller tyre.. might take it to 151 @ 6300rpm....
With smaller tyres, the Wagon R will hit less than 151kmph (true speed) at 6050rpm in 4th.

Quote:
Originally Posted by [b
Quote[/b] ]Anyways.. we were talking about tyre pressures!... if that would change the speeds in gears (don't think abt top speed as it will surely suffer b'coz of low pressure).
Yes, you will have to take the rolling resistance of both the tyres into consideration before making any conclusions. Practically speaking, it's impossible to get accurate figures.

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Old 8th November 2004, 04:16   #21
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Here's a nice calculator which should help.

speed/rpm calculator

Just play around with the wheel diameter (keeping all the other variables, unchanged) and see how it makes a diff.

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Old 8th November 2004, 04:16   #22
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Quote:
Originally Posted by [b
Quote[/b] (Shan2nu @ Nov. 08 2004,01:35)]now you say that if i use a smaller wheel the rpm in 5th will rise to 6500rpm with the engine producing 105 bhp, right? But the true speed will still not increase here. Boc the smaller wheel will need to have a higher rpm than the bigger whel at 195kmph.
1).. now see.. 105bhp@6500rpm (195kmph true speed).
2) original was 100bhp@6000rpm (195kmph true speed).

in case 2) the car isn't able to move forward as it doesn't produce enough bhp at 6001rpm to cross 195kmph.

in case 1) the car should go ahead as its producing more bhp required to cut through the air than the car requires at that speed!... *6501rpm is giving 105.1bhp.. which is more than in case 2)

So i say... for most cars which hit redline or nearabout in 4th .. top speed won't increase.

But the 5th gear speed will increase!





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Old 8th November 2004, 04:21   #23
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Quote:
Originally Posted by [b
Quote[/b] ]in case 1) the car should go ahead as its producing more bhp required to cut through the air than the car requires at that speed!... *6501rpm is giving 105.1bhp.. which is more than in case 1)
That's bcoz the engine is using up the 5 xtra bhp, in order to make the smaller tyres revv that extra bit more so that the true speed reaches 195kmph. In both cases, the car is reaching it's limit at 195kmph. the only way the car can hit 200kmph (true speed), is if the overall weight of the car is reduced.

Quote:
Originally Posted by [b
Quote[/b] ]So i say... for most cars which hit redline or nearabout in 4th .. top speed won't increase.
Any car which hits top speed in 4th at redline will have a lesser top speed if the wheel diameter is reduced. At 7100 rpm in 4th the Vtec wheel rotates at a certain rpm, now a smaller wheel in its place will rotate at the same speed but since it's of a smaller circumference, it covers a lesser dist for every rotation, so the car's speed at 7100 rpm is less too.

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Old 8th November 2004, 04:29   #24
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Quote:
Originally Posted by [b
Quote[/b] (Shan2nu @ Nov. 08 2004,01:51)]That's bcoz the engine is using up the 5 xtra bhp, in order to make the smaller tyres revv that extra bit more so that the true speed reaches 195kmph.
I say the 5 extra bhp is still available for acceleration. its not being used.

Now why would the small tyres require more bhp? to reach a certain rpm (wheel rpm)
its like.. If you are in a smaller gear... its easier to revv as it takes less bhp... and the extra bhp gives u faster acceleration!
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Old 8th November 2004, 04:37   #25
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Quote:
Originally Posted by [b
Quote[/b] ]its like.. If you are in a smaller gear... its easier to revv as it takes less bhp... and the extra bhp gives u faster acceleration!
Youre' right, but in 1st gear, when the car is producing 106 bhp at 6800 rpm, the speed of the car is in the 50s, where as in 2nd gear, 6800 rpm, 106 bhp, the car has reached 95+kmph. What speed the smaller gear reches with 106 bhp, the bigger one can reach with a much lesser figure. So the smaller gear uses up more horse power.

Here the gear sizes make a diff, in the same way, the wheel size also makes a diff.

Technically, that 5 bhp in top gear is not useable at all. It can only be used if the weight of the car is reduced or if the frictional and aerodynamical resistance is reduced.

Engine power is like RAM in your comp, stop one programme and the RAM is diverted to the other running programmes making them work faster.

Practically, we can never come to a conclusion. This whole discussion is based on therotical conditions/ calculations.

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Old 8th November 2004, 04:47   #26
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Why do you think, making a car lighter increases it's acceleration and top speed. That's bcoz, the power that the engine was using up in pushing the extra kgs, is now being utilised in propelling the car against aerodynamical resistance.

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Old 8th November 2004, 04:54   #27
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Quote:
Originally Posted by [b
Quote[/b] (Shan2nu @ Nov. 08 2004,02:07)]Youre' right, but in 1st gear, when the car is producing 106 bhp at 6800 rpm, the speed of the car is in the 50s, where as in 2nd gear, 6800 rpm, 106 bhp, the car has reached 95+kmph. What speed the smaller gear reches with 106 bhp, the bigger one can reach with a much lesser figure. So the smaller gear uses up more horse power.
.. i am seeing it the other way...
1st gear is going 30kmph ... say 4500rpm @70bhp
2nd gear is going 30kmph ... say 2000rpm @45bhp

now
1) is 1st gear using that 70bhp? I say its using the extra bhp for faster acceleration. than 2nd.

2) Assume a strong wind* starts to blow against the car the car isn't able to go forward in 2nd gear at 45bhp and 2000rpm. BUT if it was a smaller gear it'll go forward till it reaches the point where the power required = power produced.

So same with tyres.

ok ok .. this was my final post .. my net is gone down anyways.

ps: enjoyed the chat

*the wind.. is really strong!!
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Old 8th November 2004, 05:09   #28
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Quote:
Originally Posted by [b
Quote[/b] ]2) Assume a strong wind* starts to blow against the car the car isn't able to go forward in 2nd gear at 45bhp and 2000rpm. BUT if it was a smaller gear it'll go forward till it reaches the point where the power required = power produced.
Now the variables are being changed. However, even if there is wind blowing from the other direction, i don't see why a car shouln't be able to hit 95+kmph at 6800 rpm in 2nd gear. ofcourse it will be much more relaxed compared to the 1st gear, but that has nothing to do with tops speed. Finally, the 2nd gear will have a higher top speed and that is what wer're talkin here.

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Old 8th November 2004, 06:15   #29
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Quote:
Originally Posted by [b
Quote[/b] ] i don't see why a car shouln't be able to hit 95+kmph at 6800 rpm in 2nd gear
Sorry man I can't explain my point of view to you anymore! I fail
but you can try yourself!
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Old 8th November 2004, 17:31   #30
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If top speed could have been increased by reducing wheel size, i'd have 3 inch wheels on my Vtec. Then maybe id be able to hit 250+kmph. Your theory will actually prove my statment right. But you and i know, that it's just not possible.

Using smaller wheels, is like using shorter gears. It doesn't matter at which end the size reduces, the effect will be the same. Smaller wheels will give you faster acceleration throught the gears but your top speed has to decrease.

Ask any professional tuner and he'll tell you the same thing.

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