BHP..and top end.

Bike_craze1 · Trackday Trickster Joined: 23 Sep 2008 Karma :

What other factors do you have to consider about a bikes top end apart from BHP? I mean Derbi GPR has the same BHP as an Aprilia RS125 but the RS goes another 10mph so what other factors involve a bikes top end?
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Yamaha R125 - Cagiva Mito 125 - Honda CBR 600
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Bike Test passed: 15/04/09

Keir · World Chat Champion Joined: 11 Apr 2006 Karma :

gearing, aerodynamics, wheel size.....
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MarJay · But it's British! Joined: 15 Sep 2003 Karma :

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Aerodynamics are the main factor. Although you do need to be geared to get to those speeds.

To be honest I would doubt the Derbi (in full power form) is really any slower than a current RS125 (in full power form), if they have the same amount of power. But the RS was produced in the mid 1990s with 1/4~1/3 more power, and they were somewhat quicker.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

ms51ves3 · Super Spammer Joined: 08 Jun 2007 Karma :

Bike_craze1 · Trackday Trickster Joined: 23 Sep 2008 Karma :

moog · Spanner Monkey Joined: 10 Jul 2005 Karma :

You also have to consider what the rev ceiling is for the engine and where it makes it's peak power and torque.
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Hutchysrs50 · World Chat Champion Joined: 15 Sep 2008 Karma :

also depends on how much the riders lie about there emence 125s hitting 150mph uphill.

also take into consideration the rs might have warp drive fitted.
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Gilera SC 125 - Current

moog · Spanner Monkey Joined: 10 Jul 2005 Karma :

emence?

Think you might mean immense.

But yeah...you are quite correct. My CBR125 would go no faster than 70mph...75mph down a hill (and it was a steep hill) with wind assistance. My mate's would do 90mph apparently but, when he rode with me, it suddenly wouldn't go faster than 70mph.

So basically there's the "bullshit" factor to be considered too.
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"A process which led from the amoeba to man appeared to the philosophers to be obviously a progress...though whether the amoeba would agree with this opinion is not known."

"I've made an odd discovery. Every time I talk to a savant I feel quite sure that happiness is no longer a possibility. Yet when I talk with my gardener, I'm convinced of the opposite."

Hutchysrs50 · World Chat Champion Joined: 15 Sep 2008 Karma :

jonchivers · Nova Slayer Joined: 09 Mar 2008 Karma :

weight of the bike and rider?
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rotax81 · Nearly there... Joined: 10 Sep 2008 Karma :

it takes a bit of everything to get a higher top speed.
a very narrow powerband high in the revs (the motor tuned to its optimum in that area) , the right gearing and good aerodynamics is what you are looking for but i bet a bike that has a more linear power curve gets around quicker Very Happy

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Pete. · Super Spammer Joined: 22 Aug 2006 Karma :

In a roundy-race yes that's undoubtably true but for top-speed what you need is top power near the top of the rev-range.

Aerodynamics is what you need to go fast - if you had a perfectly frictionless shape you would need but a tiny amount of power to go infinately fast. As speed increases the amount of power needed to go the same amount faster goes up by a huge ratio so the best gains are to be had from reducing drag.

The best single value-for-money improvement I saw was a guy fitting a £20 fairing to an old GSX running nitrous - I think he gained about 40mph, I'll check the records.

Edit: Naked GSX1100 (w/nitrous): 164.98mph
£20 fairing GSX1100 (w/nitrous): 191.3mph (later made 200.2mph with strong tailwind)

Aeros rule Thumbs Up

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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good Very Happy

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Weight is basically irrelevant to top speed (just means you take longer to get there).

What you need is torque at high rpm (which is power, as power is torque x rpm). High torque but low revving means you need higher gearing which negates much of the torque.

Here is a graph of thrust against speed in each gear for a 'busa:-

https://www.bikegraph.co.uk/TorqueThrustGraphMulti.php?x_max=938&y_max=501&Id1=13&FrontSprocket1=17&RearSprocket1=40&drag=yes&cd=3.37

The black line is the amount of drag. Where the thrust line crosses the drag line is the theoretical top speed.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing