RPM to speed relation

dancata · Nov 9, 2009

Hi all,

When I bought the bike('95 1100gs 110k km done so far), 2 years ago, I used to have 90 km/h in 5th @ 3000 rpm and 120 km/h at 4000 rpm fix.

After a valve adjustment at a local service (first and last time I got my bike in a shop, I do my own service now) I observed 120 km/h were only reached when I had 4200 rpm... And the rpm to speed relation was getting worse, and now I have 4400 rpm for 120 km/h.

The front wheel has only been removed since for 3 times only, for tire changing. Valves play is set ok, so is the rocker arm play. Is there something I should watch out for? Does this happen to any of you?

Thanks.

sockpuppet · Nov 9, 2009

I assume your running the tyres with the same tread depth (i.e. not full worn vs. not worn at all) and the same tyre pressures.

I'd say its either the speedo or the tacho need calibrating as the gearing hasn't changed so its "impossible" for the engine to have to spin faster to go the same speed. I'd get a friend (with a decent speedo) to ride with you so you can see if your speedo is reading the accurate kmh.

Schtum · Nov 9, 2009

sockpuppet said:
so its "impossible" for the engine to have to spin faster to go the same speed.

Unless the clutch is slipping..... :nenau

carrera2 · Nov 10, 2009

Does it sound/feel like it's revving more?

Could simply be a faulty rev counter.

MattW · Nov 10, 2009

My guess would be different tyres / tread depth / tyre pressures affecting the rolling diameter of the wheels. The revcounter just counts ignition pulses so it'll either work or not - I guess it could be drag in the speedo?

If you have a sat nav try using that to check your speedo (but don't expect it to agree with the speedo reading - my speedo consistently reads high which is normal).

MarkShelley · Nov 10, 2009

MattW said:
My guess would be different tyres / tread depth / tyre pressures affecting the rolling diameter of the wheels. The revcounter just counts ignition pulses so it'll either work or not - I guess it could be drag in the speedo?

If you have a sat nav try using that to check your speedo (but don't expect it to agree with the speedo reading - my speedo consistently reads high which is normal).

I second that. My 1100 reads 5mph over at all speeds above about 15mph.

dancata · Nov 10, 2009

It's the same front and back tire, Anakee. Clutch is not slipping, as I would know if it were, and has been recently changed, so we can rule out the clutch.
I am aware that comparing my speedometer to a satnav would not indicate the same speed, but my concern is the discrepance in different rpm's at the same speed after a while.

carrera2 · Nov 10, 2009

MattW said:
The revcounter just counts ignition pulses so it'll either work or not

I know of rev counters that over-read by 30% on cars. I assume the principle is the same here.

littleredrooster · Nov 10, 2009

The only possible explanations are wheel-spin (!) or clutch slip for that amount of difference. There is a fixed relationship between the engine and the wheels and the revs/speed ratio will always be the same.
(Assuming neither instrument is dying)

RPM to speed relation

dancata

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sockpuppet

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Schtum

Iconoclast

carrera2

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MattW

Well-known member

MarkShelley

Very similar to Paolo Maldini

dancata

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carrera2

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littleredrooster

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