changing wheel/tyre size and speedo adjustment?

[masha]

Does anyone also know if it is possible to change overall diameter of wheels & tyres on 7s without having to adjust the speedo calibration and if so how is adjustment best done.

 

I'm running 13" revolution alloys on my Supersprint and I'd like to fit CR500s after a modified sump conversion but present tyres are 185/75/13 CR500s are 185/55/13.

 

Any advice on changing over appreciated.

[mike ashurst]

If check your speedo (eg timed measured mile) you'll probably find that it's quite a long way out anyway, so changing the rolling radius will just exacerbate the existing problem. 7 speedos are famous for there inaccuracy.

 

It might sound silly, but a pushbike speedo can be fitted using a sensor on the front upright and a magnet attached to the hub which allows you to change the wheels and have an accurate speedo whilst ignorring the standard one. I've done it, and it definitely works. Theres a thread dealing with it somewhere if you can't find it try looking here:-

 

http://www.strangely.org/

 

But remember to alter the fixing position to the upright/hub rather than the stay/wheel.

 

Mike

[masha]

Thanks Mike - I'll try that:

 

One further question for you or anyone else viewing is:

 

- what would happen to handling if I fit 175/55/13s up front and leave my 175/75/13s at the back?

 

All four tyres are similar direction treads, but distinctly different side-wall (more at the back) and therefore bigger outside circumference than the fronts.

 

Thanks - I guess I'll get through the MOT which is pending because treads are good, but there's the sidewall issue and safe handling cornering to consider.

 

Matthew

[stevefoster]

With the existing speedo drive from the gearbox the size of the rears is what changes the speedo gearing I believe.

Do like me and many others and fit a bike speedo on the front and it the circumference there that is important.

Bike speedo's are cheap and brill'! The Sigma 1200 for example allows for independant logging of 2 different wheel sizes, say low profile slicks for the track and another setting for higher profile road tyres. You just toggle between them. It also allows for certain data to be summed for both settings i.e total distance covered. You can reset the total milage covered so it doesn't have to start from zero when the battery runs out.

Jason B and I calibrated our bike speedo's on a measured mile...v. accurate indeed.

See the web link above to Strangely....