Evening all

Since i built the stylus i have had a strange vibration at higher speeds, but only in 5th gear. Its the sort of vibration you would expect from a prop or half shaft being out of balance rather than an an engine vibration.

It doesnt matter what speed i do in 4th (or any other gear) , the red line in 4th is 140 mph, there is no vibration, but as soon as i put in 5th at anything above about 70 mph, i get the vibration. I rarely notice it on the road and just pootling about it doesnt do it but on certain tracks with longer straights i can get to the end of 4th gear which means having to use 5th and the vibration which at those speeds is quite disconcerting, doesnt feel good. It also does it if i coast at higher speeds in 5th.

Initially i thought it was the gearbox so i changed it for a different one. I have also since changed the engine, the half shafts, had new prop made by a prop person,new CW&P, new wheels and tyres and it still does it. I cannot think of anything else but some body mentioned today it may be a harmonic balancing issue.

Anybody got any ideas ?

Engine mounts?

the engine mounts are solid as they are on most modified rx7's. The vibration is definitely speed related and not engine revs related.

It could be harmonics with the differing rotational components. Whats your diff ratio?

a good read on here regarding set up and harmonics.
http://www.iedls.com/Education-Zone.asp?CID=6

thanks guys. when first built it was a 3.09 diff but i then changed to a 3.77. I obviously didnt notice as much with the old diff ratio as it would do over 80 in 2nd gear so i never really got to use 5th in anger.

that is very interesting. I will have to have another look at the angles of my prop shaft. It lines up quite well in relation to height but it could be better in relation to sideways position.

It is the only in 5th gear even when coasting aspects that puzzle me ----

when say coasting, i mean with the clutch pedal up but off the throttle so effectively decelerating using engine braking. I am going to have a good look at my prop angle and will feedback. from a side view the diff nose pretty much points at the splne entry on the gearbox but from a rear view the diff is off center to the gearbox. I need to check if the off set is equal at both ends and double check the height and anle