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  #1  
Old 27-04-2016, 03:57 PM
dunmurryboy dunmurryboy is offline
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Default Yamaha xj900 trike

Hi just purchased a xj900 as a project rear end comes from a Mazda mx5 but seems severely under geared feels like it could pull another two gears can anybody recommend a replacement or any other way round this thank you
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Old 27-04-2016, 09:27 PM
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This is a big problem for shaft driven bike engines powering car final drives.

In very round figures because they do vary.

Most bike gearboxes run at half engine speed because the gear on the clutch is rougly twice the size of the crankshaft gear. Because of this the final drive ratio is about two to one. This gives an overall reduction of about 4 to 1

Inconveniently most front engined rear wheel drive cars run the gearboxes at one to one. and have a final drive ratio of roughly 4 to 1

When you bolt the bike engine shaft to the car back axle you have a reduction of eight to one.

The simplest solution is huge rear wheels and tyres, but this puts a lot of strain on hub bearings wheel bolts/ studs and suspension components. Also it reduces the effectiveness of the brakes.
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Old 27-04-2016, 09:58 PM
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As Harry said bigger wheels steel rims tend to run lighter than alloys if you don't go too wide you should be ok keep to the width of the mazda tyres, but as Harry also said fitting bigger wheels will take more stopping if you have got the mazda master cylinder and servo I would think you will be ok you will have to get a bit imaginative with the plumbing but it can be done you may not need the servo but it's not a bad idea

Sorry I can't tell you what rims will fit the mazda hubs run a hub round a few breakers and try some van rims but they must fit perfectly with all the studs centered in the mounting holes, even the slightest bit out will lead to snapped studs and lost wheels

Cheers Dai

EDIT Remember you can always go down a size or two on the tyres to take a bit of stress of the brakes studs etc, also car tyres are a lot lighter than van one's, budget brand tend to be lighter than premium just don't go for low profile
You may find that tyres with a high profile on the rims you have will up the gearing enough presuming you have the low profile tyres from the mazda still fitted

EDIT another thing if you change from alloys to steel rims you will need open top wheel nuts, the studs will be fine, you could cut the dome off the nuts you have but your better off getting new

A mate of mine was running a GS 850 G engine with a escort diff it was low geared but he didn't get left behind that had 14" rims with high profile tyres fitted
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Last edited by wiskers; 28-04-2016 at 07:54 AM. Reason: Bit after the word edit
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Old 28-04-2016, 04:31 PM
dunmurryboy dunmurryboy is offline
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Thanks for reply I'm running 18" rear wheels and 17" front the front tyre is a motorcycle rear 200x55x17 needs changed worn out
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Old 28-04-2016, 05:17 PM
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Harry over to you

Both my trikes have run car engines

Would have thought running 18" rims should be ok
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Old 28-04-2016, 07:38 PM
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Quote:
Originally Posted by dunmurryboy View Post
Thanks for reply I'm running 18" rear wheels and 17" front the front tyre is a motorcycle rear 200x55x17 needs changed worn out
The front tyre size makes no difference to the gearing. What size tyres have you got on those 18 inch rears?
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Old 01-05-2016, 05:21 PM
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Sorry for not getting back about tyre sizes will find out tomorrow and get back to you ASAP they are big
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Old 03-05-2016, 11:12 AM
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235x?x18 rear tyres fitted
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Old 05-05-2016, 08:47 AM
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Nice big rims there. But to make any sense of it we need the figure where you have a question mark
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Old 06-05-2016, 07:15 AM
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I have no idea what is being talked about here, but I'm intrigued as to what it's all about, and waiting to see the outcome....
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Old 06-05-2016, 08:03 AM
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I have no idea what is being talked about here, but I'm intrigued as to what it's all about, and waiting to see the outcome....
We talk about gear ratios, but what actually matters is how far you travel for each turn of the engine crankshaft.
This is a combination of the overall gear ratio (a combination of primary drive reduction gearbox reduction and final drive reduction) But then this is multiplied by the Diameter of the driving wheels.

The technical term is rolling radius and is roughly the distance from the ground to the centre of the wheel.

The tyre size is marked as (for example) 235/50-18. This means the tyre is 235mm wide (It is measured on a standard sized wheel rim and called a nominal size as it varies depending on the rim width.) 18 inches is the diameter of rim that it will fit. 50 is the aspect ratio and is the differnce between the tyre width and the distance between the rim and the tread expressed as a percentage.

So in the example above the distance from the ground to the hub centre will be half the rim diameter plus half the tyre width (50% aspect ratio) so is 9 inches plus 117.5 mm.

This is only a rough guide to actual rolling radius as tyre flex and things like tread squirm have an effect, but its good enough for what we need.
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Old 07-05-2016, 05:36 AM
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You are now mixing inches and metric!!!!! - 9 inches + 117.5mm????

I'm totally mamboozled by the science, I was useless at maths and physics at school. But quite good at chemistry for some unknown reason!

I've forgotten what the original question to the thread was all about, I'll have to go back over the thread.....
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Old 07-05-2016, 08:37 AM
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You are now mixing inches and metric!!!!! - 9 inches + 117.5mm????
.
Yeah tyres used to be measured in inches cross ply tyres still are, but in order to distinguish between tyre types radial tyres have their width in metric. Therefore as side wall height is measured as a percentage of width we end up with a combination of measurements.

25mm to one inch is good enough for tyre calculations.
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Old 11-05-2016, 08:04 PM
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Sorry about delay about tyre size 285x50x18 rears any idea about tyre pressures thanks
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Old 15-05-2016, 11:03 AM
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All you can do is check for clearance under the rear mudguards and see how much bigger you can go tyre wise. 285x50x18 The 18 bit is the rim diameter which will have to stay the same or change the wheels. You may be able to increase the width a little Thats the 285 bit. But where you can really go bigger is the aspect ratio this is the sidewall height the bigger you can get the better.

To get a big improvement you may have to change or move the muguards.
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Old 18-05-2016, 11:42 PM
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Thanks for all the replies its a lot better but still not 100% been messing with tyre pressures running ,14.5 psi rear 10 psi front
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Old 19-05-2016, 05:58 AM
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Blimey! They're almost flat then? Any chance of a photo of this machine?
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Old 26-05-2016, 02:07 AM
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Sorry about delay I am not at my home very often but will get pics up asap
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Old 26-05-2016, 11:01 AM
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Could be a bit late in this, but here goes anyway.
My xs11 trike had a Mk1 Capri rear axle, this gave the choice of all the available ford Capri/escort diffs (inc LSDs) so plenty of gearing options, and had 10.5" wide rubber on 15" alloys. The gearing was slightly short,, but it launched form lights like a whippet on steroids, but would still pull 125 win some revs left. originally, it had a servo assist master cylinder, but TBH, the plumbing for the vacuum takeoff was a nightmare, and could get a bit sketchy, I swapped it out for a non servo master cylinder, that made the rear brakes MUCH better. Just before it was sold I fitted a pair of 4 pot calipers on the front.



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Old 27-05-2016, 06:08 AM
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Nice Burglar Alarm!
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Old 27-05-2016, 10:53 AM
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Nice Burglar Alarm!
Most effective anti theft device I ever had.
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  #22  
Old 06-06-2016, 05:45 AM
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Sorry about delay I am not at my home very often but will get pics up asap
Well, where's the pics then?
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Old 11-07-2016, 11:37 PM
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Sorry for no pics been in hospital for six weeks but out now so plenty of spare time will get some pics up ..
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Old 13-07-2016, 04:38 AM
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Jeez, what happened?
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  #25  
Old 13-07-2016, 08:14 AM
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Tyre sizing guide.

Although some people mistakenly think the aspect ratio is the actual depth of the tyre wall in mm, it isnt. The profile as we call it is the mathematical ratio of the tyre actual height as a percentage of the width.

Tyre height mm/tyre width mm
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Old 13-07-2016, 08:16 AM
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Quote:
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Sorry for no pics been in hospital for six weeks but out now so plenty of spare time will get some pics up ..
6 weeks is quite some stay.
I hope you are feeling a bit better soon.
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