Front sprocket position

[3narf]

Hi, it occurred to me that when transplanting an engine into a frame for which it wasn't intended the only really important dimension is the front sprocket position. As it must make a difference to how the bike squats under power etc is there a formula for working out the best position, relative to the swingarm pivot and rear wheel spindle? 

Thanks

[Damnthistinleg]

Don't forget swingarm length, swingarm angle, tyre size, tyre type, engine power, suspension characteristics, gear ratios, type of corners on each track, riding style, etc., etc., etc..

If being technical, it's one of those things where there isn't truly a right answer, just varying degrees of wrong.

In reality though, something close to the standard position will probably do fine unless you're going well beyond the design parameters of the chassis.

[3narf]

It would be good if I could find a sufficiently detailed pic of a Sachs XTC125 to see where the sprocket sat initially. 

[Superdunc]

Huge can of worms. It's a subject that has 10,000s  pages of research done on the theory.

Let's keep it simple.

Look at most race bikes side on, then project a line through the rear wheel spindle and swing arm pivot, and you will see that it meets the front sprocket just above the output shaft. That's at ride height with no rider.

Set your bike with the projected line between 10 and 20mm above the output shaft centre line, and you wont be wrong.

[GSXR814]

https://images.app.goo.gl/XSLPNDF1hBUa6AzZA

Should give you a reasonable idea

[3narf]

24 minutes ago, Superdunc said:

Huge can of worms. It's a subject that has 10,000s  pages of research done on the theory.

Let's keep it simple.

Look at most race bikes side on, then project a line through the rear wheel spindle and swing arm pivot, and you will see that it meets the front sprocket just above the output shaft. That's at ride height with no rider.

Set your bike with the projected line between 10 and 20mm above the output shaft centre line, and you wont be wrong.

Bloody brilliant, thanks! That's my starting point, then. 

[Damnthistinleg]

If you're shoving a 150bhp motor in a shitty spindly 125 frame however, all bets are off.

[John21]

All welds are off too.

[TLRS]

8 hours ago, John21 said:

All welds are off too.

 

Should we also try and say something clever about crank position? Shouldn’t that be close to center of mass, heightwise?

[2moto]

17 hours ago, Superdunc said:

Huge can of worms. It's a subject that has 10,000s  pages of research done on the theory.

Let's keep it simple.

Look at most race bikes side on, then project a line through the rear wheel spindle and swing arm pivot, and you will see that it meets the front sprocket just above the output shaft. That's at ride height with no rider.

Set your bike with the projected line between 10 and 20mm above the output shaft centre line, and you wont be wrong.

I'm all for keeping it simple but that's a bit too simplistic, I'm afraid. You really can't answer this question without the context of engine power. 10-20 mm is a huge range.

[John21]

This is indeed a huge topic and there is no formula for a an instant solution as far as I am aware.

@3narf I strongly recommend getting John Bradley's "the Racing Motorcycle" vol.1

It will give you a good solid base on the matters of motorcycle design and geometry and will set you on your way.

 

[hawkati]

1 hour ago, 2moto said:

I'm all for keeping it simple but that's a bit too simplistic, I'm afraid. You really can't answer this question without the context of engine power. 10-20 mm is a huge range.

the more power the less offset from that line then? putting it, er, simply? 

[Superdunc]

24 minutes ago, hawkati said:

the more power the less offset from that line then? putting it, er, simply? 

You also need to consider c.o.g. weight distribution front to rear, wheelbase, swing arm length, even changing the rear sprocket size changes the optimum position.

[2moto]

1 hour ago, Superdunc said:

You also need to consider c.o.g. weight distribution front to rear, wheelbase, swing arm length, even changing the rear sprocket size changes the optimum position.

I thought we were trying to keep things simple! 😅

[hawkati]

2 hours ago, Superdunc said:

You also need to consider c.o.g. weight distribution front to rear, wheelbase, swing arm length, even changing the rear sprocket size changes the optimum position.

That's where i'm at with the blade engine - dithering about where to sling it. The encouraging thing is it's ever so slightly smaller and a couple of kgs lighter than the old skool 600 engine it had so with a bit of an angle grinder frenzy i can have a bit of room to shift it around if the first attempt at engine mounts proves no good. 

[Evilchicken0]

I guess the question is what engine is going in ???

[TLRS]

Don’t forget to factor in motivation. Motogp level of perfection is probably not necessary for having fun building and riding it. So chose priorities wisely.

[wavey]

The million yen question! There's a reason almost all proper race bikes have an adjustable Swingarm pivot and adjustable engine positioning. The front sprocket position has a massive influence on the anti squat percentage (too much anti squat and it'll break traction mid corner and be unstable, not enough it'll squat, not generate enough grip, run wide and be generally shit) swinger length, linkage ratio, shock spring rates, preload, centre of gravity position, final drive ratios and more will also be involved.... 

John Bradley books are a good start to finding an answer... Or find some bikes of similar power and weight that are reputable in the handling stakes and get a measurement off those. 

Then you've only got centre of gravity, inclination and all the ancillaries to worry about 😁

[3narf]

On 12/26/2020 at 6:12 PM, Damnthistinleg said:

If you're shoving a 150bhp motor in a shitty spindly 125 frame however, all bets are off.

Don't worry! It'll be around a 3rd of that, which will make it more fun stuffing it up the inside or riding round the outside of 150bhp bikes at Cadwell Gooseneck!

[3narf]

On 12/28/2020 at 11:34 AM, TLRS said:

Don’t forget to factor in motivation. Motogp level of perfection is probably not necessary for having fun building and riding it. So chose priorities wisely.

Oh yes! And pulling the whole thing apart to make new engine mounts isn't the end of the world.

[3narf]

On 12/28/2020 at 11:21 AM, Evilchicken0 said:

I guess the question is what engine is going in ???

Single cylinder 4-stroke, iro 50 - 70bhp, so it shouldn't be tied in knots by the power.

[2639]

On 12/27/2020 at 11:19 AM, John21 said:

This is indeed a huge topic and there is no formula for a an instant solution as far as I am aware.

@3narf I strongly recommend getting John Bradley's "the Racing Motorcycle" vol.1

It will give you a good solid base on the matters of motorcycle design and geometry and will set you on your way.

 

I concur with the recomendation of the Bradley books. I also suggest Motorcycle Design and Technology by Gaetano Cocco (isbn 88 7911 189 2).

Buying the books worth reading will cost about 150 quid all up ( Id add the Robinson trilogy). One of the books says something like... if this book saves one morning of testing its paid for itself. I think its one of ther JR ones. Never a truer word spoken. It can hard enough working out whats up with some basics when everything is new.

That said... once upon a time I put a XT engine in my AR50. This was deep into death trap territory but still got me the length of Cumbria. And back again. The head was too big so it was all I could do to get a chain run in line and not have the exhaust act as a front brake. No one died. But if your serious about having some fun at decent speed it's going to need to be to a higher standard than my first go (all be it aged 17).

What about making it with a grid pattern of engine mounts, or some sort of shimming arrangement so things can be moved easily. By that token, a sectional ehxaust, a wiring harness thats deliberately long and an airbox partly made of rubber could speed up development.

Wavey mentioning swing arm adjustability is touching up an intersting variable too. Don't think this is for gp level riders only. I run a final drive of 2.94 or 3 to 1. Do the same drive ratio with different sprockets and significant changes in traction and feel become apparent. 

My primary recommendation whilst building projects is... to have a bog stock - just add petrol - CBR6 sat in the corner of the shed ready to ride. 

[Mark/Foggy]

Just a little reassurance here, to get things in perspective.

Given what you are building, max sprocket sizes are a necessity of what you should be aiming for. You definitely want to minimises all chain losses.

In my thoughts this already helps you. I think it gets much more unstable in big engines that have relatively limited space for big front sprockets, and want to put relatively huge torque into the equation.

I mentioned in a build that I was working on a while back and @Dunc pointed out that on 125s the output shaft is almost directly in line with the neutral axes of swing arm position.

Have a little look at that triangle made between rivet centre at the top of the output sprocket, swing arm centre and a rivet at top center of the rear sprocket.

Now imagine that is in tension. so it's a fixed line. Think about how that line would change as the suspension goes through it's travel. It is a vector of the forces.

If that triangle ever reaches a flat line, your engine will be trying to lift the suspension. If it flips the triangle, you will be in squat.

 

[hawkati]

1 hour ago, Mark/Foggy said:

Just a little reassurance here, to get things in perspective.

Given what you are building, max sprocket sizes are a necessity of what you should be aiming for. You definitely want to minimises all chain losses.

In my thoughts this already helps you. I think it gets much more unstable in big engines that have relatively limited space for big front sprockets, and want to put relatively huge torque into the equation.

I mentioned in a build that I was working on a while back and @Dunc pointed out that on 125s the output shaft is almost directly in line with the neutral axes of swing arm position.

Have a little look at that triangle made between rivet centre at the top of the output sprocket, swing arm centre and a rivet at top center of the rear sprocket.

Now imagine that is in tension. so it's a fixed line. Think about how that line would change as the suspension goes through it's travel. It is a vector of the forces.

If that triangle ever reaches a flat line, your engine will be trying to lift the suspension. If it flips the triangle, you will be in squat.

 

Would you be suggesting the centre front sprocket should be slightly below a straight line from swing arm pivot dead centre to rear sprocket dead centre?

[4tgp]

17 hours ago, Mark/Foggy said:

Just a little reassurance here, to get things in perspective.

Given what you are building, max sprocket sizes are a necessity of what you should be aiming for. You definitely want to minimises all chain losses.

In my thoughts this already helps you. I think it gets much more unstable in big engines that have relatively limited space for big front sprockets, and want to put relatively huge torque into the equation.

I mentioned in a build that I was working on a while back and @Dunc pointed out that on 125s the output shaft is almost directly in line with the neutral axes of swing arm position.

Have a little look at that triangle made between rivet centre at the top of the output sprocket, swing arm centre and a rivet at top center of the rear sprocket.

Now imagine that is in tension. so it's a fixed line. Think about how that line would change as the suspension goes through it's travel. It is a vector of the forces.

If that triangle ever reaches a flat line, your engine will be trying to lift the suspension. If it flips the triangle, you will be in squat.

 

Good point about sprocket sizes.  Depending on circuit my Rotax single was about 19T front and 34T rear.

Oh, one other point.....the Rotax designed BMW rear engine mount is also the swingarm spindle.

[Superdunc]

41 minutes ago, 4tgp said:

Good point about sprocket sizes.  Depending on circuit my Rotax single was about 19T front and 34T rear.

Oh, one other point.....the Rotax designed BMW rear engine mount is also the swingarm spindle.

Well that simplifies things.

 

[4tgp]

3 hours ago, Superdunc said:

Well that simplifies things.

 

and it looks like the 2 stroke versions of the Sachs are the same

[spannerman]

On 12/27/2020 at 12:12 PM, Superdunc said:

You also need to consider c.o.g. weight distribution front to rear, wheelbase, swing arm length, even changing the rear sprocket size changes the optimum position.

It takes a long time just going by trial and error, even with advice off someone who's done it it's still a headache

[hawkati]

This is rapidly becoming my favourite thread evvrrrr...

[Bram43]

Bwahahahahahaha....I am so walking away from this thread before I end up back in the psychiatrist’s office*.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

*I can recommend a good one that specializes on such matters, but a few bottles of rum and having someone else do it would be cheaper