Effect on steering of moving calipers from in front to behind forks

[Simon_London]

I wonder if anyone would like to comment?

I reversed the forks on my G5 cafe racer and mounted 4 pot Brembo calipers. Brakes are great and retain the original M/C and linked system.

At the time I noticed steering was effected with a slight reluctance to turn before dropping in slightly unnervingly. I put this down to squared off tyres.

Steering head bearings are good and properly adjusted.

I changed the tyres  (correct 110/90 rear and 100/90 front Avons) and if anything, the issue is slightly more pronounced.

Discs are heavier - HMB floating.

So I put the original Brembo P08s back on and hey presto some improvement.

However I think I will try front mounting them again to see waht happens.

I wonder if the practice of front mounting the calipers on 70s Tonti bikes was actually using their their best position and that the change to rear mounting was really an asthaetic decision.

[pete roper]

When did you stop studying physics at school and how much do you understand about how a motorcycle steers?

Pete

[Simon_London]

Pete, I can appreciate that altering weight distribution will effectt the steering. You don't need to study physics to realise that. (Although I did until my md 30s).

However I am interested in people's practical experience. Alot of Guzzi caf racers have 4 pot caipers fitted and they work great, braking wise.

I am also interested in Guzzi's decision to move the calipers rearwards. My old Mk2 Le Mans  didn't steer as wells as my old Mk1 which I always put down to the fairing but I now wonder about the caliper position as well.

[lucky phil]

I wonder if anyone would like to comment?

I reversed the forks on my G5 cafe racer and mounted 4 pot Brembo calipers. Brakes are great and retain the original M/C and linked system.

At the time I noticed steering was effected with a slight reluctance to turn before dropping in slightly unnervingly. I put this down to squared off tyres.

Steering head bearings are good and properly adjusted.

I changed the tyres  (correct 110/90 rear and 100/90 front Avons) and if anything, the issue is slightly more pronounced.

Discs are heavier - HMB floating.

So I put the original Brembo P08s back on and hey presto some improvement.

However I think I will try front mounting them again to see waht happens.

I wonder if the practice of front mounting the calipers on 70s Tonti bikes was actually using their their best position and that the change to rear mounting was really an asthaetic decision.

If you can feel the theoretical differing effect of the front/rear caliper position on a bike like a Guzzi that steers like an ocean liner I'd be truly amazed and suspect you should be racing in MotoGP.

Ciao

[TOMB]

My 1978 G5 had the disc calipers brake calipers mounted to the front of the forks my cx100 LeMans 1980 had them mounted in the rear pretty much the same motorcycle except for the cosmetics
TOMB

[pete roper]

If you can feel the theoretical differing effect of the front/rear caliper position on a bike like a Guzzi that steers like an ocean liner I'd be truly amazed and suspect you should be racing in MotoGP.

Ciao

With P8's it is barely noticeable. With axial mount 4 spots it is. I've done it 'Both Ways' and with the longer, more massive, calipers you can feel a substantial difference. Lots of other factors in as well like wheel mass etc.

Pete

[Dukedesmo]

Discs are heavier - HMB floating.

Heavier discs will make it turn slower, as will heavier wheels - I replaced the wheels on my 916 with light weight Marchesini wheels that are significantly lighter than the original and it turns so much quicker.

I doubt moving the calipers either side of the fork would make any noticeable difference?

[mtiberio]

this is why I never run more than one front rotor and caliper on a guzzi. unecessary, and heavy. Also get some nice EBC floating rotors with aluminum centers. Much lighter. I have the 4 piston caliper on my eldovert behind the fork leg because the caliper is made to go behind. That is the pistons are different sizes, and the pad will wear tapered if you put it on backwards.

[Idontwantapickle]

this is why I never run more than one front rotor and caliper on a guzzi. unecessary, and heavy. Also get some nice EBC floating rotors with aluminum centers. Much lighter. I have the 4 piston caliper on my eldovert behind the fork leg because the caliper is made to go behind. That is the pistons are different sizes, and the pad will wear tapered if you put it on backwards.

:1:
One well thought out disk should be enough on a street bike.
 

[redrider]

Good read. We all were told decades ago that moving the calipers to the rear reduced "unsprung weight". Not being an engineer or physicist, I interpret that as hitting a bump and the suspension is forced to push the mass of the caliper, along with the rest of the wheel upwards as opposed to lifting the caliper mass when positioned behind the fork leg. Compared to the total mass of the front assembly, the mass of the calipers is miniscule. Lots of variables. What else can I learn? Enlighten me.

[rodekyll]

I'll go with Phil on this one.  I've been accused of "princess and thee pea" pickiness about non-existant and nonconsequential 'differences' after doing things with my bike.  It feels different.  Either change it back or get over it and ride.

[Triple Jim]

Good read. We all were told decades ago that moving the calipers to the rear reduced "unsprung weight". Not being an engineer or physicist, I interpret that as hitting a bump and the suspension is forced to push the mass of the caliper, along with the rest of the wheel upwards as opposed to lifting the caliper mass when positioned behind the fork leg. Compared to the total mass of the front assembly, the mass of the calipers is miniscule. Lots of variables. What else can I learn? Enlighten me.

Clearly, unspring weight does not change by moving that weight around, and no one ever told me it does.  The advantage to calipers mounted behind the fork tubes is less rotational inertia about the steering axis, since the calipers are closer to that axis.  This should make the front end feel a little easier to turn when the bars are moved.  Long ago, I moved my Kawasaki H2's caliper to the rear, and did not feel any difference, but I'd like to hear what others think.

If something gives way under hard braking and a caliper tries to come loose, having it rear mounted might allow it to jam in place instead of rotating around the disk and ripping loose, but that's an unlikely scenario.

[Dukedesmo]

If something gives way under hard braking and a caliper tries to come loose, having it rear mounted might allow it to jam in place instead of rotating around the disk and ripping loose, but that's an unlikely scenario.

I would imagine that this is one reason that calipers have moved to the rear in the last 30 years or so, meaning the mounts can be machined lighter due to requiring less material to hold the caliper in place, that and the reduction of the amount of crap/water that can get into the pads.

[Rusnak_322]

any difference you feel is only in your head.
there is zero difference, both are sprung weight, moving it 3" fore or aft is not going to be something that you can feel. 

hell, the front wheel with tire from my Guzzi weighs more then both wheels and tires on my Ducati and the Ducati has considerably wider tires.

[Simon_London]

Thanks for the input.

Certainly not in my head Rusnak. Very noticeacle with 4 pots, less so with PO8s, but never the less apparent.

My floating front discs are heavier thans stock. I wonder what effect this my be having. I might switch to the old discs to compare.

Wheels are Borranis, much lighter thans cast Guzzi wheels.

[Rusnak_322]

I don't know why I am arguing this, but...

Switching from 4 pots to P08s will be oz of weight, maybe a pound total. You could feel that, very slightly maybe, but whether they are on the front of the fork leg or the rear is irrelevant. You will have minimal mass centralization, but not that you would be able to notice when riding. Heavy or lighter disks will make a noticeable difference in themselves, but that is not what you are talking about.

The brake caliper will move up and down with the fork lower, the springs and dampening wont know or care if the caliper is on the front or rear. Turning, the wheel moves so little and so slow that the weight being lower or higher on the fork leg wont matter during slow or high speed turning. when you swap front to back, I am guessing you are swapping fork legs from side to side. the axel is centered in the fork lower, so that will not change geometry at all.

I just see no way that it can change how the bike will feel or act when in motion. 

[canuck750]

Reminds me of when I took some Guzzi 850T heads into my local machine shop last year, this time I decided to go for porting and polishing, maybe bigger valves, new inserts, stiffer springs and then bigger carbs and exhaust.

The machinist listened carefully, he is a vintage dirt track and moto cross racer in the local senior class, loves old bikes and making horse power.

I asked for his honest opinion, is it worth it, will it make a noticeable difference, what else should I consider?

He politely looked me in the eye and said I could get better results if I lost ten pounds around my gut and worked out.

Now that's wisdom.

[redrider]

I did some digging and unsprung weight is the weight of suspension components, wheels, tires and brakes not supported by the suspension proper. The chassis, engine and such are sprung. They are isolated from the unsprung weight by the springs. I do remember reading the road tests years ago where it was explained that unsprung weight was reduced by moving the calipers. Hogwash is my conclusion. Another case of some-one making an unsubstantiated claim and having that claim interpreted as fact and thus entering the book of common but not necessarily true knowledge. After all, Journalists throw words at paper. Truth is not always included.

[mtiberio]

As stated by another, it is reduction in moment of inertia. Moving weight closer to centerline of steering stem...

[oldbike54]

I did some digging and unsprung weight is the weight of suspension components, wheels, tires and brakes not supported by the suspension proper. The chassis, engine and such are sprung. They are isolated from the unsprung weight by the springs. I do remember reading the road tests years ago where it was explained that unsprung weight was reduced by moving the calipers. Hogwash is my conclusion. Another case of some-one making an unsubstantiated claim and having that claim interpreted as fact and thus entering the book of common but not necessarily true knowledge. After all, Journalists throw words at paper. Truth is not always included.

 You may be remembering an article about the ATK which had the rear brake mounted at the front of the swing arm , actually at the same point as the counter shaft sprocket , or an article about cars with inboard brakes . Both effectively reduce unsprung weight .

 Dusty

[Dukedesmo]

If anything having the calipers behind the fork leg makes them lower thus lowering the CG but, like folding down the rear seats in your 'hatchback' I can't see it making any noticeable difference.

Some years ago I embarked on a programme of upgrades on my Ducati 916, I reduced the weight, tuned the engine, upgraded the suspension and brakes but the biggest single improvement was fitting lighter wheels, totally transformed the way it changes direction.

In hindsight if I could do only one of the mods I would choose the wheels, turned out better overall than even a 15hp power hike.

IMO, without a doubt the best performance upgrade on a bike is less weight, especially of parts that rotate...

[Muzz]

I did some digging and unsprung weight is the weight of suspension components, wheels, tires and brakes not supported by the suspension proper.

That is how I understand it.

I have always thought (with absolutely no scientific backup) that by moving the caliper to the rear would mean the braking forces would be transferred to a point higher up the fork and closer to the bottom clamp, leading to a more stable bike when under braking.

The lower the unsprung weight the better the handling and the more complaint the suspension. Really noticeable on a mountain bike.

[Simon_London]

I'm interested in your comments. For all your doubters simply changing the 4 pots back to Brembo P08s improved matters considerably, as much as I liked the more powerful braking. Before switching the calipers around to the front I will change the discs back to stock as the current set are a fair bit heavier and see what effect that has.

Its not the weight of the 4 pots that is effecting the steering, but that the weight is carried lower.

[lucky phil]

As stated by another, it is reduction in moment of inertia. Moving weight closer to centerline of steering stem...

This is absolutely correct and was the original reason for the relocation and standardisation of nearly all bikes back in the 70's. Its totally logical and makes sense however I'd be amazed if anyone could actually feel any tangible difference in the real world on something like a Guzzi.
From my recollection it was more to do with steering and fork stability not actual turn in or cornering.  For you to feel a difference due to the effect rear mounted caliper have over front mounted with regards to moment of inertia the steering has to actually be moving and in normal riding and cornering it for all intents and purposes at any genuine speed doesn't.
It was more about reducing steering oscillation and possible tank slappers and weave from memory.
Opinions about "feel" are never to be trusted in these situations as the only worthwhile evaluation is a blind test IMHO.
Plenty of racers have gone out on an identically set up bike and come back in quicker and totally happy with the new set up after being told of changes that weren't actually done.

Ciao 

[Simon_London]

An update for those interested.

I switched the forks round and put the P8s back on (in front of the forks).

Most definitely improves steering (in my opinion). More neutral and quicker. Doesn't surprise me in hind sight. Big chunks of metal placed in a different position on a turning fork would seem likely to change the feel. At the time when I made the mods, that there might be an effect on steering didn't occur to me.

Sorry if this offends all the outraged posters on this thread.

[Xlratr]

But the main reason for keeping the calipers up front should be that it just looks better! 

I always thought my Laverda 3c looked much better than the later models, same for my T3. Now, I wonder if I can relocate them on the Stelvio! 

[wirespokes]

I've wondered about this as well. There was a time in the 80s when BMWs positioned the calipers in front of the forks and I always wondered why. And then why did they go back to the rear mounts?

The most logical reason I could come up with was the rear mount being more stable. As the forks turn, the caliper has to swing upwards a little. Perhaps that helps reduce a tank slapper - though I must admit, I still had a tank slapper on a bike with rear mounted calipers.

From an engineering viewpoint it seems much better to have the caliper pushing the slider rather than pulling it when braking. Somehow, front mounted doesn't look right to me, though I like the looks of the T3.