Steering Stem- the weak link

[8541Hawk]

The stock front end on a VTR has gotten a pretty bad reputation as being "flexy" or weak.
Personally I can remember way back when we would have killed for a 41mm set of forks, but again I am old =).

If the issue was with the fork tubes, you would think someone would have sourced a set of higher quality aftermarket tubes.
Though in reality the forks are actually pretty good. The issue is the triple clamp and even more so the steering stem.

So lets look at what I see as the main issue.

Here is a stock VTR stem:



and for comparison this is what you get with a SP or Blade triple:



Then pull out the measuring stick and you get this:
Stock -


SP-


and the final visual, the difference between the pins needed for a pit bull stand to lift each front end:



Which leads to my opinion that you can get very good results with the stock forks if you address the triple clamp issue.

[popkat]

I don't see an issue with stem flex, looks to me like the SP one is aluminium where as the Storm is steel, the aluminium one would need to be thicker to have the same strength as steel.

You'd more likely have an issue with the bottom yoke (tree) flexing, this could be improved with a more substantial item. even then I think you'd need to push on very hard to notice and that's beyond what most Firestorm riders would put their bike through.
I have Firestorm front end including yokes on my VFR race bike, I may have to sort something in the future as I'm monitoring it, if I raise the forks up (with the caps off) the tyre easily clears the front of the lower fairing but in race use I have a slight tyre mark on the lower, this is either the forks flexing backwards under full braking or the bottom yoke flexing (or both), the bike rides well but as said it's something I may have to think about modifying later or have a new one made.


.

[kenmoore]

Before I did my USD I experienced flex in my front end under very hard brakes with CBR F4i calipers.

I fitted a fork brace and the problem was almost gone, not completely but almost.

I have a CBR600rr lower triple now and it is much more substantial than the original storm but uses the original size bearings.

Even though I now have a USD setup after my experience I think it is the tripple clamp that causes the problem, it's just not strong enough when pushing the front hard under brakes.

[AMCQ46]

like popkat pointed out, the storm has a steel yoke spindle and the Blade is aluminium, so to compensate the reduction in material strength it will need to be thicker. ... but I haven't done the sums to see if the sizes make them the same strength in bending or if the blade one is stronger... so no conclusion, just an observation.

From my Aprillia USD conversion I can report:
1) that Aprillia use a steel stem, but it is a much bigger OD than the Honda [with bigger bearings] but a much thinner wall thickness than the Strorm.. but that looks to be stronger in twist & bending than either of the Honda parts.
2) both the Blade and the Aprillia have Aluminium bottom yokes compared to the cast [forged?] steel Firestorm, but they are significantly thicker sections, with longer overlap onto the stem, and have a double the clamp area onto the fork leg and 2 bolts to control the clamping compared to 1 on the storm.

thinking about the mechanics of keeping the forks inline under braking and steering forces, I tend to think that the yoke bending strength and the clamping area to the forkleg is more important than the stem. but I stress [no engineering pun intended ] this is my opinion, I haven't sat down and got my old university books out to do the calculations

[sirch345]

Personally I can remember way back when we would have killed for a 41mm set of forks, but again I am old =).

+1

I'm old enough to remember that too

The stock front end on a VTR has gotten a pretty bad reputation as being "flexy" or weak.
Personally I can remember way back when we would have killed for a 41mm set of forks, but again I am old =).

If the issue was with the fork tubes, you would think someone would have sourced a set of higher quality aftermarket tubes.
Though in reality the forks are actually pretty good. The issue is the triple clamp and even more so the steering stem.

So lets look at what I see as the main issue.

Here is a stock VTR stem:



and for comparison this is what you get with a SP or Blade triple:



Then pull out the measuring stick and you get this:
Stock -


SP-


and the final visual, the difference between the pins needed for a pit bull stand to lift each front end:



Which leads to my opinion that you can get very good results with the stock forks if you address the triple clamp issue.

An interesting topic this, thanks for posting up the nice clear photo's too Mike
Good to hear what others opinions are as well.

thinking about the mechanics of keeping the forks inline under braking and steering forces, I tend to think that the yoke bending strength and the clamping area to the forkleg is more important than the stem. but I stress [no engineering pun intended ] this is my opinion, I haven't sat down and got my old university books out to do the calculations

You'd better get your books out Al as it would be interesting to know what the difference in strength is between the two steering stems, as the SP or Blade stem does look like it could be made from aluminium I agree unlike the VTR (Firestorm/Superhawk) which is steel as we know.

Chris.

[8541Hawk]

What I can add to this is that yes it is comparing steel to aluminum but the question really should be which aluminum?

Personally I believe they use 7075 for the stems (one of the clues is that it is chrome plated) and might be stronger than one would think.
Also remember we are dealing with torsional or twisting loads.
Another thing that I look at is the torque setting for the nut holding everything together.
Both the VTR and SP1 (which use the same stem nut) have the same torque value. So wall thickness aside the Al stem threads are as strong as the steel units.

All I can say is there is something "weak" in the stock front end. Even with a fork brace it is pretty easy to get thing to start twisting.
I don't believe it is the forks as I have had much heavier bikes with much thinner forks that didn't have the issue.
It's not the frame because as soon as you do a front end swap (which replaces the triple) the issue goes away.

So the only thing left is the triple clamp. I personally think the stock clamp is made of cheese but that is just me

[tony.mon]

Front end swaps are often for shorter fork leg lengths, so the leverage is slightly less.
But the Blade or blade swaps sometimes also have larger discs, and arguably better calipers.
There's also the spindle diameter at the fork bottom to take into account, as a more rigid spindle will reduce the distortion at the bottom of the fork but will also affect the distortion at the yoke.

Not an easy calculation to make, with all of these factors.

[rollingthunderx2]

I always thought of the stem as just something that held the bearings, and allowed their tensioning, if you wanted a stronger, thicker wall tube, you could press in a solid piece of alloy, or just a bottom piece the height of the bearing......and a top piece inside the bearing part of the stem...but I reallly dont think the stem is flexing, and replacing the whole front end but with a similar stem, and getting an improvement would make the stem an unlikely culprit.

[Flatline]

Hawk, if it's the triple as you suggest, what's the solution?

[fabiostar]

well to add confusion. iv just swapped both my front ends.

the blue one got a blade front end including blade stem.... a big improvement over standard. blade yokes,legs and stem.

now the yellow one got 7r forks and pretty huge yokes compared to the storm items, but iv used the storm stem pressed into the 7r yokes.

now from the rides iv had, which arnt the same as spanking it on dry summer roads, the yellow bike has a much stiffer solid feeling front end..

[VTRDark]

now from the rides iv had, which arnt the same as spanking it on dry summer roads, the yellow bike has a much stiffer solid feeling front end..

That must be the little Blue pills having an effect then

[fabiostar]

now from the rides iv had, which arnt the same as spanking it on dry summer roads, the yellow bike has a much stiffer solid feeling front end..

That must be the little Blue pills having an effect then

lol arf arf arf dont be telling the wife lol

[freeridenick]

Has anyone fitted a Blade lower yolk with spacers on standard forks? The yolks are cheap and this seems to me to be a simple and very cost effective way of stiffening the front end. So simple I'm assuming there are downsides I'm not aware of.

[popkat]

The standard yoke is hollow on the underside, you could plate it from underneath which would add strength, you would need to remove the yoke as the heat from welding would melt the steering bearing grease, would also need to be held in a jug so it doesn't distort when welding.

It's possible also to make a plate like a fork brace that goes flush to the bottom yoke, clamp it to the fork tubes then drill and bolt it the bottom yoke. the other option is to have a custom ally bottom yoke made, could be a nice think 3 bolt item, this is the expensive option but find a good friendly engineer and it could cost about £200 all in.


.

[Stephan]

The standard yoke is hollow on the underside, you could plate it from underneath which would add strength, you would need to remove the yoke as the heat from welding would melt the steering bearing grease, would also need to be held in a jug so it doesn't distort when welding.

It's possible also to make a plate like a fork brace that goes flush to the bottom yoke, clamp it to the fork tubes then drill and bolt it the bottom yoke. the other option is to have a custom ally bottom yoke made, could be a nice think 3 bolt item, this is the expensive option but find a good friendly engineer and it could cost about £200 all in.

I was able to do it for that price, but it is maybe still bloody expensive for most of the folks. To do complete usd front end swap seems to be better option, however I wouldn´t be able to do it under 500 gpb including full service, correct springs and valve adjustments. I still think this is good way how to enhance forks from Roger, where there are some money put in.

http://www.vtr1000.org/phpBB3/viewtopic ... 38&t=37985